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GitHub - derive4j/derive4j: Java 8 annotation processor and framework for deriving algebraic data types constructors, pattern-matching, folds, optics and typeclasses.
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9.70375 17.1287 10.02 16.8537C7.5725 16.5787 5.015 15.63 5.015 11.4225C5.015 10.2262 5.44125 9.23625 6.1425 8.46625C6.0325 8.19125 5.6475 7.06375 6.2525 5.55125C6.2525 5.55125 7.17375 5.2625 9.2775 6.67875C10.1575 6.43125 11.0925 6.3075 12.0275 6.3075C12.9625 6.3075 13.8975 6.43125 14.7775 6.67875C16.8813 5.24875 17.8025 5.55125 17.8025 5.55125C18.4075 7.06375 18.0225 8.19125 17.9125 8.46625C18.6138 9.23625 19.04 10.2125 19.04 11.4225C19.04 15.6437 16.4688 16.5787 14.0213 16.8537C14.42 17.1975 14.7638 17.8575 14.7638 18.8887C14.7638 20.36 14.75 21.5425 14.75 21.9137C14.75 22.2025 14.9563 22.5462 15.5063 22.4362C19.8513 20.9787 23 16.8537 23 12C23 5.9225 18.0775 1 12 1Z"></path> </svg> </a> <div class="flex-1 flex-order-2 text-right"> <a href="/login?return_to=https%3A%2F%2Fgithub.com%2Fderive4j%2Fderive4j" class="HeaderMenu-link HeaderMenu-button d-inline-flex d-lg-none flex-order-1 f5 no-underline border color-border-default rounded-2 px-2 py-1 color-fg-inherit 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js-header-menu-item"> <button type="button" class="HeaderMenu-link border-0 width-full width-lg-auto px-0 px-lg-2 py-lg-2 no-wrap d-flex flex-items-center flex-justify-between js-details-target" aria-expanded="false"> Product <svg opacity="0.5" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-chevron-down HeaderMenu-icon ml-1"> <path d="M12.78 5.22a.749.749 0 0 1 0 1.06l-4.25 4.25a.749.749 0 0 1-1.06 0L3.22 6.28a.749.749 0 1 1 1.06-1.06L8 8.939l3.72-3.719a.749.749 0 0 1 1.06 0Z"></path> </svg> </button> <div class="HeaderMenu-dropdown dropdown-menu rounded m-0 p-0 pt-2 pt-lg-4 position-relative position-lg-absolute left-0 left-lg-n3 pb-2 pb-lg-4 d-lg-flex flex-wrap dropdown-menu-wide"> <div class="HeaderMenu-column px-lg-4 border-lg-right mb-4 mb-lg-0 pr-lg-7"> <div class="border-bottom pb-3 pb-lg-0 border-lg-bottom-0"> <ul class="list-style-none f5" > <li> <a class="HeaderMenu-dropdown-link d-block 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0v-2a1 1 0 0 1 1-1Zm-5 0a1 1 0 0 1 1 1v2a1 1 0 0 1-2 0v-2a1 1 0 0 1 1-1Z"></path> </svg> <div> <div class="color-fg-default h4">GitHub Copilot</div> Write better code with AI </div> </a></li> <li> <a class="HeaderMenu-dropdown-link d-block no-underline position-relative py-2 Link--secondary d-flex flex-items-center Link--has-description pb-lg-3" data-analytics-event="{"location":"navbar","action":"github_advanced_security","context":"product","tag":"link","label":"github_advanced_security_link_product_navbar"}" href="https://github.com/security/advanced-security"> <svg aria-hidden="true" height="24" viewBox="0 0 24 24" version="1.1" width="24" data-view-component="true" class="octicon octicon-shield-check color-fg-subtle mr-3"> <path d="M16.53 9.78a.75.75 0 0 0-1.06-1.06L11 13.19l-1.97-1.97a.75.75 0 0 0-1.06 1.06l2.5 2.5a.75.75 0 0 0 1.06 0l5-5Z"></path><path d="m12.54.637 8.25 2.675A1.75 1.75 0 0 1 22 4.976V10c0 6.19-3.771 10.704-9.401 12.83a1.704 1.704 0 0 1-1.198 0C5.77 20.705 2 16.19 2 10V4.976c0-.758.489-1.43 1.21-1.664L11.46.637a1.748 1.748 0 0 1 1.08 0Zm-.617 1.426-8.25 2.676a.249.249 0 0 0-.173.237V10c0 5.46 3.28 9.483 8.43 11.426a.199.199 0 0 0 .14 0C17.22 19.483 20.5 15.461 20.5 10V4.976a.25.25 0 0 0-.173-.237l-8.25-2.676a.253.253 0 0 0-.154 0Z"></path> </svg> <div> <div class="color-fg-default h4">GitHub Advanced Security</div> Find and fix vulnerabilities </div> </a></li> <li> <a class="HeaderMenu-dropdown-link d-block no-underline position-relative py-2 Link--secondary d-flex flex-items-center Link--has-description pb-lg-3" data-analytics-event="{"location":"navbar","action":"actions","context":"product","tag":"link","label":"actions_link_product_navbar"}" href="https://github.com/features/actions"> <svg aria-hidden="true" height="24" viewBox="0 0 24 24" version="1.1" width="24" data-view-component="true" class="octicon octicon-workflow color-fg-subtle mr-3"> <path d="M1 3a2 2 0 0 1 2-2h6.5a2 2 0 0 1 2 2v6.5a2 2 0 0 1-2 2H7v4.063C7 16.355 7.644 17 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1.75 1.75v7.5A1.75 1.75 0 0 1 18.75 13H5.25a1.75 1.75 0 0 1-1.75-1.75Zm-2 12c0-.966.784-1.75 1.75-1.75h17.5c.966 0 1.75.784 1.75 1.75v4a1.75 1.75 0 0 1-1.75 1.75H3.25a1.75 1.75 0 0 1-1.75-1.75ZM5.25 3.5a.25.25 0 0 0-.25.25v7.5c0 .138.112.25.25.25h13.5a.25.25 0 0 0 .25-.25v-7.5a.25.25 0 0 0-.25-.25Zm-2 12a.25.25 0 0 0-.25.25v4c0 .138.112.25.25.25h17.5a.25.25 0 0 0 .25-.25v-4a.25.25 0 0 0-.25-.25Z"></path><path d="M10 17.75a.75.75 0 0 1 .75-.75h6.5a.75.75 0 0 1 0 1.5h-6.5a.75.75 0 0 1-.75-.75Zm-4 0a.75.75 0 0 1 .75-.75h.5a.75.75 0 0 1 0 1.5h-.5a.75.75 0 0 1-.75-.75Z"></path> </svg> <div> <div class="color-fg-default h4">Codespaces</div> Instant dev environments </div> </a></li> <li> <a class="HeaderMenu-dropdown-link d-block no-underline position-relative py-2 Link--secondary d-flex flex-items-center Link--has-description pb-lg-3" data-analytics-event="{"location":"navbar","action":"issues","context":"product","tag":"link","label":"issues_link_product_navbar"}" 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2h16.5c.966 0 1.75.784 1.75 1.75v16.5A1.75 1.75 0 0 1 20.25 22H3.75A1.75 1.75 0 0 1 2 20.25Zm1.75-.25a.25.25 0 0 0-.25.25v16.5c0 .138.112.25.25.25h16.5a.25.25 0 0 0 .25-.25V3.75a.25.25 0 0 0-.25-.25Z"></path> </svg> <div> <div class="color-fg-default h4">Code Search</div> Find more, search less </div> </a></li> </ul> </div> </div> <div class="HeaderMenu-column px-lg-4"> <div class="border-bottom pb-3 pb-lg-0 border-lg-bottom-0 border-bottom-0"> <span class="d-block h4 color-fg-default my-1" id="product-explore-heading">Explore</span> <ul class="list-style-none f5" aria-labelledby="product-explore-heading"> <li> <a class="HeaderMenu-dropdown-link d-block no-underline position-relative py-2 Link--secondary" data-analytics-event="{"location":"navbar","action":"all_features","context":"product","tag":"link","label":"all_features_link_product_navbar"}" href="https://github.com/features"> All features </a></li> <li> <a class="HeaderMenu-dropdown-link d-block no-underline position-relative 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height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-device-desktop"> <path d="M14.25 1c.966 0 1.75.784 1.75 1.75v7.5A1.75 1.75 0 0 1 14.25 12h-3.727c.099 1.041.52 1.872 1.292 2.757A.752.752 0 0 1 11.25 16h-6.5a.75.75 0 0 1-.565-1.243c.772-.885 1.192-1.716 1.292-2.757H1.75A1.75 1.75 0 0 1 0 10.25v-7.5C0 1.784.784 1 1.75 1ZM1.75 2.5a.25.25 0 0 0-.25.25v7.5c0 .138.112.25.25.25h12.5a.25.25 0 0 0 .25-.25v-7.5a.25.25 0 0 0-.25-.25ZM9.018 12H6.982a5.72 5.72 0 0 1-.765 2.5h3.566a5.72 5.72 0 0 1-.765-2.5Z"></path> </svg> </template> <div class="position-relative"> <ul role="listbox" class="ActionListWrap QueryBuilder-ListWrap" aria-label="Suggestions" data-action=" combobox-commit:query-builder#comboboxCommit mousedown:query-builder#resultsMousedown " data-target="query-builder.resultsList" data-persist-list=false id="query-builder-test-results" ></ul> </div> <div class="FormControl-inlineValidation" id="validation-9a1643b3-a2ea-4e8a-ab50-1bca64167930" hidden="hidden"> <span class="FormControl-inlineValidation--visual"> <svg aria-hidden="true" height="12" viewBox="0 0 12 12" version="1.1" width="12" data-view-component="true" class="octicon octicon-alert-fill"> <path d="M4.855.708c.5-.896 1.79-.896 2.29 0l4.675 8.351a1.312 1.312 0 0 1-1.146 1.954H1.33A1.313 1.313 0 0 1 .183 9.058ZM7 7V3H5v4Zm-1 3a1 1 0 1 0 0-2 1 1 0 0 0 0 2Z"></path> </svg> </span> <span></span> </div> </div> <div data-target="query-builder.screenReaderFeedback" aria-live="polite" aria-atomic="true" class="sr-only"></div> </query-builder></form> <div class="d-flex flex-row color-fg-muted px-3 text-small color-bg-default search-feedback-prompt"> <a target="_blank" href="https://docs.github.com/search-github/github-code-search/understanding-github-code-search-syntax" data-view-component="true" class="Link color-fg-accent text-normal ml-2">Search syntax tips</a> <div class="d-flex flex-1"></div> </div> </div> </div> </div> </modal-dialog></div> </div> <div data-action="click:qbsearch-input#retract" class="dark-backdrop position-fixed" hidden data-target="qbsearch-input.darkBackdrop"></div> <div class="color-fg-default"> <dialog-helper> <dialog data-target="qbsearch-input.feedbackDialog" data-action="close:qbsearch-input#handleDialogClose cancel:qbsearch-input#handleDialogClose" id="feedback-dialog" aria-modal="true" aria-labelledby="feedback-dialog-title" aria-describedby="feedback-dialog-description" data-view-component="true" class="Overlay Overlay-whenNarrow Overlay--size-medium Overlay--motion-scaleFade Overlay--disableScroll"> <div data-view-component="true" class="Overlay-header"> <div class="Overlay-headerContentWrap"> <div class="Overlay-titleWrap"> <h1 class="Overlay-title " id="feedback-dialog-title"> Provide feedback </h1> </div> <div class="Overlay-actionWrap"> <button data-close-dialog-id="feedback-dialog" aria-label="Close" type="button" data-view-component="true" class="close-button 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seriously.</p> <textarea name="feedback" class="form-control width-full mb-2" style="height: 120px" id="feedback"></textarea> <input name="include_email" id="include_email" aria-label="Include my email address so I can be contacted" class="form-control mr-2" type="checkbox"> <label for="include_email" style="font-weight: normal">Include my email address so I can be contacted</label> </form></div> </scrollable-region> <div data-view-component="true" class="Overlay-footer Overlay-footer--alignEnd"> <button data-close-dialog-id="feedback-dialog" type="button" data-view-component="true" class="btn"> Cancel </button> <button form="code-search-feedback-form" data-action="click:qbsearch-input#submitFeedback" type="submit" data-view-component="true" class="btn-primary btn"> Submit feedback </button> </div> </dialog></dialog-helper> <custom-scopes data-target="qbsearch-input.customScopesManager"> <dialog-helper> <dialog data-target="custom-scopes.customScopesModalDialog" data-action="close:qbsearch-input#handleDialogClose cancel:qbsearch-input#handleDialogClose" id="custom-scopes-dialog" aria-modal="true" aria-labelledby="custom-scopes-dialog-title" aria-describedby="custom-scopes-dialog-description" data-view-component="true" class="Overlay Overlay-whenNarrow Overlay--size-medium Overlay--motion-scaleFade Overlay--disableScroll"> <div data-view-component="true" class="Overlay-header Overlay-header--divided"> <div class="Overlay-headerContentWrap"> <div class="Overlay-titleWrap"> <h1 class="Overlay-title " id="custom-scopes-dialog-title"> Saved searches </h1> <h2 id="custom-scopes-dialog-description" class="Overlay-description">Use saved searches to filter your results more quickly</h2> </div> <div class="Overlay-actionWrap"> <button data-close-dialog-id="custom-scopes-dialog" aria-label="Close" type="button" data-view-component="true" class="close-button Overlay-closeButton"><svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-x"> <path d="M3.72 3.72a.75.75 0 0 1 1.06 0L8 6.94l3.22-3.22a.749.749 0 0 1 1.275.326.749.749 0 0 1-.215.734L9.06 8l3.22 3.22a.749.749 0 0 1-.326 1.275.749.749 0 0 1-.734-.215L8 9.06l-3.22 3.22a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042L6.94 8 3.72 4.78a.75.75 0 0 1 0-1.06Z"></path> </svg></button> </div> </div> </div> <scrollable-region data-labelled-by="custom-scopes-dialog-title"> <div data-view-component="true" class="Overlay-body"> <div data-target="custom-scopes.customScopesModalDialogFlash"></div> <div hidden class="create-custom-scope-form" data-target="custom-scopes.createCustomScopeForm"> <!-- '"` --><!-- </textarea></xmp> --></option></form><form id="custom-scopes-dialog-form" data-turbo="false" action="/search/custom_scopes" accept-charset="UTF-8" method="post"><input type="hidden" data-csrf="true" name="authenticity_token" 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0 1 1.5 0Zm14.28 2.53-5.25 5.25a.75.75 0 0 1-1.06 0L7 7.06 4.28 9.78a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042l3.25-3.25a.75.75 0 0 1 1.06 0L10 7.94l4.72-4.72a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042Z"></path> </svg> </span> <span data-view-component="true" class="ActionListItem-label"> Insights </span> </a> </li> </ul> </div></action-list> </div> </div></anchored-position> </focus-group> </action-menu></div> </nav> </div> <turbo-frame id="repo-content-turbo-frame" target="_top" data-turbo-action="advance" class=""> <div id="repo-content-pjax-container" class="repository-content " > <h1 class='sr-only'>derive4j/derive4j</h1> <div class="clearfix container-xl px-md-4 px-lg-5 px-3"> <div> <div style="max-width: 100%" data-view-component="true" class="Layout Layout--flowRow-until-md react-repos-overview-margin Layout--sidebarPosition-end Layout--sidebarPosition-flowRow-end"> <div data-view-component="true" class="Layout-main"> <script crossorigin="anonymous" 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annotation processor for deriving algebraic data types constructors, pattern matching and more!\u003c/h1\u003e\u003ca id=\"user-content-derive4j-java-8-annotation-processor-for-deriving-algebraic-data-types-constructors-pattern-matching-and-more\" class=\"anchor\" aria-label=\"Permalink: Derive4J: Java 8 annotation processor for deriving algebraic data types constructors, pattern matching and more!\" href=\"#derive4j-java-8-annotation-processor-for-deriving-algebraic-data-types-constructors-pattern-matching-and-more\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e\u003ca href=\"https://travis-ci.org/derive4j/derive4j\" rel=\"nofollow\"\u003e\u003cimg src=\"https://camo.githubusercontent.com/319c1f2c5b8eb073bf284bbda44c12318bc24e566d217b5de67c189b72cc56ad/68747470733a2f2f7472617669732d63692e6f72672f646572697665346a2f646572697665346a2e7376673f6272616e63683d6d6173746572\" alt=\"Travis\" data-canonical-src=\"https://travis-ci.org/derive4j/derive4j.svg?branch=master\" style=\"max-width: 100%;\"\u003e\u003c/a\u003e\n\u003ca href=\"https://codecov.io/github/derive4j/derive4j\" rel=\"nofollow\"\u003e\u003cimg src=\"https://camo.githubusercontent.com/eee537df185d83087ad205e94d9744034a13176edd794a3397c3a0bc01a5d34e/68747470733a2f2f636f6465636f762e696f2f6769746875622f646572697665346a2f646572697665346a2f6272616e63682f6d61737465722f67726170682f62616467652e737667\" alt=\"codecov.io\" data-canonical-src=\"https://codecov.io/github/derive4j/derive4j/branch/master/graph/badge.svg\" style=\"max-width: 100%;\"\u003e\u003c/a\u003e\n\u003ca href=\"http://search.maven.org/#search%7Cga%7C1%7Corg.derive4j.derive4j\" rel=\"nofollow\"\u003e\u003cimg src=\"https://camo.githubusercontent.com/ee4c68fca4275a128c243717fe6a6d75dc4b0af73dec0e85d9f94d499a815115/68747470733a2f2f696d672e736869656c64732e696f2f6d6176656e2d63656e7472616c2f762f6f72672e646572697665346a2f646572697665346a2e737667\" alt=\"Maven Central\" data-canonical-src=\"https://img.shields.io/maven-central/v/org.derive4j/derive4j.svg\" style=\"max-width: 100%;\"\u003e\u003c/a\u003e\n\u003ca href=\"https://gitter.im/derive4j/derive4j\" rel=\"nofollow\"\u003e\u003cimg src=\"https://camo.githubusercontent.com/ef3705254e766b5edea93f49291c6d9239f29b942cfdb84f3296d0e37898b067/68747470733a2f2f6261646765732e6769747465722e696d2f4a6f696e253230436861742e737667\" alt=\"Gitter Chat\" data-canonical-src=\"https://badges.gitter.im/Join%20Chat.svg\" style=\"max-width: 100%;\"\u003e\u003c/a\u003e\u003c/p\u003e\n\u003cp dir=\"auto\"\u003e\u003cstrong\u003etl;dr\u003c/strong\u003e \u003ca href=\"https://gist.github.com/jbgi/d6035910e55b5b45d1e18553530d9d72\"\u003eShow me how to write, say, the \u003ccode\u003eEither\u003c/code\u003e sum type with Derive4J!\u003c/a\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTable of contents\u003c/h2\u003e\u003ca id=\"user-content-table-of-contents\" class=\"anchor\" aria-label=\"Permalink: Table of contents\" href=\"#table-of-contents\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003e\u003ca href=\"#example-a-visitor-for-http-request\"\u003eExample: a 'Visitor' for HTTP Request\u003c/a\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003e\u003ca href=\"#constructors\"\u003eConstructors\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#equals-hashcode-tostring\"\u003eequals, hashCode, toString?\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#pattern-matching-syntaxes\"\u003ePattern matching syntaxes\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#accessors-getters\"\u003eAccessors (getters)\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#functional-setters-withers\"\u003eFunctional setters ('withers')\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#first-class-laziness\"\u003eFirst class laziness\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#flavours\"\u003eFlavours\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#optics-functional-lenses\"\u003eOptics (functional lenses)\u003c/a\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#smart-constructors\"\u003eSmart constructors\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#static-methods-export\"\u003eStatic methods export\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#updating-deeply-nested-immutable-data-structure\"\u003eUpdating deeply nested immutable data structure\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#popular-use-case-domain-specific-languages\"\u003ePopular use-case: domain specific languages\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#catamorphisms\"\u003eCatamorphisms\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#extensible-algebraic-data-types\"\u003eExtensible algebraic data types\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#but-what-exactly-is-generated\"\u003eBut what exactly is generated?\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#parametric-polymorphism\"\u003eParametric polymorphism\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#generalized-algebraic-data-types\"\u003eGeneralized Algebraic Data Types\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#dry-annotation-configuration\"\u003eDRY annotation configuration\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#use-it-in-your-project\"\u003eUse it in your project\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/derive4j/derive4j/milestones?state=closed\"\u003eChangelog\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#contributing\"\u003eContributing\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"#contact\"\u003eContact\u003c/a\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003e\u003cstrong\u003eCaution\u003c/strong\u003e: if you are not familiar with Algebraic Data Types or the \"visitor pattern\" then you may want to \u003ca href=\"#further-reading\"\u003elearn a bit about them\u003c/a\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eSo, what can this project do for us, poor functional programmers stuck with a legacy language called Java?\nA good deal of what is commonly available in better languages like Haskell, including:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003estructural pattern matching on Algebraic data types, with compile-time exhaustiveness/redundancy check,\u003c/li\u003e\n\u003cli\u003elaziness (a value can be a memoized \u003ca href=\"https://wiki.haskell.org/Thunk\" rel=\"nofollow\"\u003ethunk\u003c/a\u003e),\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/derive4j/derive4j-fj\"\u003eautomatic type class derivation\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://wiki.haskell.org/GADT\" rel=\"nofollow\"\u003eGeneralised algebraic data types\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003ecombinators implementing \u003ca href=\"http://julien-truffaut.github.io/Monocle/optics/lens.html\" rel=\"nofollow\"\u003elenses\u003c/a\u003e, \u003ca href=\"http://julien-truffaut.github.io/Monocle/optics/prism.html\" rel=\"nofollow\"\u003eprisms\u003c/a\u003e and \u003ca href=\"http://julien-truffaut.github.io/Monocle/optics/optional.html\" rel=\"nofollow\"\u003eoptionals\u003c/a\u003e.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eAlgebraic data types come in two flavours, product types and sum types. This readme focus on sum types because it is the more interesting case; product types being the well known common case in Java, but Derive4J handles product types in exactly the same fashion (ie. through a visitor interface with a single abstract method).\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eExample: a 'Visitor' for HTTP Request\u003c/h1\u003e\u003ca id=\"user-content-example-a-visitor-for-http-request\" class=\"anchor\" aria-label=\"Permalink: Example: a 'Visitor' for HTTP Request\" href=\"#example-a-visitor-for-http-request\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eLet's say we want to model an HTTP request. For the sake of the example let's say that an HTTP request can either be\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003ea GET on a given \u003ccode\u003epath\u003c/code\u003e\u003c/li\u003e\n\u003cli\u003ea DELETE on a given \u003ccode\u003epath\u003c/code\u003e\u003c/li\u003e\n\u003cli\u003ea POST of a content \u003ccode\u003ebody\u003c/code\u003e on a given \u003ccode\u003epath\u003c/code\u003e\u003c/li\u003e\n\u003cli\u003ea PUT of a content \u003ccode\u003ebody\u003c/code\u003e on a given \u003ccode\u003epath\u003c/code\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eand nothing else!\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eYou could then use the \u003ca href=\"http://logji.blogspot.ch/2012/02/correcting-visitor-pattern.html\" rel=\"nofollow\"\u003ecorrected visitor pattern\u003c/a\u003e and write the following class in Java:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"package org.derive4j.example;\n\n/** A data type to model an http request. */\n@Data\npublic abstract class Request {\n\n /** the Request 'visitor' interface, R being the return type\n * used by the 'accept' method : */\n interface Cases\u0026lt;R\u0026gt; {\n // A request can either be a 'GET' (of a path):\n R GET(String path);\n // or a 'DELETE' (of a path):\n R DELETE(String path);\n // or a 'PUT' (on a path, with a body):\n R PUT(String path, String body);\n // or a 'POST' (on a path, with a body):\n R POST(String path, String body);\n // and nothing else!\n }\n\n // the 'accept' method of the visitor pattern:\n public abstract \u0026lt;R\u0026gt; R match(Cases\u0026lt;R\u0026gt; cases);\n\n /**\n * Alternatively and equivalently to the visitor pattern above, if you prefer a more FP style,\n * you can define a catamorphism instead. (see examples)\n * (most useful for standard data type like Option, Either, List...)\n */\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003epackage\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e;\n\n\u003cspan class=\"pl-c\"\u003e/** A data type to model an http request. */\u003c/span\u003e\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e {\n\n \u003cspan class=\"pl-c\"\u003e/** the Request 'visitor' interface, R being the return type\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * used by the 'accept' method : */\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-c\"\u003e// A request can either be a 'GET' (of a path):\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e);\n \u003cspan class=\"pl-c\"\u003e// or a 'DELETE' (of a path):\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e);\n \u003cspan class=\"pl-c\"\u003e// or a 'PUT' (on a path, with a body):\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e);\n \u003cspan class=\"pl-c\"\u003e// or a 'POST' (on a path, with a body):\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e);\n \u003cspan class=\"pl-c\"\u003e// and nothing else!\u003c/span\u003e\n }\n\n \u003cspan class=\"pl-c\"\u003e// the 'accept' method of the visitor pattern:\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e);\n\n \u003cspan class=\"pl-c\"\u003e/**\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * Alternatively and equivalently to the visitor pattern above, if you prefer a more FP style,\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * you can define a catamorphism instead. (see examples)\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * (most useful for standard data type like Option, Either, List...)\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e */\u003c/span\u003e\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eConstructors\u003c/h2\u003e\u003ca id=\"user-content-constructors\" class=\"anchor\" aria-label=\"Permalink: Constructors\" href=\"#constructors\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWithout Derive4J, you would have to create subclasses of \u003ccode\u003eRequest\u003c/code\u003e for all four cases. That is, write at the minimum something like:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" public static Request GET(String path) {\n return new Request() {\n @Override\n public \u0026lt;R\u0026gt; R match(Cases\u0026lt;R\u0026gt; cases) {\n return cases.GET(path);\n }\n };}\"\u003e\u003cpre\u003e \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-k\"\u003enew\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e() {\n \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eOverride\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e);\n }\n };}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003efor each case.\nBut thanks to the \u003ccode\u003e@Data\u003c/code\u003e annotation, Derive4j will do that for you! That is, it will generate a \u003ccode\u003eRequests\u003c/code\u003e class (the name is configurable, the class is generated by default in \u003ccode\u003etarget/generated-sources/annotations\u003c/code\u003e when using Maven) with four static factory methods (what we call '\u003cem\u003econstructors\u003c/em\u003e' in FP):\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" public static Request GET(String path) {...}\n public static Request DELETE(String path) {...}\n public static Request PUT(String path, String body) {...}\n public static Request POST(String path, String body) {...}\"\u003e\u003cpre\u003e \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e) {...}\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e) {...}\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) {...}\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) {...}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eYou can also ask Derive4J to generate null checks with:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"@Data(arguments = ArgOption.checkedNotNull)\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003earguments\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eArgOption\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003echeckedNotNull\u003c/span\u003e)\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eequals, hashCode, toString?\u003c/h2\u003e\u003ca id=\"user-content-equals-hashcode-tostring\" class=\"anchor\" aria-label=\"Permalink: equals, hashCode, toString?\" href=\"#equals-hashcode-tostring\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e\u003ca href=\"https://github.com/derive4j/derive4j/issues/50\" data-hovercard-type=\"issue\" data-hovercard-url=\"/derive4j/derive4j/issues/50/hovercard\"\u003eDerive4J philosophy is to be as safe and consistent as possible\u003c/a\u003e. That is why Object.{equals, hashCode, toString} are not implemented by generated classes by default (they are best kept ignored as they break parametricity). Nonetheless, as a concession to legacy, it is possible to force Derive4J to implement them, by declaring them abstract. Eg by adding the following in your annotated class:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" @Override\n public abstract int hashCode();\n @Override\n public abstract boolean equals(Object obj);\n @Override\n public abstract String toString();\"\u003e\u003cpre\u003e \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eOverride\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eint\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ehashCode\u003c/span\u003e();\n \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eOverride\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eboolean\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eequals\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eObject\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eobj\u003c/span\u003e);\n \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eOverride\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-en\"\u003etoString\u003c/span\u003e();\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe safer solution would be to never use those methods and use 'type classes' instead, eg. \u003ca href=\"https://github.com/functionaljava/functionaljava/blob/master/core/src/main/java/fj/Equal.java\"\u003eEqual\u003c/a\u003e, \u003ca href=\"https://github.com/functionaljava/functionaljava/blob/master/core/src/main/java/fj/Hash.java\"\u003eHash\u003c/a\u003e and \u003ca href=\"https://github.com/functionaljava/functionaljava/blob/master/core/src/main/java/fj/Show.java\"\u003eShow\u003c/a\u003e.\nThe project \u003ca href=\"https://github.com/derive4j/derive4j-fj\"\u003eDerive4J for Functional Java\u003c/a\u003e aims to generate them automatically.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003ePattern matching syntaxes\u003c/h2\u003e\u003ca id=\"user-content-pattern-matching-syntaxes\" class=\"anchor\" aria-label=\"Permalink: Pattern matching syntaxes\" href=\"#pattern-matching-syntaxes\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eNow let's say that you want a function that returns the body size of a \u003ccode\u003eRequest\u003c/code\u003e. Without Derive4J you would write something like:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" static final Function\u0026lt;Request, Integer\u0026gt; getBodySize = request -\u0026gt; \n request.match(new Cases\u0026lt;Integer\u0026gt;() {\n public Integer GET(String path) {\n return 0;\n }\n public Integer DELETE(String path) {\n return 0;\n }\n public Integer PUT(String path, String body) {\n return body.length();\n }\n public Integer POST(String path, String body) {\n return body.length();\n }\n });\"\u003e\u003cpre\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003egetBodySize\u003c/span\u003e = \u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e -\u0026gt; \n \u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-k\"\u003enew\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt;() {\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e;\n }\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e;\n }\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e();\n }\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e();\n }\n });\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWith Derive4J you can do that a lot less verbosely, thanks to a generated fluent \u003ca href=\"http://www.deadcoderising.com/pattern-matching-syntax-comparison-in-scala-haskell-ml/\" rel=\"nofollow\"\u003estructural pattern matching\u003c/a\u003e syntaxes! And it does exhaustivity check! (you must handle all cases). The above can be rewritten into:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"static final Function\u0026lt;Request, Integer\u0026gt; getBodySize = Requests.cases()\n .GET_(0) // shortcut for .Get(path -\u0026gt; 0)\n .DELETE_(0)\n .PUT((path, body) -\u0026gt; body.length())\n .POST((path, body) -\u0026gt; body.length())\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003egetBodySize\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003eGET_\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e) \u003cspan class=\"pl-c\"\u003e// shortcut for .Get(path -\u0026gt; 0)\u003c/span\u003e\n .\u003cspan class=\"pl-c1\"\u003eDELETE_\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e)\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e())\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e())\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eor even (because you don't care of GET and DELETE cases):\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"static final Function\u0026lt;Request, Integer\u0026gt; getBodySize = Requests.cases()\n .PUT((path, body) -\u0026gt; body.length())\n .POST((path, body) -\u0026gt; body.length())\n .otherwise_(0)\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003egetBodySize\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e())\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e())\n .\u003cspan class=\"pl-en\"\u003eotherwise_\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e)\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eDerive4j also allows to match directly against a value:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"static int getBodyLength(Request request) {\n return Requests.caseOf(request)\n .PUT((path, body) -\u0026gt; body.length())\n .POST((path, body) -\u0026gt; body.length())\n .otherwise_(0)\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eint\u003c/span\u003e \u003cspan class=\"pl-en\"\u003egetBodyLength\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecaseOf\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e)\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e())\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003elength\u003c/span\u003e())\n .\u003cspan class=\"pl-en\"\u003eotherwise_\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e)\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAccessors (getters)\u003c/h2\u003e\u003ca id=\"user-content-accessors-getters\" class=\"anchor\" aria-label=\"Permalink: Accessors (getters)\" href=\"#accessors-getters\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eNow, pattern matching every time you want to inspect an instance of \u003ccode\u003eRequest\u003c/code\u003e is a bit tedious. For this reason Derive4J generates 'getter' static methods for all fields. For the \u003ccode\u003epath\u003c/code\u003e and \u003ccode\u003ebody\u003c/code\u003e fields, Derive4J will generate the following methods in the \u003ccode\u003eRequests\u003c/code\u003e class:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" public static String getPath(Request request){\n return Requests.cases()\n .GET(path -\u0026gt; path)\n .DELETE(path -\u0026gt; path)\n .PUT((path, body) -\u0026gt; path)\n .POST((path, body) -\u0026gt; path)\n .apply(request);\n }\n // return an Optional because the body is not present in the GET and DELETE cases:\n static Optional\u0026lt;String\u0026gt; getBody(Request request){\n return Requests.cases()\n .PUT((path, body) -\u0026gt; body)\n .POST((path, body) -\u0026gt; body)\n .otherwiseEmpty()\n .apply(request);\n }\"\u003e\u003cpre\u003e \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-en\"\u003egetPath\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e){\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)\n .\u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)\n .\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e);\n }\n \u003cspan class=\"pl-c\"\u003e// return an Optional because the body is not present in the GET and DELETE cases:\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eOptional\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003egetBody\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e){\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e)\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e)\n .\u003cspan class=\"pl-en\"\u003eotherwiseEmpty\u003c/span\u003e()\n .\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e);\n }\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e(Actually the generated code is equivalent but more efficient)\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eUsing the generated \u003ccode\u003egetBody\u003c/code\u003e methods, we can rewrite our \u003ccode\u003egetBodySize\u003c/code\u003e function into:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"static final Function\u0026lt;Request, Integer\u0026gt; getBodySize = request -\u0026gt;\n Requests.getBody(request)\n .map(String::length)\n .orElse(0);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003egetBodySize\u003c/span\u003e = \u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e -\u0026gt;\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003egetBody\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003erequest\u003c/span\u003e)\n .\u003cspan class=\"pl-en\"\u003emap\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003elength\u003c/span\u003e)\n .\u003cspan class=\"pl-en\"\u003eorElse\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFunctional setters ('withers')\u003c/h2\u003e\u003ca id=\"user-content-functional-setters-withers\" class=\"anchor\" aria-label=\"Permalink: Functional setters ('withers')\" href=\"#functional-setters-withers\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe most painful part of immutable data structures (like the one generated by Derive4J) is updating them. Scala case classes have \u003ccode\u003ecopy\u003c/code\u003e methods for that. Derive4J generates similar modifier and setter methods in the \u003ccode\u003eRequests\u003c/code\u003e class:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" public static Function\u0026lt;Request, Request\u0026gt; setPath(String newPath){\n return Requests.cases()\n .GET(path -\u0026gt; Requests.GET(newPath))\n .DELETE(path -\u0026gt; Requests.DELETE(newPath))\n .PUT((path, body) -\u0026gt; Requests.PUT(newPath, body))\n .POST((path, body) -\u0026gt; Requests.POST(newPath, body)));\n }\n public static Function\u0026lt;Request, Request\u0026gt; modPath(Function\u0026lt;String, String\u0026gt; pathMapper){\n return Requests.cases()\n .GET(path -\u0026gt; Requests.GET(pathMapper.apply(path)))\n .DELETE(path -\u0026gt; Requests.DELETE(pathMapper.apply(path)))\n .PUT((path, body) -\u0026gt; Requests.PUT(pathMapper.apply(path), body))\n .POST((path, body) -\u0026gt; Requests.POST(pathMapper.apply(path), body)));\n }\n public static Function\u0026lt;Request, Request\u0026gt; setBody(String newBody){\n return Requests.cases()\n .GET(path -\u0026gt; Requests.GET(path)) // identity function for GET\n .DELETE(path -\u0026gt; Requests.DELETE(path)) // and DELETE cases.\n .PUT((path, body) -\u0026gt; Requests.PUT(path, newBody))\n .POST((path, body) -\u0026gt; Requests.POST(path, newBody)));\n }\n ...\"\u003e\u003cpre\u003e \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003esetPath\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003enewPath\u003c/span\u003e){\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003enewPath\u003c/span\u003e))\n .\u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003enewPath\u003c/span\u003e))\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003enewPath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e))\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003enewPath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e)));\n }\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003emodPath\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003epathMapper\u003c/span\u003e){\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epathMapper\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)))\n .\u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epathMapper\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)))\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epathMapper\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e), \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e))\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epathMapper\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e), \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e)));\n }\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003esetBody\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003enewBody\u003c/span\u003e){\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)) \u003cspan class=\"pl-c\"\u003e// identity function for GET\u003c/span\u003e\n .\u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eDELETE\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e)) \u003cspan class=\"pl-c\"\u003e// and DELETE cases.\u003c/span\u003e\n .\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003ePUT\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003enewBody\u003c/span\u003e))\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003enewBody\u003c/span\u003e)));\n }\n ...\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eBy returning a function, modifiers and setters allow for a lightweight syntax when \u003ca href=\"#updating-deeply-nested-immutable-data-structure\"\u003eupdating deeply nested immutable data structures\u003c/a\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFirst class laziness\u003c/h2\u003e\u003ca id=\"user-content-first-class-laziness\" class=\"anchor\" aria-label=\"Permalink: First class laziness\" href=\"#first-class-laziness\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eLanguages like Haskell provide laziness by default, which simplifies a lot of algorithms. In traditional Java you would have to declare a method argument as \u003ccode\u003eSupplier\u0026lt;Request\u0026gt;\u003c/code\u003e (and do memoization) to emulate laziness. With Derive4J that is no more necessary as it generates a lazy constructor that gives you transparent lazy evaluation for all consumers of your data type:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" // the requestExpression will be lazy-evaluated on the first call\n // to the 'match' method of the returned Request instance:\n public static Request lazy(Supplier\u0026lt;Request\u0026gt; requestExpression) {\n ...\n }\"\u003e\u003cpre\u003e \u003cspan class=\"pl-c\"\u003e// the requestExpression will be lazy-evaluated on the first call\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// to the 'match' method of the returned Request instance:\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e \u003cspan class=\"pl-en\"\u003elazy\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eSupplier\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003erequestExpression\u003c/span\u003e) {\n ...\n }\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eHave a look at \u003ca href=\"https://github.com/derive4j/derive4j/blob/master/examples/src/main/java/org/derive4j/example/List.java\"\u003eList\u003c/a\u003e for how to implement a lazy cons list in Java using Derive4J (you may also want to see the associated \u003ca href=\"https://gist.github.com/jbgi/43c1bd0ab67e3f4b9634\"\u003egenerated code\u003c/a\u003e).\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFlavours\u003c/h2\u003e\u003ca id=\"user-content-flavours\" class=\"anchor\" aria-label=\"Permalink: Flavours\" href=\"#flavours\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIn the example above, we have used the default \u003ccode\u003eJDK\u003c/code\u003e flavour. Also available are \u003ccode\u003eFJ\u003c/code\u003e (\u003ca href=\"https://github.com/functionaljava/\"\u003eFunctional Java\u003c/a\u003e),\n\u003ccode\u003eFugue\u003c/code\u003e (\u003ca href=\"https://bitbucket.org/atlassian/fugue\" rel=\"nofollow\"\u003eFugue\u003c/a\u003e),\n\u003ccode\u003eJavaslang\u003c/code\u003e/\u003ccode\u003eVavr\u003c/code\u003e (\u003ca href=\"http://www.vavr.io/\" rel=\"nofollow\"\u003eVavr\u003c/a\u003e),\n\u003ccode\u003eHighJ\u003c/code\u003e (\u003ca href=\"https://github.com/DanielGronau/highj\"\u003eHighJ\u003c/a\u003e),\n\u003ccode\u003eGuava\u003c/code\u003e and\n\u003ccode\u003eCyclops\u003c/code\u003e (\u003ca href=\"http://cyclops-react.io/\" rel=\"nofollow\"\u003eCyclops-react\u003c/a\u003e) flavours.\nWhen using those alternative flavours, Derive4J will use eg. the specific \u003ccode\u003eOption\u003c/code\u003e implementations from those projects instead of the jdk \u003ccode\u003eOptional\u003c/code\u003e class.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eOptics (functional lenses)\u003c/h2\u003e\u003ca id=\"user-content-optics-functional-lenses\" class=\"anchor\" aria-label=\"Permalink: Optics (functional lenses)\" href=\"#optics-functional-lenses\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIf you are not familiar with optics, have a look at \u003ca href=\"https://github.com/julien-truffaut/Monocle\"\u003eMonocle\u003c/a\u003e (for Scala, but \u003ca href=\"https://github.com/functionaljava/functionaljava/\"\u003eFunctional Java\u003c/a\u003e provides similar abstraction).\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eUsing Derive4J generated code, defining optics is a breeze (you need to use the \u003ccode\u003eFJ\u003c/code\u003e flavour by specifying \u003ccode\u003e@Data(flavour = Flavour.FJ)\u003c/code\u003e:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" /**\n * Lenses: optics focused on a field present for all data type constructors\n * (getter cannot 'failed'):\n */\n public static final Lens\u0026lt;Request, String\u0026gt; _path = lens(\n Requests::getPath,\n Requests::setPath);\n /**\n * Optional: optics focused on a field that may not be present for all constructors\n * (getter return an 'Option'):\n */\n public static final Optional\u0026lt;Request, String\u0026gt; _body = optional(\n Requests::getBody,\n Requests::setBody);\n /**\n * Prism: optics focused on a specific constructor:\n */\n public static final Prism\u0026lt;Request, String\u0026gt; _GET = prism(\n // Getter function\n Requests.cases()\n .GET(fj.data.Option::some)\n .otherwise(Option::none),\n // Reverse Get function (aka constructor)\n Requests::GET);\n\n // If there is more than one field, we use a tuple as the prism target:\n public static final Prism\u0026lt;Request, P2\u0026lt;String, String\u0026gt;\u0026gt; _POST = prism(\n // Getter:\n Requests.cases()\n .POST((path, body) -\u0026gt; p(path, body))\n .otherwiseNone(),\n // reverse get (construct a POST request given a P2\u0026lt;String, String\u0026gt;):\n p2 -\u0026gt; Requests.POST(p2._1(), p2._2()));\n}\"\u003e\u003cpre\u003e \u003cspan class=\"pl-c\"\u003e/**\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * Lenses: optics focused on a field present for all data type constructors\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * (getter cannot 'failed'):\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e */\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eLens\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003e_path\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003elens\u003c/span\u003e(\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003egetPath\u003c/span\u003e,\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003esetPath\u003c/span\u003e);\n \u003cspan class=\"pl-c\"\u003e/**\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * Optional: optics focused on a field that may not be present for all constructors\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * (getter return an 'Option'):\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e */\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eOptional\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003e_body\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003eoptional\u003c/span\u003e(\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003egetBody\u003c/span\u003e,\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003esetBody\u003c/span\u003e);\n \u003cspan class=\"pl-c\"\u003e/**\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e * Prism: optics focused on a specific constructor:\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e */\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003ePrism\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-c1\"\u003e_GET\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003eprism\u003c/span\u003e(\n \u003cspan class=\"pl-c\"\u003e// Getter function\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003efj\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003edata\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eOption\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003esome\u003c/span\u003e)\n .\u003cspan class=\"pl-en\"\u003eotherwise\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eOption\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003enone\u003c/span\u003e),\n \u003cspan class=\"pl-c\"\u003e// Reverse Get function (aka constructor)\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e::\u003cspan class=\"pl-c1\"\u003eGET\u003c/span\u003e);\n\n \u003cspan class=\"pl-c\"\u003e// If there is more than one field, we use a tuple as the prism target:\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003ePrism\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eRequest\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eP2\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e\u0026gt;\u0026gt; \u003cspan class=\"pl-c1\"\u003e_POST\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003eprism\u003c/span\u003e(\n \u003cspan class=\"pl-c\"\u003e// Getter:\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n .\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-en\"\u003ep\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epath\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ebody\u003c/span\u003e))\n .\u003cspan class=\"pl-en\"\u003eotherwiseNone\u003c/span\u003e(),\n \u003cspan class=\"pl-c\"\u003e// reverse get (construct a POST request given a P2\u0026lt;String, String\u0026gt;):\u003c/span\u003e\n \u003cspan class=\"pl-s1\"\u003ep2\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eRequests\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003ePOST\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003ep2\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003e_1\u003c/span\u003e(), \u003cspan class=\"pl-s1\"\u003ep2\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003e_2\u003c/span\u003e()));\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eSmart constructors\u003c/h1\u003e\u003ca id=\"user-content-smart-constructors\" class=\"anchor\" aria-label=\"Permalink: Smart constructors\" href=\"#smart-constructors\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eSometimes you want to validate the constructors parameters before returning an instance of a type. When using the \u003ccode\u003eSmart\u003c/code\u003e visibity (\u003ccode\u003e@Data(@Derive(withVisibility = Visibility.Smart))\u003c/code\u003e), Derive4J will not expose \"raw\" constructors and setter as public, but will use package private visibility for those methods instead (getters will still be public).\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThen you expose a public static factory method that will do the necessary validation of the arguments before returning an instance (typically wrapped in a \u003ccode\u003eOption\u003c/code\u003e/\u003ccode\u003eEither\u003c/code\u003e/\u003ccode\u003eValidation\u003c/code\u003e), and that public factory will be the only way to get an instance of that type.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eSee usage of this feature in \u003ca href=\"https://github.com/derive4j/derive4j/blob/master/examples/src/main/java/org/derive4j/example/PersonName.java#L49\"\u003ePersonName\u003c/a\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eStatic methods export\u003c/h1\u003e\u003ca id=\"user-content-static-methods-export\" class=\"anchor\" aria-label=\"Permalink: Static methods export\" href=\"#static-methods-export\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIt is generally considered good style to keep static methods and instance methods separated, especially because overloads could cause ambiguities on usage as method references.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe Java file generated by derive4j contains only static methods, so it makes sense to use this class as main entry point for the \u003ccode\u003estatic\u003c/code\u003e part of the data type API.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eTo this end, Derive4J support re-exporting of your own manually-written static methods as part of the generated class API. It can do so in two ways (that can be combined):\u003c/p\u003e\n\u003col dir=\"auto\"\u003e\n\u003cli\u003eby specifying that the main generated class must extends a given class eg. \u003ccode\u003eMyStaticMethods.class\u003c/code\u003e, thus exposing all its static methods through inheritance.\u003c/li\u003e\n\u003cli\u003eby annotating your package-private static methods with \u003ccode\u003e@ExportAsPublic\u003c/code\u003e: Derive4J will generate public forwarding methods in the generated class, and, as bonus, it will memoize the result of nullary methods.\u003c/li\u003e\n\u003c/ol\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"@Data(@Derive(extend = MyStaticMethods.class))\npublic abstract class List\u0026lt;A\u0026gt; {\n // package-private static class with public static methods:\n static abstract class MyStaticMethods {\n public static \u0026lt;A\u0026gt; List\u0026lt;A\u0026gt; singleton(A a) {\n return Lists.cons(a, Lists.nil())\n }\n }\n // Or use the annotation, either in the above MyStaticMethods class\n // or directly in the data type class:\n @ExportAsPublic\n static \u0026lt;A\u0026gt; List\u0026lt;A\u0026gt; singleton(A a) {\n return Lists.cons(a, Lists.nil())\n }\n}\n\npublic static void main(final String[] args) {\n // enjoy single access points for all static methods:\n List\u0026lt;String\u0026gt; a = Lists.singleton(\u0026quot;a\u0026quot;);\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eDerive\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eextend\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eMyStaticMethods\u003c/span\u003e.\u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e))\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eList\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-c\"\u003e// package-private static class with public static methods:\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eMyStaticMethods\u003c/span\u003e {\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eList\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003esingleton\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ea\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eLists\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003econs\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003ea\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eLists\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003enil\u003c/span\u003e())\n }\n }\n \u003cspan class=\"pl-c\"\u003e// Or use the annotation, either in the above MyStaticMethods class\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// or directly in the data type class:\u003c/span\u003e\n \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eExportAsPublic\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eList\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003esingleton\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ea\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eLists\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003econs\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003ea\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eLists\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003enil\u003c/span\u003e())\n }\n}\n\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e(\u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e[] \u003cspan class=\"pl-s1\"\u003eargs\u003c/span\u003e) {\n \u003cspan class=\"pl-c\"\u003e// enjoy single access points for all static methods:\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eList\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ea\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eLists\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003esingleton\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e\"a\"\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eSee usage of this feature in \u003ca href=\"https://github.com/derive4j/derive4j/blob/master/examples/src/main/java/org/derive4j/example/PersonName.java#L49\"\u003ePersonName\u003c/a\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eUpdating deeply nested immutable data structure\u003c/h1\u003e\u003ca id=\"user-content-updating-deeply-nested-immutable-data-structure\" class=\"anchor\" aria-label=\"Permalink: Updating deeply nested immutable data structure\" href=\"#updating-deeply-nested-immutable-data-structure\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eLet's say you want to model a CRM. Each client is a \u003ccode\u003ePerson\u003c/code\u003e who can be contacted by email, by telephone or by postal mail. With Derive4J you could write the following:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"import org.derive4j.*;\nimport java.util.function.BiFunction;\n\n@Data\npublic abstract class Address {\n public abstract \u0026lt;R\u0026gt; R match(@FieldNames({\u0026quot;number\u0026quot;, \u0026quot;street\u0026quot;}) \n \t\t\t BiFunction\u0026lt;Integer, String, R\u0026gt; Address);\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.*;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejava\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eutil\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003efunction\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eBiFunction\u003c/span\u003e;\n\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eAddress\u003c/span\u003e {\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eFieldNames\u003c/span\u003e({\u003cspan class=\"pl-s\"\u003e\"number\"\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e\"street\"\u003c/span\u003e}) \n \t\t\t \u003cspan class=\"pl-smi\"\u003eBiFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eAddress\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"import org.derive4j.Data;\n\n@Data\npublic abstract class Contact {\n interface Cases\u0026lt;R\u0026gt; {\n R byEmail(String email);\n R byPhone(String phoneNumber);\n R byMail(Address postalAddress);\n }\n public abstract \u0026lt;R\u0026gt; R match(Cases\u0026lt;R\u0026gt; cases);\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eData\u003c/span\u003e;\n\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eContact\u003c/span\u003e {\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ebyEmail\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eemail\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ebyPhone\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ephoneNumber\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ebyMail\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eAddress\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003epostalAddress\u003c/span\u003e);\n }\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"import org.derive4j.*;\nimport java.util.function.BiFunction;\n\n@Data\npublic abstract class Person {\n public abstract \u0026lt;R\u0026gt; R match(@FieldNames({\u0026quot;name\u0026quot;, \u0026quot;contact\u0026quot;})\n BiFunction\u0026lt;String, Contact, R\u0026gt; Person);\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.*;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejava\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eutil\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003efunction\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eBiFunction\u003c/span\u003e;\n\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003ePerson\u003c/span\u003e {\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eFieldNames\u003c/span\u003e({\u003cspan class=\"pl-s\"\u003e\"name\"\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e\"contact\"\u003c/span\u003e})\n \u003cspan class=\"pl-smi\"\u003eBiFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eContact\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ePerson\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eBut now we have a problem: All the clients have been imported from a legacy database with an off-by-one error for the street number! We must create a function that increments each \u003ccode\u003ePerson\u003c/code\u003e's street number (if it exists) by one. And we have to do this without modifying the original data structure (because it is immutable).\nWith Derive4J, writing such a function is trivial:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"import java.util.Optional;\nimport java.util.function.Function;\n\nimport static org.derive4j.example.Addresss.Address;\nimport static org.derive4j.example.Addresss.getNumber;\nimport static org.derive4j.example.Addresss.modNumber;\nimport static org.derive4j.example.Contacts.getPostalAddress;\nimport static org.derive4j.example.Contacts.modPostalAddress;\nimport static org.derive4j.example.Persons.Person;\nimport static org.derive4j.example.Persons.getContact;\nimport static org.derive4j.example.Persons.modContact;\n\n public static void main(String[] args) {\n\n Person joe = Person(\u0026quot;Joe\u0026quot;, Contacts.byMail(Address(10, \u0026quot;Main St\u0026quot;)));\n\n Function\u0026lt;Person, Person\u0026gt; incrementStreetNumber = modContact(\n \t\t\t\t\t\t modPostalAddress(\n \t\t\t\t\t\t modNumber(number -\u0026gt; number + 1)));\n \n // correctedJoe is a copy of joe with the street number incremented:\n Person correctedJoe = incrementStreetNumber.apply(joe);\n\n Optional\u0026lt;Integer\u0026gt; newStreetNumber = getPostalAddress(getContact(correctedJoe))\n .map(postalAddress -\u0026gt; getNumber(postalAddress));\n\n System.out.println(newStreetNumber); // print \u0026quot;Optional[11]\u0026quot; !!\n }\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejava\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eutil\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eOptional\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejava\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eutil\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003efunction\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eFunction\u003c/span\u003e;\n\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eAddresss\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eAddress\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eAddresss\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003egetNumber\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eAddresss\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003emodNumber\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eContacts\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003egetPostalAddress\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eContacts\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003emodPostalAddress\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ePersons\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ePerson\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ePersons\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003egetContact\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ePersons\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003emodContact\u003c/span\u003e;\n\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e[] \u003cspan class=\"pl-s1\"\u003eargs\u003c/span\u003e) {\n\n \u003cspan class=\"pl-smi\"\u003ePerson\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejoe\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003ePerson\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e\"Joe\"\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eContacts\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ebyMail\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eAddress\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e10\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e\"Main St\"\u003c/span\u003e)));\n\n \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003ePerson\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003ePerson\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eincrementStreetNumber\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003emodContact\u003c/span\u003e(\n \t\t\t\t\t\t \u003cspan class=\"pl-en\"\u003emodPostalAddress\u003c/span\u003e(\n \t\t\t\t\t\t \u003cspan class=\"pl-en\"\u003emodNumber\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003enumber\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-s1\"\u003enumber\u003c/span\u003e + \u003cspan class=\"pl-c1\"\u003e1\u003c/span\u003e)));\n \n \u003cspan class=\"pl-c\"\u003e// correctedJoe is a copy of joe with the street number incremented:\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003ePerson\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ecorrectedJoe\u003c/span\u003e = \u003cspan class=\"pl-s1\"\u003eincrementStreetNumber\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003ejoe\u003c/span\u003e);\n\n \u003cspan class=\"pl-smi\"\u003eOptional\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003enewStreetNumber\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003egetPostalAddress\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003egetContact\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003ecorrectedJoe\u003c/span\u003e))\n .\u003cspan class=\"pl-en\"\u003emap\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epostalAddress\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-en\"\u003egetNumber\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003epostalAddress\u003c/span\u003e));\n\n \u003cspan class=\"pl-smi\"\u003eSystem\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eout\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eprintln\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003enewStreetNumber\u003c/span\u003e); \u003cspan class=\"pl-c\"\u003e// print \"Optional[11]\" !!\u003c/span\u003e\n }\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003ePopular use-case: domain specific languages\u003c/h1\u003e\u003ca id=\"user-content-popular-use-case-domain-specific-languages\" class=\"anchor\" aria-label=\"Permalink: Popular use-case: domain specific languages\" href=\"#popular-use-case-domain-specific-languages\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAlgebraic data types are particulary well fitted for creating DSLs. A calculator for arithmetic expressions could be built like this:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"import java.util.function.Function;\nimport org.derive4j.Data;\nimport static org.derive4j.example.Expressions.*;\n\n@Data\npublic abstract class Expression {\n\n\tinterface Cases\u0026lt;R\u0026gt; {\n\t\tR Const(Integer value);\n\t\tR Add(Expression left, Expression right);\n\t\tR Mult(Expression left, Expression right);\n\t\tR Neg(Expression expr);\n\t}\n\t\n\tpublic abstract \u0026lt;R\u0026gt; R match(Cases\u0026lt;R\u0026gt; cases);\n\n\tprivate static Function\u0026lt;Expression, Integer\u0026gt; eval = Expressions\n\t\t.cases()\n\t\t\t.Const(value -\u0026gt; value)\n\t\t\t.Add((left, right) -\u0026gt; eval(left) + eval(right))\n\t\t\t.Mult((left, right) -\u0026gt; eval(left) * eval(right))\n\t\t\t.Neg(expr -\u0026gt; -eval(expr));\n\n\tpublic static Integer eval(Expression expression) {\n\t\treturn eval.apply(expression);\n\t}\n\n\tpublic static void main(String[] args) {\n\t\tExpression expr = Add(Const(1), Mult(Const(2), Mult(Const(3), Const(3))));\n\t\tSystem.out.println(eval(expr)); // (1+(2*(3*3))) = 19\n\t}\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejava\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eutil\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003efunction\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eFunction\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eData\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eexample\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eExpressions\u003c/span\u003e.*;\n\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e {\n\n\t\u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; {\n\t\t\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eConst\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e);\n\t\t\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eAdd\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e);\n\t\t\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eMult\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e);\n\t\t\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eNeg\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e);\n\t}\n\t\n\t\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e);\n\n\t\u003cspan class=\"pl-k\"\u003eprivate\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eeval\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eExpressions\u003c/span\u003e\n\t\t.\u003cspan class=\"pl-en\"\u003ecases\u003c/span\u003e()\n\t\t\t.\u003cspan class=\"pl-en\"\u003eConst\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e)\n\t\t\t.\u003cspan class=\"pl-en\"\u003eAdd\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e) + \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e))\n\t\t\t.\u003cspan class=\"pl-en\"\u003eMult\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e) * \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e))\n\t\t\t.\u003cspan class=\"pl-en\"\u003eNeg\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e -\u0026gt; -\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e));\n\n\t\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eexpression\u003c/span\u003e) {\n\t\t\u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eeval\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eexpression\u003c/span\u003e);\n\t}\n\n\t\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e[] \u003cspan class=\"pl-s1\"\u003eargs\u003c/span\u003e) {\n\t\t\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003eAdd\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eConst\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e1\u003c/span\u003e), \u003cspan class=\"pl-en\"\u003eMult\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eConst\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e2\u003c/span\u003e), \u003cspan class=\"pl-en\"\u003eMult\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eConst\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e3\u003c/span\u003e), \u003cspan class=\"pl-en\"\u003eConst\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e3\u003c/span\u003e))));\n\t\t\u003cspan class=\"pl-smi\"\u003eSystem\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eout\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eprintln\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e)); \u003cspan class=\"pl-c\"\u003e// (1+(2*(3*3))) = 19\u003c/span\u003e\n\t}\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eCatamorphisms\u003c/h1\u003e\u003ca id=\"user-content-catamorphisms\" class=\"anchor\" aria-label=\"Permalink: Catamorphisms\" href=\"#catamorphisms\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eare generated for recursively defined datatypes. So that you can rewrite the above \u003ccode\u003eeval\u003c/code\u003e method into:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"\tpublic static Integer eval(Expression expression) {\n\t\tExpressions\n\t\t .cata(\n\t\t value -\u0026gt; value,\n\t\t (left, right) -\u0026gt; left + right,\n\t\t (left, right) -\u0026gt; left * right,\n\t\t expr -\u0026gt; -expr,\n\t\t Supplier::get\n\t\t )\n\t\t .apply(expression)\n\t}\"\u003e\u003cpre\u003e\t\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eexpression\u003c/span\u003e) {\n\t\t\u003cspan class=\"pl-smi\"\u003eExpressions\u003c/span\u003e\n\t\t .\u003cspan class=\"pl-en\"\u003ecata\u003c/span\u003e(\n\t\t \u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e,\n\t\t (\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e + \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e,\n\t\t (\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e * \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e,\n\t\t \u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e -\u0026gt; -\u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e,\n\t\t \u003cspan class=\"pl-smi\"\u003eSupplier\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003eget\u003c/span\u003e\n\t\t )\n\t\t .\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eexpression\u003c/span\u003e)\n\t}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe last parameter (\u003ccode\u003eSupplier::get\u003c/code\u003e above) specify how recursive calls are suspended.\nUsing \u003ccode\u003eSupplier::get\u003c/code\u003e means that the computation is not suspended: for deep structures it may blow the stack!\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eTo be safe, use the \u003ccode\u003elazy\u003c/code\u003e, (or \u003ccode\u003edelay\u003c/code\u003e or \u003ccode\u003esuspend\u003c/code\u003e or \u003ccode\u003edefer\u003c/code\u003e...) constructor of your result type, such as the lazy constructor generated by Derive4J.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eIf no such constructor is available then your safe option is to use a \u003ccode\u003eTrampoline\u003c/code\u003e, such as the one provided by FunctionalJava:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"public static Integer stackSafeEval(Expression expression) {\n Expressions.cata(\n value -\u0026gt; Trampoline.pure(value),\n (left, right) -\u0026gt; left.zipWith(right, (l, r) -\u0026gt; l + r),\n (left, right) -\u0026gt; left.zipWith(right, (l, r) -\u0026gt; l * r),\n expr -\u0026gt; expr.map(i -\u0026gt; -i),\n Trampoline::suspend\n ).f(expression).run();\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e \u003cspan class=\"pl-en\"\u003estackSafeEval\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpression\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eexpression\u003c/span\u003e) {\n \u003cspan class=\"pl-smi\"\u003eExpressions\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecata\u003c/span\u003e(\n \u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-smi\"\u003eTrampoline\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003epure\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e),\n (\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ezipWith\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e, (\u003cspan class=\"pl-s1\"\u003el\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003er\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003el\u003c/span\u003e + \u003cspan class=\"pl-s1\"\u003er\u003c/span\u003e),\n (\u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003eleft\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ezipWith\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eright\u003c/span\u003e, (\u003cspan class=\"pl-s1\"\u003el\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003er\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003el\u003c/span\u003e * \u003cspan class=\"pl-s1\"\u003er\u003c/span\u003e),\n \u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-s1\"\u003eexpr\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003emap\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003ei\u003c/span\u003e -\u0026gt; -\u003cspan class=\"pl-s1\"\u003ei\u003c/span\u003e),\n \u003cspan class=\"pl-smi\"\u003eTrampoline\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003esuspend\u003c/span\u003e\n ).\u003cspan class=\"pl-en\"\u003ef\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eexpression\u003c/span\u003e).\u003cspan class=\"pl-en\"\u003erun\u003c/span\u003e();\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eExtensible algebraic data types\u003c/h1\u003e\u003ca id=\"user-content-extensible-algebraic-data-types\" class=\"anchor\" aria-label=\"Permalink: Extensible algebraic data types\" href=\"#extensible-algebraic-data-types\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAlgebraic data types defined as fix-point (aka initial algebra) of an object algebras can enjoy \u003ca href=\"https://www.cs.utexas.edu/~wcook/Drafts/2012/ecoop2012.pdf\" rel=\"nofollow\"\u003etheir extensibility properties\u003c/a\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eWhen the data type is not inductive the extensibility property comes directly from covariance\u003c/h2\u003e\u003ca id=\"user-content-when-the-data-type-is-not-inductive-the-extensibility-property-comes-directly-from-covariance\" class=\"anchor\" aria-label=\"Permalink: When the data type is not inductive the extensibility property comes directly from covariance\" href=\"#when-the-data-type-is-not-inductive-the-extensibility-property-comes-directly-from-covariance\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eEg. an event type for an inventory service:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" @Data\n interface EventV1 {\n\n interface Cases\u0026lt;R\u0026gt; {\n R newItem(Long ref, String itemName);\n R itemRemoved(Long ref);\n }\n\n \u0026lt;R\u0026gt; R match(Cases\u0026lt;R\u0026gt; cases);\n }\"\u003e\u003cpre\u003e \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eEventV1\u003c/span\u003e {\n\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003enewItem\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eLong\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eref\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eitemName\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eitemRemoved\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eLong\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eref\u003c/span\u003e);\n }\n\n \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e);\n }\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThen comes a new version of the service, with enriched events and new cases.\nIf the visitor for the new event type extend the old visitor interface then old events can be easily converted to new events, without change to the old classes:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\" @Data\n interface EventV2 {\n\n interface Cases\u0026lt;R\u0026gt; extends EventV1.Cases\u0026lt;R\u0026gt; { // extends V1 with:\n\n // new `initialStock` field in `newItem` event:\n R newItem(Long ref, String itemName, int initialStock);\n // default to 0 for old events:\n @Override\n default R newItem(Long ref, String itemName) {\n return newItem(ref, itemName, 0);\n }\n // new event:\n R itemRenamed(Long ref, String newName);\n }\n\n \u0026lt;R\u0026gt; R match(Cases\u0026lt;R\u0026gt; cases);\n\n static EventV2 fromV1(EventV1 v1Event) {\n // Events are (polymorphic) functions!\n // And functions are contra-variant in type argument,\n // thus we can use method reference to convert from V1 to V2:\n return v1Event::match;\n }\n }\"\u003e\u003cpre\u003e \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eEventV2\u003c/span\u003e {\n\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-k\"\u003eextends\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eEventV1\u003c/span\u003e.\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; { \u003cspan class=\"pl-c\"\u003e// extends V1 with:\u003c/span\u003e\n\n \u003cspan class=\"pl-c\"\u003e// new `initialStock` field in `newItem` event:\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003enewItem\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eLong\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eref\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eitemName\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eint\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003einitialStock\u003c/span\u003e);\n \u003cspan class=\"pl-c\"\u003e// default to 0 for old events:\u003c/span\u003e\n \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eOverride\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003edefault\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003enewItem\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eLong\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eref\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eitemName\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-en\"\u003enewItem\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eref\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eitemName\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e);\n }\n \u003cspan class=\"pl-c\"\u003e// new event:\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eitemRenamed\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eLong\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eref\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003enewName\u003c/span\u003e);\n }\n\n \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e);\n\n \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eEventV2\u003c/span\u003e \u003cspan class=\"pl-en\"\u003efromV1\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eEventV1\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ev1Event\u003c/span\u003e) {\n \u003cspan class=\"pl-c\"\u003e// Events are (polymorphic) functions!\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// And functions are contra-variant in type argument,\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// thus we can use method reference to convert from V1 to V2:\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ev1Event\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003ematch\u003c/span\u003e;\n }\n }\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eExtensible inductive data types via hylomorphisms\u003c/h2\u003e\u003ca id=\"user-content-extensible-inductive-data-types-via-hylomorphisms\" class=\"anchor\" aria-label=\"Permalink: Extensible inductive data types via hylomorphisms\" href=\"#extensible-inductive-data-types-via-hylomorphisms\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAka solving the expression problem via object-algebras used as visitor.\nFor this, we need to slightly change the visitor of the above \u003ccode\u003eExpression\u003c/code\u003e so that a type variable (\u003ccode\u003eE\u003c/code\u003e) is used instead of the self-reference:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"@Data\ninterface Exp {\n\n interface ExpAlg\u0026lt;E, R\u0026gt; {\n R Lit(int lit);\n R Add(E e1, E e2);\n }\n\n \u0026lt;R\u0026gt; R accept(ExpAlg\u0026lt;Exp, R\u0026gt; alg);\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eExp\u003c/span\u003e {\n\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eExpAlg\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eE\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eLit\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eint\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003elit\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eAdd\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eE\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ee1\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eE\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ee2\u003c/span\u003e);\n }\n\n \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eaccept\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpAlg\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eExp\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ealg\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWhen data types are defined is such a way (as a fix-point of the algebra), Derive4J generate (by default) an instance of the visitor/algebra that can serve as factory (aka. anamorphism).\nUsing this factory as an argument to compatible catamorphism (thus creating a hylomorphism) we obtain a conversion function from one ADT to another.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eEg. we can create a new data type that add a multiplication case to the above data type, and still be able to maximally reuse the existing code without modification:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"@Data\ninterface ExpMul {\n\n interface ExpMulAlg\u0026lt;E, R\u0026gt; extends Exp.ExpAlg\u0026lt;E, R\u0026gt; {\n R Mul(E e1, E e2);\n }\n\n \u0026lt;R\u0026gt; R accept(ExpMulAlg\u0026lt;ExpMul, R\u0026gt; alg);\n\n static Function\u0026lt;Exp, ExpMul\u0026gt; fromExp() {\n ExpMulAlg\u0026lt;ExpMul, ExpMul\u0026gt; factory = ExpMuls.factory();\n return Exps.cata(factory, ExpMuls::lazy);\n }\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eExpMul\u003c/span\u003e {\n\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eExpMulAlg\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eE\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-k\"\u003eextends\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eExp\u003c/span\u003e.\u003cspan class=\"pl-smi\"\u003eExpAlg\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eE\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eMul\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eE\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ee1\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eE\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ee2\u003c/span\u003e);\n }\n\n \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eaccept\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eExpMulAlg\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eExpMul\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ealg\u003c/span\u003e);\n\n \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eExp\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eExpMul\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003efromExp\u003c/span\u003e() {\n \u003cspan class=\"pl-smi\"\u003eExpMulAlg\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eExpMul\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eExpMul\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003efactory\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eExpMuls\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003efactory\u003c/span\u003e();\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eExps\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecata\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003efactory\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eExpMuls\u003c/span\u003e::\u003cspan class=\"pl-s1\"\u003elazy\u003c/span\u003e);\n }\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eTo ensure smooth extensibility across compilation unit (or even during incremental compilation), it is best to \u003cstrong\u003euse the \u003ccode\u003e-parameters\u003c/code\u003e option of javac\u003c/strong\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eBut what exactly is generated?\u003c/h1\u003e\u003ca id=\"user-content-but-what-exactly-is-generated\" class=\"anchor\" aria-label=\"Permalink: But what exactly is generated?\" href=\"#but-what-exactly-is-generated\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis is a very legitimate question. Here is the \u003ca href=\"https://gist.github.com/jbgi/31b891f00566feb301f8100762ee8511#file-expmuls-java\"\u003e\u003ccode\u003eExpMuls.java\u003c/code\u003e\u003c/a\u003e file that is generated for the above \u003ccode\u003e@Data ExpMul\u003c/code\u003e type.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eParametric polymorphism\u003c/h1\u003e\u003ca id=\"user-content-parametric-polymorphism\" class=\"anchor\" aria-label=\"Permalink: Parametric polymorphism\" href=\"#parametric-polymorphism\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e... works as expected. For example, you can write the following:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"import java.util.function.Function;\nimport java.util.function.Supplier;\nimport org.derive4j.Data;\n\n@Data\npublic abstract class Option\u0026lt;A\u0026gt; {\n\n public abstract \u0026lt;R\u0026gt; R cata(Supplier\u0026lt;R\u0026gt; none, Function\u0026lt;A, R\u0026gt; some);\n\n public final \u0026lt;B\u0026gt; Option\u0026lt;B\u0026gt; map(final Function\u0026lt;A, B\u0026gt; mapper) {\n return Options.modSome(mapper).apply(this);\n }\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejava\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eutil\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003efunction\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eFunction\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003ejava\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eutil\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003efunction\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eSupplier\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eData\u003c/span\u003e;\n\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eOption\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; {\n\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ecata\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eSupplier\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003enone\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003esome\u003c/span\u003e);\n\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eB\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eOption\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eB\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003emap\u003c/span\u003e(\u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eFunction\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eB\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003emapper\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eOptions\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003emodSome\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003emapper\u003c/span\u003e).\u003cspan class=\"pl-en\"\u003eapply\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003ethis\u003c/span\u003e);\n }\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe generated modifier method \u003ccode\u003emodSome\u003c/code\u003e allows polymorphic update and is incidentaly the functor for our \u003ccode\u003eOption\u003c/code\u003e!\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eGeneralized Algebraic Data Types\u003c/h1\u003e\u003ca id=\"user-content-generalized-algebraic-data-types\" class=\"anchor\" aria-label=\"Permalink: Generalized Algebraic Data Types\" href=\"#generalized-algebraic-data-types\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eGADTs are also supported out of the box by Derive4J (within the limitations of the Java type system). Here is how you can translate the example from \u003ca href=\"http://www.cs.ox.ac.uk/ralf.hinze/publications/With.pdf\" rel=\"nofollow\"\u003eFun with phantom types\u003c/a\u003e:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"import org.derive4j.hkt.TypeEq;\n\n@Data\npublic abstract class Term\u0026lt;T\u0026gt; {\n interface Cases\u0026lt;A, R\u0026gt; {\n R Zero(TypeEq\u0026lt;Integer, A\u0026gt; id);\n R Succ(Term\u0026lt;Integer\u0026gt; pred, TypeEq\u0026lt;Integer, A\u0026gt; id);\n R Pred(Term\u0026lt;Integer\u0026gt; succ, TypeEq\u0026lt;Integer, A\u0026gt; id);\n R IsZero(Term\u0026lt;Integer\u0026gt; a, TypeEq\u0026lt;Boolean, A\u0026gt; id);\n R If(Term\u0026lt;Boolean\u0026gt; cond, Term\u0026lt;A\u0026gt; then, Term\u0026lt;A\u0026gt; otherwise);\n }\n\n public abstract \u0026lt;X\u0026gt; X match(Cases\u0026lt;T, X\u0026gt; cases);\n\n public static \u0026lt;T\u0026gt; T eval(final Term\u0026lt;T\u0026gt; term) {\n\n return Terms.caseOf(term).\n Zero(id -\u0026gt; id.coerce(0)).\n Succ((t, id) -\u0026gt; id.coerce(eval(t) + 1)).\n Pred((t, id) -\u0026gt; id.coerce(eval(t) - 1)).\n IsZero((t, id) -\u0026gt; id.coerce(eval(t) == 0)).\n If((cond, then, otherwise) -\u0026gt; eval(cond)\n ? eval(then)\n : eval(otherwise));\n }\n\n public static void main(final String[] args) {\n\n Term\u0026lt;Integer\u0026gt; one = Succ(Zero());\n out.println(eval(one)); // \u0026quot;1\u0026quot;\n out.println(eval(IsZero(one))); // \u0026quot;false\u0026quot;\n // IsZero(IsZero(one)); // does not compile:\n // \u0026quot;The method IsZero(Term\u0026lt;Integer\u0026gt;) in the type Term\u0026lt;T\u0026gt; is not\n // applicable for the arguments (Term\u0026lt;Boolean\u0026gt;)\u0026quot;\n out.println(eval(If(IsZero(one), Zero(), one))); // \u0026quot;1\u0026quot;\n Term\u0026lt;Boolean\u0026gt; True = IsZero(Zero());\n Term\u0026lt;Boolean\u0026gt; False = IsZero(one);\n out.println(eval(If(True, True, False))); // \u0026quot;true\u0026quot;\n // out.println(prettyPrint(If(True, True, False), 0)); // \u0026quot;if IsZero(0)\n // then IsZero(0)\n // else IsZero(Succ(0))\u0026quot;\n }\n}\n\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eimport\u003c/span\u003e \u003cspan class=\"pl-s1\"\u003eorg\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ederive4j\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ehkt\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003eTypeEq\u003c/span\u003e;\n\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eT\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-k\"\u003einterface\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e\u0026gt; {\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eZero\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eTypeEq\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eSucc\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003epred\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eTypeEq\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ePred\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003esucc\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eTypeEq\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eIsZero\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ea\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eTypeEq\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eBoolean\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e);\n \u003cspan class=\"pl-smi\"\u003eR\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eIf\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eBoolean\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003econd\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ethen\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eA\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eotherwise\u003c/span\u003e);\n }\n\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eX\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eX\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ematch\u003c/span\u003e(\u003cspan class=\"pl-smi\"\u003eCases\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eT\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eX\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003ecases\u003c/span\u003e);\n\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u0026lt;\u003cspan class=\"pl-smi\"\u003eT\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-smi\"\u003eT\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eT\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eterm\u003c/span\u003e) {\n\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eTerms\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecaseOf\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eterm\u003c/span\u003e).\n \u003cspan class=\"pl-en\"\u003eZero\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e -\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecoerce\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e)).\n \u003cspan class=\"pl-en\"\u003eSucc\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003et\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecoerce\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003et\u003c/span\u003e) + \u003cspan class=\"pl-c1\"\u003e1\u003c/span\u003e)).\n \u003cspan class=\"pl-en\"\u003ePred\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003et\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecoerce\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003et\u003c/span\u003e) - \u003cspan class=\"pl-c1\"\u003e1\u003c/span\u003e)).\n \u003cspan class=\"pl-en\"\u003eIsZero\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003et\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-s1\"\u003eid\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003ecoerce\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003et\u003c/span\u003e) == \u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e)).\n \u003cspan class=\"pl-en\"\u003eIf\u003c/span\u003e((\u003cspan class=\"pl-s1\"\u003econd\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003ethen\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eotherwise\u003c/span\u003e) -\u0026gt; \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003econd\u003c/span\u003e)\n ? \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003ethen\u003c/span\u003e)\n : \u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eotherwise\u003c/span\u003e));\n }\n\n \u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e(\u003cspan class=\"pl-k\"\u003efinal\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eString\u003c/span\u003e[] \u003cspan class=\"pl-s1\"\u003eargs\u003c/span\u003e) {\n\n \u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eInteger\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eone\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003eSucc\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eZero\u003c/span\u003e());\n \u003cspan class=\"pl-s1\"\u003eout\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eprintln\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eone\u003c/span\u003e)); \u003cspan class=\"pl-c\"\u003e// \"1\"\u003c/span\u003e\n \u003cspan class=\"pl-s1\"\u003eout\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eprintln\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eIsZero\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eone\u003c/span\u003e))); \u003cspan class=\"pl-c\"\u003e// \"false\"\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// IsZero(IsZero(one)); // does not compile:\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// \"The method IsZero(Term\u0026lt;Integer\u0026gt;) in the type Term\u0026lt;T\u0026gt; is not\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// applicable for the arguments (Term\u0026lt;Boolean\u0026gt;)\"\u003c/span\u003e\n \u003cspan class=\"pl-s1\"\u003eout\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eprintln\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eIf\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eIsZero\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eone\u003c/span\u003e), \u003cspan class=\"pl-en\"\u003eZero\u003c/span\u003e(), \u003cspan class=\"pl-s1\"\u003eone\u003c/span\u003e))); \u003cspan class=\"pl-c\"\u003e// \"1\"\u003c/span\u003e\n \u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eBoolean\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eTrue\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003eIsZero\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eZero\u003c/span\u003e());\n \u003cspan class=\"pl-smi\"\u003eTerm\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-smi\"\u003eBoolean\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-s1\"\u003eFalse\u003c/span\u003e = \u003cspan class=\"pl-en\"\u003eIsZero\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eone\u003c/span\u003e);\n \u003cspan class=\"pl-s1\"\u003eout\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eprintln\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eeval\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eIf\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eTrue\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eTrue\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003eFalse\u003c/span\u003e))); \u003cspan class=\"pl-c\"\u003e// \"true\"\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// out.println(prettyPrint(If(True, True, False), 0)); // \"if IsZero(0)\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// then IsZero(0)\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// else IsZero(Succ(0))\"\u003c/span\u003e\n }\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eFor GADT you will need to add a dependency on \u003ca href=\"https://github.com/derive4j/hkt\"\u003ederive4j/hkt\u003c/a\u003e which provides \u003ccode\u003eTypeEq\u0026lt;A, B\u0026gt;\u003c/code\u003e: a witness of the equality of two types, \u003ccode\u003eA\u003c/code\u003e and \u003ccode\u003eB\u003c/code\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eDRY annotation configuration\u003c/h1\u003e\u003ca id=\"user-content-dry-annotation-configuration\" class=\"anchor\" aria-label=\"Permalink: DRY annotation configuration\" href=\"#dry-annotation-configuration\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eBy default the \u003ccode\u003e@Data\u003c/code\u003e annotation triggers the generation of \u003ca href=\"/derive4j/derive4j/blob/master/annotation/src/main/java/org/derive4j/Make.java#L22\"\u003eeverything which is available\u003c/a\u003e, in a file whose name is the English plural of the annotated class. But you may want to restrict the scope of what is generated, or change the name of the file, and you usually want all you ADTs to use the same flavour. You may even dislike the name of the annotation because it clashes with another framework...\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor example, let's say that you want to always use the \u003ccode\u003eFJ\u003c/code\u003e flavour (FunctionalJava), make the generated code package private in a class suffixed by \u003ccode\u003eImpl\u003c/code\u003e and only generate the pattern matching syntax and the constructors. Then all you have to do is to create the following annotation:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"@Data(flavour = Flavour.FJ, value = @Derive(\n inClass = \u0026quot;{ClassName}Impl\u0026quot;,\n withVisibility = Visibility.Package,\n make = { Make.constructors, Make.caseOfMatching }\n))\npublic @interface myADT {}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eData\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003eflavour\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eFlavour\u003c/span\u003e.\u003cspan class=\"pl-c1\"\u003eFJ\u003c/span\u003e, \u003cspan class=\"pl-s1\"\u003evalue\u003c/span\u003e = \u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eDerive\u003c/span\u003e(\n \u003cspan class=\"pl-s1\"\u003einClass\u003c/span\u003e = \u003cspan class=\"pl-s\"\u003e\"{ClassName}Impl\"\u003c/span\u003e,\n \u003cspan class=\"pl-s1\"\u003ewithVisibility\u003c/span\u003e = \u003cspan class=\"pl-smi\"\u003eVisibility\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ePackage\u003c/span\u003e,\n \u003cspan class=\"pl-s1\"\u003emake\u003c/span\u003e = { \u003cspan class=\"pl-smi\"\u003eMake\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003econstructors\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eMake\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003ecaseOfMatching\u003c/span\u003e }\n))\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e @interface \u003cspan class=\"pl-s1\"\u003emyADT\u003c/span\u003e {}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAnd you annotate your classes with \u003ccode\u003e@myADT\u003c/code\u003e instead of \u003ccode\u003e@Data\u003c/code\u003e, saving on that configuration every time.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eBut now for some of your ADTs you may want to also generate getters and functional setters. In order to not lose the benefits of your \u003ccode\u003e@myADT\u003c/code\u003e, derive4j allows you to do this:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-java notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"@myADT\n@Derive(make = { Make.getters, Make.modifiers }) // add-up to the @myADT configuration\npublic abstract class Adt {...}\n\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003emyADT\u003c/span\u003e\n\u003cspan class=\"pl-c1\"\u003e@\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eDerive\u003c/span\u003e(\u003cspan class=\"pl-s1\"\u003emake\u003c/span\u003e = { \u003cspan class=\"pl-smi\"\u003eMake\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003egetters\u003c/span\u003e, \u003cspan class=\"pl-smi\"\u003eMake\u003c/span\u003e.\u003cspan class=\"pl-s1\"\u003emodifiers\u003c/span\u003e }) \u003cspan class=\"pl-c\"\u003e// add-up to the @myADT configuration\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003epublic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eabstract\u003c/span\u003e \u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-smi\"\u003eAdt\u003c/span\u003e {...}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eUse it in your project\u003c/h1\u003e\u003ca id=\"user-content-use-it-in-your-project\" class=\"anchor\" aria-label=\"Permalink: Use it in your project\" href=\"#use-it-in-your-project\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eDerive4J should be declared as a compile-time only dependency (not needed at runtime). So while derive4j is (L)GPL-licensed, the generated code is not linked to derive4j, and thus \u003cstrong\u003ederive4j can be used in any project (proprietary or not)\u003c/strong\u003e.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eMaven:\u003c/h2\u003e\u003ca id=\"user-content-maven\" class=\"anchor\" aria-label=\"Permalink: Maven:\" href=\"#maven\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cdiv class=\"highlight highlight-text-xml notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"\u0026lt;dependency\u0026gt;\n \u0026lt;groupId\u0026gt;org.derive4j\u0026lt;/groupId\u0026gt;\n \u0026lt;artifactId\u0026gt;derive4j\u0026lt;/artifactId\u0026gt;\n \u0026lt;version\u0026gt;1.1.1\u0026lt;/version\u0026gt;\n \u0026lt;optional\u0026gt;true\u0026lt;/optional\u0026gt;\n\u0026lt;/dependency\u0026gt;\"\u003e\u003cpre\u003e\u0026lt;\u003cspan class=\"pl-ent\"\u003edependency\u003c/span\u003e\u0026gt;\n \u0026lt;\u003cspan class=\"pl-ent\"\u003egroupId\u003c/span\u003e\u0026gt;org.derive4j\u0026lt;/\u003cspan class=\"pl-ent\"\u003egroupId\u003c/span\u003e\u0026gt;\n \u0026lt;\u003cspan class=\"pl-ent\"\u003eartifactId\u003c/span\u003e\u0026gt;derive4j\u0026lt;/\u003cspan class=\"pl-ent\"\u003eartifactId\u003c/span\u003e\u0026gt;\n \u0026lt;\u003cspan class=\"pl-ent\"\u003eversion\u003c/span\u003e\u0026gt;1.1.1\u0026lt;/\u003cspan class=\"pl-ent\"\u003eversion\u003c/span\u003e\u0026gt;\n \u0026lt;\u003cspan class=\"pl-ent\"\u003eoptional\u003c/span\u003e\u0026gt;true\u0026lt;/\u003cspan class=\"pl-ent\"\u003eoptional\u003c/span\u003e\u0026gt;\n\u0026lt;/\u003cspan class=\"pl-ent\"\u003edependency\u003c/span\u003e\u0026gt;\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eGradle\u003c/h2\u003e\u003ca id=\"user-content-gradle\" class=\"anchor\" aria-label=\"Permalink: Gradle\" href=\"#gradle\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cdiv class=\"snippet-clipboard-content notranslate position-relative overflow-auto\" data-snippet-clipboard-copy-content=\"compile(group: 'org.derive4j', name: 'derive4j', version: '1.1.1', ext: 'jar')\"\u003e\u003cpre class=\"notranslate\"\u003e\u003ccode\u003ecompile(group: 'org.derive4j', name: 'derive4j', version: '1.1.1', ext: 'jar')\n\u003c/code\u003e\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eor better using the \u003ca href=\"https://github.com/tbroyer/gradle-apt-plugin\"\u003egradle-apt-plugin\u003c/a\u003e:\u003c/p\u003e\n\u003cdiv class=\"snippet-clipboard-content notranslate position-relative overflow-auto\" data-snippet-clipboard-copy-content=\"compileOnly \u0026quot;org.derive4j:derive4j-annotation:1.1.1\u0026quot;\napt \u0026quot;org.derive4j:derive4j:1.1.1\u0026quot;\"\u003e\u003cpre class=\"notranslate\"\u003e\u003ccode\u003ecompileOnly \"org.derive4j:derive4j-annotation:1.1.1\"\napt \"org.derive4j:derive4j:1.1.1\"\n\u003c/code\u003e\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eContributing\u003c/h2\u003e\u003ca id=\"user-content-contributing\" class=\"anchor\" aria-label=\"Permalink: Contributing\" href=\"#contributing\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eBug reports and feature requests are welcome, as well as contributions to improve documentation.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eRight now the codebase is not ready for external contribution (many blocks of code are more complicated than they should be). So you might be better off waiting for the resolution of \u003ca href=\"https://github.com/derive4j/derive4j/issues/2\" data-hovercard-type=\"issue\" data-hovercard-url=\"/derive4j/derive4j/issues/2/hovercard\"\u003e#2\u003c/a\u003e before trying to dig into the codebase.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eContact\u003c/h2\u003e\u003ca id=\"user-content-contact\" class=\"anchor\" aria-label=\"Permalink: Contact\" href=\"#contact\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e\u003ca href=\"mailto:jb@giraudeau.info\"\u003ejb@giraudeau.info\u003c/a\u003e, \u003ca href=\"https://twitter.com/jb9i\" rel=\"nofollow\"\u003e@jb9i\u003c/a\u003e or use the project GitHub issues.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFurther reading\u003c/h2\u003e\u003ca id=\"user-content-further-reading\" class=\"anchor\" aria-label=\"Permalink: Further reading\" href=\"#further-reading\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003e\u003ca href=\"https://gracious-hypatia-aac58b.netlify.com/\" rel=\"nofollow\"\u003eEncoding FP in Java\u003c/a\u003e (and links)\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"http://eng.wealthfront.com/2015/02/pattern-matching-in-java-with-visitor.html\" rel=\"nofollow\"\u003ehttp://eng.wealthfront.com/2015/02/pattern-matching-in-java-with-visitor.html\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://en.wikipedia.org/wiki/Algebraic_data_type\" rel=\"nofollow\"\u003ehttps://en.wikipedia.org/wiki/Algebraic_data_type\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://en.wikipedia.org/wiki/Tagged_union\" rel=\"nofollow\"\u003ehttps://en.wikipedia.org/wiki/Tagged_union\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"http://blog.higher-order.com/blog/2009/08/21/structural-pattern-matching-in-java/\" rel=\"nofollow\"\u003ehttp://blog.higher-order.com/blog/2009/08/21/structural-pattern-matching-in-java/\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"http://tomasp.net/blog/types-and-math.aspx/\" rel=\"nofollow\"\u003ehttp://tomasp.net/blog/types-and-math.aspx/\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"http://fsharpforfunandprofit.com/posts/type-size-and-design/\" rel=\"nofollow\"\u003ehttp://fsharpforfunandprofit.com/posts/type-size-and-design/\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://codewords.recurse.com/issues/three/algebra-and-calculus-of-algebraic-data-types\" rel=\"nofollow\"\u003ehttps://codewords.recurse.com/issues/three/algebra-and-calculus-of-algebraic-data-types\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"http://chris-taylor.github.io/blog/2013/02/10/the-algebra-of-algebraic-data-types/\" rel=\"nofollow\"\u003ehttp://chris-taylor.github.io/blog/2013/02/10/the-algebra-of-algebraic-data-types/\u003c/a\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eThanks\u003c/h2\u003e\u003ca id=\"user-content-thanks\" class=\"anchor\" aria-label=\"Permalink: Thanks\" href=\"#thanks\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis project has a special dedication to Tony Morris' blog post \u003ca href=\"http://blog.tmorris.net/posts/debut-with-a-catamorphism/index.html\" rel=\"nofollow\"\u003eDebut with a catamorphism\u003c/a\u003e.\nI'm also very thankful to \u003ca href=\"https://github.com/sviperll\"\u003e@sviperll\u003c/a\u003e and his \u003ca href=\"https://github.com/sviperll/adt4j/\"\u003eadt4j\u003c/a\u003e project which was the initial inspiration for Derive4J.\u003c/p\u003e\n\u003c/article\u003e","loaded":true,"timedOut":false,"errorMessage":null,"headerInfo":{"toc":[{"level":1,"text":"Derive4J: Java 8 annotation processor for deriving algebraic data types constructors, pattern matching and more!","anchor":"derive4j-java-8-annotation-processor-for-deriving-algebraic-data-types-constructors-pattern-matching-and-more","htmlText":"Derive4J: Java 8 annotation processor for deriving algebraic data types constructors, pattern matching and more!"},{"level":2,"text":"Table of contents","anchor":"table-of-contents","htmlText":"Table of contents"},{"level":1,"text":"Example: a 'Visitor' for HTTP Request","anchor":"example-a-visitor-for-http-request","htmlText":"Example: a 'Visitor' for HTTP Request"},{"level":2,"text":"Constructors","anchor":"constructors","htmlText":"Constructors"},{"level":2,"text":"equals, hashCode, toString?","anchor":"equals-hashcode-tostring","htmlText":"equals, hashCode, toString?"},{"level":2,"text":"Pattern matching syntaxes","anchor":"pattern-matching-syntaxes","htmlText":"Pattern matching syntaxes"},{"level":2,"text":"Accessors (getters)","anchor":"accessors-getters","htmlText":"Accessors (getters)"},{"level":2,"text":"Functional setters ('withers')","anchor":"functional-setters-withers","htmlText":"Functional setters ('withers')"},{"level":2,"text":"First class laziness","anchor":"first-class-laziness","htmlText":"First class laziness"},{"level":2,"text":"Flavours","anchor":"flavours","htmlText":"Flavours"},{"level":2,"text":"Optics (functional lenses)","anchor":"optics-functional-lenses","htmlText":"Optics (functional lenses)"},{"level":1,"text":"Smart constructors","anchor":"smart-constructors","htmlText":"Smart constructors"},{"level":1,"text":"Static methods export","anchor":"static-methods-export","htmlText":"Static methods export"},{"level":1,"text":"Updating deeply nested immutable data structure","anchor":"updating-deeply-nested-immutable-data-structure","htmlText":"Updating deeply nested immutable data structure"},{"level":1,"text":"Popular use-case: domain specific languages","anchor":"popular-use-case-domain-specific-languages","htmlText":"Popular use-case: domain specific languages"},{"level":1,"text":"Catamorphisms","anchor":"catamorphisms","htmlText":"Catamorphisms"},{"level":1,"text":"Extensible algebraic data types","anchor":"extensible-algebraic-data-types","htmlText":"Extensible algebraic data types"},{"level":2,"text":"When the data type is not inductive the extensibility property comes directly from covariance","anchor":"when-the-data-type-is-not-inductive-the-extensibility-property-comes-directly-from-covariance","htmlText":"When the data type is not inductive the extensibility property comes directly from covariance"},{"level":2,"text":"Extensible inductive data types via hylomorphisms","anchor":"extensible-inductive-data-types-via-hylomorphisms","htmlText":"Extensible inductive data types via hylomorphisms"},{"level":1,"text":"But what exactly is generated?","anchor":"but-what-exactly-is-generated","htmlText":"But what exactly is generated?"},{"level":1,"text":"Parametric polymorphism","anchor":"parametric-polymorphism","htmlText":"Parametric polymorphism"},{"level":1,"text":"Generalized Algebraic Data Types","anchor":"generalized-algebraic-data-types","htmlText":"Generalized Algebraic Data Types"},{"level":1,"text":"DRY annotation configuration","anchor":"dry-annotation-configuration","htmlText":"DRY annotation configuration"},{"level":1,"text":"Use it in your project","anchor":"use-it-in-your-project","htmlText":"Use it in your project"},{"level":2,"text":"Maven:","anchor":"maven","htmlText":"Maven:"},{"level":2,"text":"Gradle","anchor":"gradle","htmlText":"Gradle"},{"level":2,"text":"Contributing","anchor":"contributing","htmlText":"Contributing"},{"level":2,"text":"Contact","anchor":"contact","htmlText":"Contact"},{"level":2,"text":"Further reading","anchor":"further-reading","htmlText":"Further reading"},{"level":2,"text":"Thanks","anchor":"thanks","htmlText":"Thanks"}],"siteNavLoginPath":"/login?return_to=https%3A%2F%2Fgithub.com%2Fderive4j%2Fderive4j"}}],"overviewFilesProcessingTime":0}},"appPayload":{"helpUrl":"https://docs.github.com","findFileWorkerPath":"/assets-cdn/worker/find-file-worker-7d7eb7c71814.js","findInFileWorkerPath":"/assets-cdn/worker/find-in-file-worker-708ec8ade250.js","githubDevUrl":null,"enabled_features":{"copilot_workspace":null,"code_nav_ui_events":false,"overview_shared_code_dropdown_button":true,"react_blob_overlay":false,"accessible_code_button":true,"github_models_repo_integration":false}}}}</script> <div data-target="react-partial.reactRoot"><style data-styled="true" data-styled-version="5.3.11">.iVEunk{margin-top:16px;margin-bottom:16px;}/*!sc*/ 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0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto"><a href="https://travis-ci.org/derive4j/derive4j" rel="nofollow"><img src="https://camo.githubusercontent.com/319c1f2c5b8eb073bf284bbda44c12318bc24e566d217b5de67c189b72cc56ad/68747470733a2f2f7472617669732d63692e6f72672f646572697665346a2f646572697665346a2e7376673f6272616e63683d6d6173746572" alt="Travis" data-canonical-src="https://travis-ci.org/derive4j/derive4j.svg?branch=master" style="max-width: 100%;"></a> <a href="https://codecov.io/github/derive4j/derive4j" rel="nofollow"><img src="https://camo.githubusercontent.com/eee537df185d83087ad205e94d9744034a13176edd794a3397c3a0bc01a5d34e/68747470733a2f2f636f6465636f762e696f2f6769746875622f646572697665346a2f646572697665346a2f6272616e63682f6d61737465722f67726170682f62616467652e737667" alt="codecov.io" data-canonical-src="https://codecov.io/github/derive4j/derive4j/branch/master/graph/badge.svg" style="max-width: 100%;"></a> <a href="http://search.maven.org/#search%7Cga%7C1%7Corg.derive4j.derive4j" rel="nofollow"><img src="https://camo.githubusercontent.com/ee4c68fca4275a128c243717fe6a6d75dc4b0af73dec0e85d9f94d499a815115/68747470733a2f2f696d672e736869656c64732e696f2f6d6176656e2d63656e7472616c2f762f6f72672e646572697665346a2f646572697665346a2e737667" alt="Maven Central" data-canonical-src="https://img.shields.io/maven-central/v/org.derive4j/derive4j.svg" style="max-width: 100%;"></a> <a href="https://gitter.im/derive4j/derive4j" rel="nofollow"><img src="https://camo.githubusercontent.com/ef3705254e766b5edea93f49291c6d9239f29b942cfdb84f3296d0e37898b067/68747470733a2f2f6261646765732e6769747465722e696d2f4a6f696e253230436861742e737667" alt="Gitter Chat" data-canonical-src="https://badges.gitter.im/Join%20Chat.svg" style="max-width: 100%;"></a></p> <p dir="auto"><strong>tl;dr</strong> <a href="https://gist.github.com/jbgi/d6035910e55b5b45d1e18553530d9d72">Show me how to write, say, the <code>Either</code> sum type with Derive4J!</a>.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Table of contents</h2><a id="user-content-table-of-contents" class="anchor" aria-label="Permalink: Table of contents" href="#table-of-contents"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <ul dir="auto"> <li><a href="#example-a-visitor-for-http-request">Example: a 'Visitor' for HTTP Request</a> <ul dir="auto"> <li><a href="#constructors">Constructors</a></li> <li><a href="#equals-hashcode-tostring">equals, hashCode, toString?</a></li> <li><a href="#pattern-matching-syntaxes">Pattern matching syntaxes</a></li> <li><a href="#accessors-getters">Accessors (getters)</a></li> <li><a href="#functional-setters-withers">Functional setters ('withers')</a></li> <li><a href="#first-class-laziness">First class laziness</a></li> <li><a href="#flavours">Flavours</a></li> <li><a href="#optics-functional-lenses">Optics (functional lenses)</a></li> </ul> </li> <li><a href="#smart-constructors">Smart constructors</a></li> <li><a href="#static-methods-export">Static methods export</a></li> <li><a href="#updating-deeply-nested-immutable-data-structure">Updating deeply nested immutable data structure</a></li> <li><a href="#popular-use-case-domain-specific-languages">Popular use-case: domain specific languages</a></li> <li><a href="#catamorphisms">Catamorphisms</a></li> <li><a href="#extensible-algebraic-data-types">Extensible algebraic data types</a></li> <li><a href="#but-what-exactly-is-generated">But what exactly is generated?</a></li> <li><a href="#parametric-polymorphism">Parametric polymorphism</a></li> <li><a href="#generalized-algebraic-data-types">Generalized Algebraic Data Types</a></li> <li><a href="#dry-annotation-configuration">DRY annotation configuration</a></li> <li><a href="#use-it-in-your-project">Use it in your project</a></li> <li><a href="https://github.com/derive4j/derive4j/milestones?state=closed">Changelog</a></li> <li><a href="#contributing">Contributing</a></li> <li><a href="#contact">Contact</a></li> </ul> <p dir="auto"><strong>Caution</strong>: if you are not familiar with Algebraic Data Types or the "visitor pattern" then you may want to <a href="#further-reading">learn a bit about them</a>.</p> <p dir="auto">So, what can this project do for us, poor functional programmers stuck with a legacy language called Java? A good deal of what is commonly available in better languages like Haskell, including:</p> <ul dir="auto"> <li>structural pattern matching on Algebraic data types, with compile-time exhaustiveness/redundancy check,</li> <li>laziness (a value can be a memoized <a href="https://wiki.haskell.org/Thunk" rel="nofollow">thunk</a>),</li> <li><a href="https://github.com/derive4j/derive4j-fj">automatic type class derivation</a></li> <li><a href="https://wiki.haskell.org/GADT" rel="nofollow">Generalised algebraic data types</a></li> <li>combinators implementing <a href="http://julien-truffaut.github.io/Monocle/optics/lens.html" rel="nofollow">lenses</a>, <a href="http://julien-truffaut.github.io/Monocle/optics/prism.html" rel="nofollow">prisms</a> and <a href="http://julien-truffaut.github.io/Monocle/optics/optional.html" rel="nofollow">optionals</a>.</li> </ul> <p dir="auto">Algebraic data types come in two flavours, product types and sum types. This readme focus on sum types because it is the more interesting case; product types being the well known common case in Java, but Derive4J handles product types in exactly the same fashion (ie. through a visitor interface with a single abstract method).</p> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Example: a 'Visitor' for HTTP Request</h1><a id="user-content-example-a-visitor-for-http-request" class="anchor" aria-label="Permalink: Example: a 'Visitor' for HTTP Request" href="#example-a-visitor-for-http-request"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Let's say we want to model an HTTP request. For the sake of the example let's say that an HTTP request can either be</p> <ul dir="auto"> <li>a GET on a given <code>path</code></li> <li>a DELETE on a given <code>path</code></li> <li>a POST of a content <code>body</code> on a given <code>path</code></li> <li>a PUT of a content <code>body</code> on a given <code>path</code></li> </ul> <p dir="auto">and nothing else!</p> <p dir="auto">You could then use the <a href="http://logji.blogspot.ch/2012/02/correcting-visitor-pattern.html" rel="nofollow">corrected visitor pattern</a> and write the following class in Java:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="package org.derive4j.example; /** A data type to model an http request. */ @Data public abstract class Request { /** the Request 'visitor' interface, R being the return type * used by the 'accept' method : */ interface Cases<R> { // A request can either be a 'GET' (of a path): R GET(String path); // or a 'DELETE' (of a path): R DELETE(String path); // or a 'PUT' (on a path, with a body): R PUT(String path, String body); // or a 'POST' (on a path, with a body): R POST(String path, String body); // and nothing else! } // the 'accept' method of the visitor pattern: public abstract <R> R match(Cases<R> cases); /** * Alternatively and equivalently to the visitor pattern above, if you prefer a more FP style, * you can define a catamorphism instead. (see examples) * (most useful for standard data type like Option, Either, List...) */ }"><pre><span class="pl-k">package</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>; <span class="pl-c">/** A data type to model an http request. */</span> <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Request</span> { <span class="pl-c">/** the Request 'visitor' interface, R being the return type</span> <span class="pl-c"> * used by the 'accept' method : */</span> <span class="pl-k">interface</span> <span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> { <span class="pl-c">// A request can either be a 'GET' (of a path):</span> <span class="pl-smi">R</span> <span class="pl-c1">GET</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>); <span class="pl-c">// or a 'DELETE' (of a path):</span> <span class="pl-smi">R</span> <span class="pl-c1">DELETE</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>); <span class="pl-c">// or a 'PUT' (on a path, with a body):</span> <span class="pl-smi">R</span> <span class="pl-c1">PUT</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>, <span class="pl-smi">String</span> <span class="pl-s1">body</span>); <span class="pl-c">// or a 'POST' (on a path, with a body):</span> <span class="pl-smi">R</span> <span class="pl-c1">POST</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>, <span class="pl-smi">String</span> <span class="pl-s1">body</span>); <span class="pl-c">// and nothing else!</span> } <span class="pl-c">// the 'accept' method of the visitor pattern:</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> <span class="pl-s1">cases</span>); <span class="pl-c">/**</span> <span class="pl-c"> * Alternatively and equivalently to the visitor pattern above, if you prefer a more FP style,</span> <span class="pl-c"> * you can define a catamorphism instead. (see examples)</span> <span class="pl-c"> * (most useful for standard data type like Option, Either, List...)</span> <span class="pl-c"> */</span> }</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Constructors</h2><a id="user-content-constructors" class="anchor" aria-label="Permalink: Constructors" href="#constructors"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Without Derive4J, you would have to create subclasses of <code>Request</code> for all four cases. That is, write at the minimum something like:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" public static Request GET(String path) { return new Request() { @Override public <R> R match(Cases<R> cases) { return cases.GET(path); } };}"><pre> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Request</span> <span class="pl-c1">GET</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>) { <span class="pl-k">return</span> <span class="pl-k">new</span> <span class="pl-smi">Request</span>() { <span class="pl-c1">@</span><span class="pl-c1">Override</span> <span class="pl-k">public</span> <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> <span class="pl-s1">cases</span>) { <span class="pl-k">return</span> <span class="pl-s1">cases</span>.<span class="pl-c1">GET</span>(<span class="pl-s1">path</span>); } };}</pre></div> <p dir="auto">for each case. But thanks to the <code>@Data</code> annotation, Derive4j will do that for you! That is, it will generate a <code>Requests</code> class (the name is configurable, the class is generated by default in <code>target/generated-sources/annotations</code> when using Maven) with four static factory methods (what we call '<em>constructors</em>' in FP):</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" public static Request GET(String path) {...} public static Request DELETE(String path) {...} public static Request PUT(String path, String body) {...} public static Request POST(String path, String body) {...}"><pre> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Request</span> <span class="pl-c1">GET</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>) {...} <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Request</span> <span class="pl-c1">DELETE</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>) {...} <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Request</span> <span class="pl-c1">PUT</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>, <span class="pl-smi">String</span> <span class="pl-s1">body</span>) {...} <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Request</span> <span class="pl-c1">POST</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>, <span class="pl-smi">String</span> <span class="pl-s1">body</span>) {...}</pre></div> <p dir="auto">You can also ask Derive4J to generate null checks with:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="@Data(arguments = ArgOption.checkedNotNull)"><pre><span class="pl-c1">@</span><span class="pl-c1">Data</span>(<span class="pl-s1">arguments</span> = <span class="pl-smi">ArgOption</span>.<span class="pl-s1">checkedNotNull</span>)</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">equals, hashCode, toString?</h2><a id="user-content-equals-hashcode-tostring" class="anchor" aria-label="Permalink: equals, hashCode, toString?" href="#equals-hashcode-tostring"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto"><a href="https://github.com/derive4j/derive4j/issues/50" data-hovercard-type="issue" data-hovercard-url="/derive4j/derive4j/issues/50/hovercard">Derive4J philosophy is to be as safe and consistent as possible</a>. That is why Object.{equals, hashCode, toString} are not implemented by generated classes by default (they are best kept ignored as they break parametricity). Nonetheless, as a concession to legacy, it is possible to force Derive4J to implement them, by declaring them abstract. Eg by adding the following in your annotated class:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" @Override public abstract int hashCode(); @Override public abstract boolean equals(Object obj); @Override public abstract String toString();"><pre> <span class="pl-c1">@</span><span class="pl-c1">Override</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-smi">int</span> <span class="pl-en">hashCode</span>(); <span class="pl-c1">@</span><span class="pl-c1">Override</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-smi">boolean</span> <span class="pl-en">equals</span>(<span class="pl-smi">Object</span> <span class="pl-s1">obj</span>); <span class="pl-c1">@</span><span class="pl-c1">Override</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-smi">String</span> <span class="pl-en">toString</span>();</pre></div> <p dir="auto">The safer solution would be to never use those methods and use 'type classes' instead, eg. <a href="https://github.com/functionaljava/functionaljava/blob/master/core/src/main/java/fj/Equal.java">Equal</a>, <a href="https://github.com/functionaljava/functionaljava/blob/master/core/src/main/java/fj/Hash.java">Hash</a> and <a href="https://github.com/functionaljava/functionaljava/blob/master/core/src/main/java/fj/Show.java">Show</a>. The project <a href="https://github.com/derive4j/derive4j-fj">Derive4J for Functional Java</a> aims to generate them automatically.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Pattern matching syntaxes</h2><a id="user-content-pattern-matching-syntaxes" class="anchor" aria-label="Permalink: Pattern matching syntaxes" href="#pattern-matching-syntaxes"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Now let's say that you want a function that returns the body size of a <code>Request</code>. Without Derive4J you would write something like:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" static final Function<Request, Integer> getBodySize = request -> request.match(new Cases<Integer>() { public Integer GET(String path) { return 0; } public Integer DELETE(String path) { return 0; } public Integer PUT(String path, String body) { return body.length(); } public Integer POST(String path, String body) { return body.length(); } });"><pre> <span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Function</span><<span class="pl-smi">Request</span>, <span class="pl-smi">Integer</span>> <span class="pl-s1">getBodySize</span> = <span class="pl-s1">request</span> -> <span class="pl-s1">request</span>.<span class="pl-en">match</span>(<span class="pl-k">new</span> <span class="pl-smi">Cases</span><<span class="pl-smi">Integer</span>>() { <span class="pl-k">public</span> <span class="pl-smi">Integer</span> <span class="pl-c1">GET</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>) { <span class="pl-k">return</span> <span class="pl-c1">0</span>; } <span class="pl-k">public</span> <span class="pl-smi">Integer</span> <span class="pl-c1">DELETE</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>) { <span class="pl-k">return</span> <span class="pl-c1">0</span>; } <span class="pl-k">public</span> <span class="pl-smi">Integer</span> <span class="pl-c1">PUT</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>, <span class="pl-smi">String</span> <span class="pl-s1">body</span>) { <span class="pl-k">return</span> <span class="pl-s1">body</span>.<span class="pl-en">length</span>(); } <span class="pl-k">public</span> <span class="pl-smi">Integer</span> <span class="pl-c1">POST</span>(<span class="pl-smi">String</span> <span class="pl-s1">path</span>, <span class="pl-smi">String</span> <span class="pl-s1">body</span>) { <span class="pl-k">return</span> <span class="pl-s1">body</span>.<span class="pl-en">length</span>(); } });</pre></div> <p dir="auto">With Derive4J you can do that a lot less verbosely, thanks to a generated fluent <a href="http://www.deadcoderising.com/pattern-matching-syntax-comparison-in-scala-haskell-ml/" rel="nofollow">structural pattern matching</a> syntaxes! And it does exhaustivity check! (you must handle all cases). The above can be rewritten into:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="static final Function<Request, Integer> getBodySize = Requests.cases() .GET_(0) // shortcut for .Get(path -> 0) .DELETE_(0) .PUT((path, body) -> body.length()) .POST((path, body) -> body.length())"><pre><span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Function</span><<span class="pl-smi">Request</span>, <span class="pl-smi">Integer</span>> <span class="pl-s1">getBodySize</span> = <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">GET_</span>(<span class="pl-c1">0</span>) <span class="pl-c">// shortcut for .Get(path -> 0)</span> .<span class="pl-c1">DELETE_</span>(<span class="pl-c1">0</span>) .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>.<span class="pl-en">length</span>()) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>.<span class="pl-en">length</span>())</pre></div> <p dir="auto">or even (because you don't care of GET and DELETE cases):</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="static final Function<Request, Integer> getBodySize = Requests.cases() .PUT((path, body) -> body.length()) .POST((path, body) -> body.length()) .otherwise_(0)"><pre><span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Function</span><<span class="pl-smi">Request</span>, <span class="pl-smi">Integer</span>> <span class="pl-s1">getBodySize</span> = <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>.<span class="pl-en">length</span>()) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>.<span class="pl-en">length</span>()) .<span class="pl-en">otherwise_</span>(<span class="pl-c1">0</span>)</pre></div> <p dir="auto">Derive4j also allows to match directly against a value:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="static int getBodyLength(Request request) { return Requests.caseOf(request) .PUT((path, body) -> body.length()) .POST((path, body) -> body.length()) .otherwise_(0) }"><pre><span class="pl-k">static</span> <span class="pl-smi">int</span> <span class="pl-en">getBodyLength</span>(<span class="pl-smi">Request</span> <span class="pl-s1">request</span>) { <span class="pl-k">return</span> <span class="pl-smi">Requests</span>.<span class="pl-en">caseOf</span>(<span class="pl-s1">request</span>) .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>.<span class="pl-en">length</span>()) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>.<span class="pl-en">length</span>()) .<span class="pl-en">otherwise_</span>(<span class="pl-c1">0</span>) }</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Accessors (getters)</h2><a id="user-content-accessors-getters" class="anchor" aria-label="Permalink: Accessors (getters)" href="#accessors-getters"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Now, pattern matching every time you want to inspect an instance of <code>Request</code> is a bit tedious. For this reason Derive4J generates 'getter' static methods for all fields. For the <code>path</code> and <code>body</code> fields, Derive4J will generate the following methods in the <code>Requests</code> class:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" public static String getPath(Request request){ return Requests.cases() .GET(path -> path) .DELETE(path -> path) .PUT((path, body) -> path) .POST((path, body) -> path) .apply(request); } // return an Optional because the body is not present in the GET and DELETE cases: static Optional<String> getBody(Request request){ return Requests.cases() .PUT((path, body) -> body) .POST((path, body) -> body) .otherwiseEmpty() .apply(request); }"><pre> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">String</span> <span class="pl-en">getPath</span>(<span class="pl-smi">Request</span> <span class="pl-s1">request</span>){ <span class="pl-k">return</span> <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">GET</span>(<span class="pl-s1">path</span> -> <span class="pl-s1">path</span>) .<span class="pl-c1">DELETE</span>(<span class="pl-s1">path</span> -> <span class="pl-s1">path</span>) .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">path</span>) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">path</span>) .<span class="pl-en">apply</span>(<span class="pl-s1">request</span>); } <span class="pl-c">// return an Optional because the body is not present in the GET and DELETE cases:</span> <span class="pl-k">static</span> <span class="pl-smi">Optional</span><<span class="pl-smi">String</span>> <span class="pl-en">getBody</span>(<span class="pl-smi">Request</span> <span class="pl-s1">request</span>){ <span class="pl-k">return</span> <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-s1">body</span>) .<span class="pl-en">otherwiseEmpty</span>() .<span class="pl-en">apply</span>(<span class="pl-s1">request</span>); }</pre></div> <p dir="auto">(Actually the generated code is equivalent but more efficient)</p> <p dir="auto">Using the generated <code>getBody</code> methods, we can rewrite our <code>getBodySize</code> function into:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="static final Function<Request, Integer> getBodySize = request -> Requests.getBody(request) .map(String::length) .orElse(0);"><pre><span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Function</span><<span class="pl-smi">Request</span>, <span class="pl-smi">Integer</span>> <span class="pl-s1">getBodySize</span> = <span class="pl-s1">request</span> -> <span class="pl-smi">Requests</span>.<span class="pl-en">getBody</span>(<span class="pl-s1">request</span>) .<span class="pl-en">map</span>(<span class="pl-smi">String</span>::<span class="pl-s1">length</span>) .<span class="pl-en">orElse</span>(<span class="pl-c1">0</span>);</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Functional setters ('withers')</h2><a id="user-content-functional-setters-withers" class="anchor" aria-label="Permalink: Functional setters ('withers')" href="#functional-setters-withers"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The most painful part of immutable data structures (like the one generated by Derive4J) is updating them. Scala case classes have <code>copy</code> methods for that. Derive4J generates similar modifier and setter methods in the <code>Requests</code> class:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" public static Function<Request, Request> setPath(String newPath){ return Requests.cases() .GET(path -> Requests.GET(newPath)) .DELETE(path -> Requests.DELETE(newPath)) .PUT((path, body) -> Requests.PUT(newPath, body)) .POST((path, body) -> Requests.POST(newPath, body))); } public static Function<Request, Request> modPath(Function<String, String> pathMapper){ return Requests.cases() .GET(path -> Requests.GET(pathMapper.apply(path))) .DELETE(path -> Requests.DELETE(pathMapper.apply(path))) .PUT((path, body) -> Requests.PUT(pathMapper.apply(path), body)) .POST((path, body) -> Requests.POST(pathMapper.apply(path), body))); } public static Function<Request, Request> setBody(String newBody){ return Requests.cases() .GET(path -> Requests.GET(path)) // identity function for GET .DELETE(path -> Requests.DELETE(path)) // and DELETE cases. .PUT((path, body) -> Requests.PUT(path, newBody)) .POST((path, body) -> Requests.POST(path, newBody))); } ..."><pre> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Function</span><<span class="pl-smi">Request</span>, <span class="pl-smi">Request</span>> <span class="pl-en">setPath</span>(<span class="pl-smi">String</span> <span class="pl-s1">newPath</span>){ <span class="pl-k">return</span> <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">GET</span>(<span class="pl-s1">path</span> -> <span class="pl-smi">Requests</span>.<span class="pl-c1">GET</span>(<span class="pl-s1">newPath</span>)) .<span class="pl-c1">DELETE</span>(<span class="pl-s1">path</span> -> <span class="pl-smi">Requests</span>.<span class="pl-c1">DELETE</span>(<span class="pl-s1">newPath</span>)) .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-smi">Requests</span>.<span class="pl-c1">PUT</span>(<span class="pl-s1">newPath</span>, <span class="pl-s1">body</span>)) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-smi">Requests</span>.<span class="pl-c1">POST</span>(<span class="pl-s1">newPath</span>, <span class="pl-s1">body</span>))); } <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Function</span><<span class="pl-smi">Request</span>, <span class="pl-smi">Request</span>> <span class="pl-en">modPath</span>(<span class="pl-smi">Function</span><<span class="pl-smi">String</span>, <span class="pl-smi">String</span>> <span class="pl-s1">pathMapper</span>){ <span class="pl-k">return</span> <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">GET</span>(<span class="pl-s1">path</span> -> <span class="pl-smi">Requests</span>.<span class="pl-c1">GET</span>(<span class="pl-s1">pathMapper</span>.<span class="pl-en">apply</span>(<span class="pl-s1">path</span>))) .<span class="pl-c1">DELETE</span>(<span class="pl-s1">path</span> -> <span class="pl-smi">Requests</span>.<span class="pl-c1">DELETE</span>(<span class="pl-s1">pathMapper</span>.<span class="pl-en">apply</span>(<span class="pl-s1">path</span>))) .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-smi">Requests</span>.<span class="pl-c1">PUT</span>(<span class="pl-s1">pathMapper</span>.<span class="pl-en">apply</span>(<span class="pl-s1">path</span>), <span class="pl-s1">body</span>)) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-smi">Requests</span>.<span class="pl-c1">POST</span>(<span class="pl-s1">pathMapper</span>.<span class="pl-en">apply</span>(<span class="pl-s1">path</span>), <span class="pl-s1">body</span>))); } <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Function</span><<span class="pl-smi">Request</span>, <span class="pl-smi">Request</span>> <span class="pl-en">setBody</span>(<span class="pl-smi">String</span> <span class="pl-s1">newBody</span>){ <span class="pl-k">return</span> <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">GET</span>(<span class="pl-s1">path</span> -> <span class="pl-smi">Requests</span>.<span class="pl-c1">GET</span>(<span class="pl-s1">path</span>)) <span class="pl-c">// identity function for GET</span> .<span class="pl-c1">DELETE</span>(<span class="pl-s1">path</span> -> <span class="pl-smi">Requests</span>.<span class="pl-c1">DELETE</span>(<span class="pl-s1">path</span>)) <span class="pl-c">// and DELETE cases.</span> .<span class="pl-c1">PUT</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-smi">Requests</span>.<span class="pl-c1">PUT</span>(<span class="pl-s1">path</span>, <span class="pl-s1">newBody</span>)) .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-smi">Requests</span>.<span class="pl-c1">POST</span>(<span class="pl-s1">path</span>, <span class="pl-s1">newBody</span>))); } ...</pre></div> <p dir="auto">By returning a function, modifiers and setters allow for a lightweight syntax when <a href="#updating-deeply-nested-immutable-data-structure">updating deeply nested immutable data structures</a>.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">First class laziness</h2><a id="user-content-first-class-laziness" class="anchor" aria-label="Permalink: First class laziness" href="#first-class-laziness"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Languages like Haskell provide laziness by default, which simplifies a lot of algorithms. In traditional Java you would have to declare a method argument as <code>Supplier<Request></code> (and do memoization) to emulate laziness. With Derive4J that is no more necessary as it generates a lazy constructor that gives you transparent lazy evaluation for all consumers of your data type:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" // the requestExpression will be lazy-evaluated on the first call // to the 'match' method of the returned Request instance: public static Request lazy(Supplier<Request> requestExpression) { ... }"><pre> <span class="pl-c">// the requestExpression will be lazy-evaluated on the first call</span> <span class="pl-c">// to the 'match' method of the returned Request instance:</span> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Request</span> <span class="pl-en">lazy</span>(<span class="pl-smi">Supplier</span><<span class="pl-smi">Request</span>> <span class="pl-s1">requestExpression</span>) { ... }</pre></div> <p dir="auto">Have a look at <a href="https://github.com/derive4j/derive4j/blob/master/examples/src/main/java/org/derive4j/example/List.java">List</a> for how to implement a lazy cons list in Java using Derive4J (you may also want to see the associated <a href="https://gist.github.com/jbgi/43c1bd0ab67e3f4b9634">generated code</a>).</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Flavours</h2><a id="user-content-flavours" class="anchor" aria-label="Permalink: Flavours" href="#flavours"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">In the example above, we have used the default <code>JDK</code> flavour. Also available are <code>FJ</code> (<a href="https://github.com/functionaljava/">Functional Java</a>), <code>Fugue</code> (<a href="https://bitbucket.org/atlassian/fugue" rel="nofollow">Fugue</a>), <code>Javaslang</code>/<code>Vavr</code> (<a href="http://www.vavr.io/" rel="nofollow">Vavr</a>), <code>HighJ</code> (<a href="https://github.com/DanielGronau/highj">HighJ</a>), <code>Guava</code> and <code>Cyclops</code> (<a href="http://cyclops-react.io/" rel="nofollow">Cyclops-react</a>) flavours. When using those alternative flavours, Derive4J will use eg. the specific <code>Option</code> implementations from those projects instead of the jdk <code>Optional</code> class.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Optics (functional lenses)</h2><a id="user-content-optics-functional-lenses" class="anchor" aria-label="Permalink: Optics (functional lenses)" href="#optics-functional-lenses"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">If you are not familiar with optics, have a look at <a href="https://github.com/julien-truffaut/Monocle">Monocle</a> (for Scala, but <a href="https://github.com/functionaljava/functionaljava/">Functional Java</a> provides similar abstraction).</p> <p dir="auto">Using Derive4J generated code, defining optics is a breeze (you need to use the <code>FJ</code> flavour by specifying <code>@Data(flavour = Flavour.FJ)</code>:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" /** * Lenses: optics focused on a field present for all data type constructors * (getter cannot 'failed'): */ public static final Lens<Request, String> _path = lens( Requests::getPath, Requests::setPath); /** * Optional: optics focused on a field that may not be present for all constructors * (getter return an 'Option'): */ public static final Optional<Request, String> _body = optional( Requests::getBody, Requests::setBody); /** * Prism: optics focused on a specific constructor: */ public static final Prism<Request, String> _GET = prism( // Getter function Requests.cases() .GET(fj.data.Option::some) .otherwise(Option::none), // Reverse Get function (aka constructor) Requests::GET); // If there is more than one field, we use a tuple as the prism target: public static final Prism<Request, P2<String, String>> _POST = prism( // Getter: Requests.cases() .POST((path, body) -> p(path, body)) .otherwiseNone(), // reverse get (construct a POST request given a P2<String, String>): p2 -> Requests.POST(p2._1(), p2._2())); }"><pre> <span class="pl-c">/**</span> <span class="pl-c"> * Lenses: optics focused on a field present for all data type constructors</span> <span class="pl-c"> * (getter cannot 'failed'):</span> <span class="pl-c"> */</span> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Lens</span><<span class="pl-smi">Request</span>, <span class="pl-smi">String</span>> <span class="pl-s1">_path</span> = <span class="pl-en">lens</span>( <span class="pl-smi">Requests</span>::<span class="pl-s1">getPath</span>, <span class="pl-smi">Requests</span>::<span class="pl-s1">setPath</span>); <span class="pl-c">/**</span> <span class="pl-c"> * Optional: optics focused on a field that may not be present for all constructors</span> <span class="pl-c"> * (getter return an 'Option'):</span> <span class="pl-c"> */</span> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Optional</span><<span class="pl-smi">Request</span>, <span class="pl-smi">String</span>> <span class="pl-s1">_body</span> = <span class="pl-en">optional</span>( <span class="pl-smi">Requests</span>::<span class="pl-s1">getBody</span>, <span class="pl-smi">Requests</span>::<span class="pl-s1">setBody</span>); <span class="pl-c">/**</span> <span class="pl-c"> * Prism: optics focused on a specific constructor:</span> <span class="pl-c"> */</span> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Prism</span><<span class="pl-smi">Request</span>, <span class="pl-smi">String</span>> <span class="pl-c1">_GET</span> = <span class="pl-en">prism</span>( <span class="pl-c">// Getter function</span> <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">GET</span>(<span class="pl-s1">fj</span>.<span class="pl-s1">data</span>.<span class="pl-s1">Option</span>::<span class="pl-s1">some</span>) .<span class="pl-en">otherwise</span>(<span class="pl-smi">Option</span>::<span class="pl-s1">none</span>), <span class="pl-c">// Reverse Get function (aka constructor)</span> <span class="pl-smi">Requests</span>::<span class="pl-c1">GET</span>); <span class="pl-c">// If there is more than one field, we use a tuple as the prism target:</span> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-k">final</span> <span class="pl-smi">Prism</span><<span class="pl-smi">Request</span>, <span class="pl-smi">P2</span><<span class="pl-smi">String</span>, <span class="pl-smi">String</span>>> <span class="pl-c1">_POST</span> = <span class="pl-en">prism</span>( <span class="pl-c">// Getter:</span> <span class="pl-smi">Requests</span>.<span class="pl-en">cases</span>() .<span class="pl-c1">POST</span>((<span class="pl-s1">path</span>, <span class="pl-s1">body</span>) -> <span class="pl-en">p</span>(<span class="pl-s1">path</span>, <span class="pl-s1">body</span>)) .<span class="pl-en">otherwiseNone</span>(), <span class="pl-c">// reverse get (construct a POST request given a P2<String, String>):</span> <span class="pl-s1">p2</span> -> <span class="pl-smi">Requests</span>.<span class="pl-c1">POST</span>(<span class="pl-s1">p2</span>.<span class="pl-en">_1</span>(), <span class="pl-s1">p2</span>.<span class="pl-en">_2</span>())); }</pre></div> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Smart constructors</h1><a id="user-content-smart-constructors" class="anchor" aria-label="Permalink: Smart constructors" href="#smart-constructors"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Sometimes you want to validate the constructors parameters before returning an instance of a type. When using the <code>Smart</code> visibity (<code>@Data(@Derive(withVisibility = Visibility.Smart))</code>), Derive4J will not expose "raw" constructors and setter as public, but will use package private visibility for those methods instead (getters will still be public).</p> <p dir="auto">Then you expose a public static factory method that will do the necessary validation of the arguments before returning an instance (typically wrapped in a <code>Option</code>/<code>Either</code>/<code>Validation</code>), and that public factory will be the only way to get an instance of that type.</p> <p dir="auto">See usage of this feature in <a href="https://github.com/derive4j/derive4j/blob/master/examples/src/main/java/org/derive4j/example/PersonName.java#L49">PersonName</a>.</p> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Static methods export</h1><a id="user-content-static-methods-export" class="anchor" aria-label="Permalink: Static methods export" href="#static-methods-export"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">It is generally considered good style to keep static methods and instance methods separated, especially because overloads could cause ambiguities on usage as method references.</p> <p dir="auto">The Java file generated by derive4j contains only static methods, so it makes sense to use this class as main entry point for the <code>static</code> part of the data type API.</p> <p dir="auto">To this end, Derive4J support re-exporting of your own manually-written static methods as part of the generated class API. It can do so in two ways (that can be combined):</p> <ol dir="auto"> <li>by specifying that the main generated class must extends a given class eg. <code>MyStaticMethods.class</code>, thus exposing all its static methods through inheritance.</li> <li>by annotating your package-private static methods with <code>@ExportAsPublic</code>: Derive4J will generate public forwarding methods in the generated class, and, as bonus, it will memoize the result of nullary methods.</li> </ol> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="@Data(@Derive(extend = MyStaticMethods.class)) public abstract class List<A> { // package-private static class with public static methods: static abstract class MyStaticMethods { public static <A> List<A> singleton(A a) { return Lists.cons(a, Lists.nil()) } } // Or use the annotation, either in the above MyStaticMethods class // or directly in the data type class: @ExportAsPublic static <A> List<A> singleton(A a) { return Lists.cons(a, Lists.nil()) } } public static void main(final String[] args) { // enjoy single access points for all static methods: List<String> a = Lists.singleton("a"); }"><pre><span class="pl-c1">@</span><span class="pl-c1">Data</span>(<span class="pl-c1">@</span><span class="pl-c1">Derive</span>(<span class="pl-s1">extend</span> = <span class="pl-smi">MyStaticMethods</span>.<span class="pl-k">class</span>)) <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">List</span><<span class="pl-smi">A</span>> { <span class="pl-c">// package-private static class with public static methods:</span> <span class="pl-k">static</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">MyStaticMethods</span> { <span class="pl-k">public</span> <span class="pl-k">static</span> <<span class="pl-smi">A</span>> <span class="pl-smi">List</span><<span class="pl-smi">A</span>> <span class="pl-en">singleton</span>(<span class="pl-smi">A</span> <span class="pl-s1">a</span>) { <span class="pl-k">return</span> <span class="pl-smi">Lists</span>.<span class="pl-en">cons</span>(<span class="pl-s1">a</span>, <span class="pl-smi">Lists</span>.<span class="pl-en">nil</span>()) } } <span class="pl-c">// Or use the annotation, either in the above MyStaticMethods class</span> <span class="pl-c">// or directly in the data type class:</span> <span class="pl-c1">@</span><span class="pl-c1">ExportAsPublic</span> <span class="pl-k">static</span> <<span class="pl-smi">A</span>> <span class="pl-smi">List</span><<span class="pl-smi">A</span>> <span class="pl-en">singleton</span>(<span class="pl-smi">A</span> <span class="pl-s1">a</span>) { <span class="pl-k">return</span> <span class="pl-smi">Lists</span>.<span class="pl-en">cons</span>(<span class="pl-s1">a</span>, <span class="pl-smi">Lists</span>.<span class="pl-en">nil</span>()) } } <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">void</span> <span class="pl-en">main</span>(<span class="pl-k">final</span> <span class="pl-smi">String</span>[] <span class="pl-s1">args</span>) { <span class="pl-c">// enjoy single access points for all static methods:</span> <span class="pl-smi">List</span><<span class="pl-smi">String</span>> <span class="pl-s1">a</span> = <span class="pl-smi">Lists</span>.<span class="pl-en">singleton</span>(<span class="pl-s">"a"</span>); }</pre></div> <p dir="auto">See usage of this feature in <a href="https://github.com/derive4j/derive4j/blob/master/examples/src/main/java/org/derive4j/example/PersonName.java#L49">PersonName</a>.</p> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Updating deeply nested immutable data structure</h1><a id="user-content-updating-deeply-nested-immutable-data-structure" class="anchor" aria-label="Permalink: Updating deeply nested immutable data structure" href="#updating-deeply-nested-immutable-data-structure"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Let's say you want to model a CRM. Each client is a <code>Person</code> who can be contacted by email, by telephone or by postal mail. With Derive4J you could write the following:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import org.derive4j.*; import java.util.function.BiFunction; @Data public abstract class Address { public abstract <R> R match(@FieldNames({"number", "street"}) BiFunction<Integer, String, R> Address); }"><pre><span class="pl-k">import</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.*; <span class="pl-k">import</span> <span class="pl-s1">java</span>.<span class="pl-s1">util</span>.<span class="pl-s1">function</span>.<span class="pl-s1">BiFunction</span>; <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Address</span> { <span class="pl-k">public</span> <span class="pl-k">abstract</span> <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-c1">@</span><span class="pl-c1">FieldNames</span>({<span class="pl-s">"number"</span>, <span class="pl-s">"street"</span>}) <span class="pl-smi">BiFunction</span><<span class="pl-smi">Integer</span>, <span class="pl-smi">String</span>, <span class="pl-smi">R</span>> <span class="pl-s1">Address</span>); }</pre></div> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import org.derive4j.Data; @Data public abstract class Contact { interface Cases<R> { R byEmail(String email); R byPhone(String phoneNumber); R byMail(Address postalAddress); } public abstract <R> R match(Cases<R> cases); }"><pre><span class="pl-k">import</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">Data</span>; <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Contact</span> { <span class="pl-k">interface</span> <span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> { <span class="pl-smi">R</span> <span class="pl-en">byEmail</span>(<span class="pl-smi">String</span> <span class="pl-s1">email</span>); <span class="pl-smi">R</span> <span class="pl-en">byPhone</span>(<span class="pl-smi">String</span> <span class="pl-s1">phoneNumber</span>); <span class="pl-smi">R</span> <span class="pl-en">byMail</span>(<span class="pl-smi">Address</span> <span class="pl-s1">postalAddress</span>); } <span class="pl-k">public</span> <span class="pl-k">abstract</span> <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> <span class="pl-s1">cases</span>); }</pre></div> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import org.derive4j.*; import java.util.function.BiFunction; @Data public abstract class Person { public abstract <R> R match(@FieldNames({"name", "contact"}) BiFunction<String, Contact, R> Person); }"><pre><span class="pl-k">import</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.*; <span class="pl-k">import</span> <span class="pl-s1">java</span>.<span class="pl-s1">util</span>.<span class="pl-s1">function</span>.<span class="pl-s1">BiFunction</span>; <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Person</span> { <span class="pl-k">public</span> <span class="pl-k">abstract</span> <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-c1">@</span><span class="pl-c1">FieldNames</span>({<span class="pl-s">"name"</span>, <span class="pl-s">"contact"</span>}) <span class="pl-smi">BiFunction</span><<span class="pl-smi">String</span>, <span class="pl-smi">Contact</span>, <span class="pl-smi">R</span>> <span class="pl-s1">Person</span>); }</pre></div> <p dir="auto">But now we have a problem: All the clients have been imported from a legacy database with an off-by-one error for the street number! We must create a function that increments each <code>Person</code>'s street number (if it exists) by one. And we have to do this without modifying the original data structure (because it is immutable). With Derive4J, writing such a function is trivial:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import java.util.Optional; import java.util.function.Function; import static org.derive4j.example.Addresss.Address; import static org.derive4j.example.Addresss.getNumber; import static org.derive4j.example.Addresss.modNumber; import static org.derive4j.example.Contacts.getPostalAddress; import static org.derive4j.example.Contacts.modPostalAddress; import static org.derive4j.example.Persons.Person; import static org.derive4j.example.Persons.getContact; import static org.derive4j.example.Persons.modContact; public static void main(String[] args) { Person joe = Person("Joe", Contacts.byMail(Address(10, "Main St"))); Function<Person, Person> incrementStreetNumber = modContact( modPostalAddress( modNumber(number -> number + 1))); // correctedJoe is a copy of joe with the street number incremented: Person correctedJoe = incrementStreetNumber.apply(joe); Optional<Integer> newStreetNumber = getPostalAddress(getContact(correctedJoe)) .map(postalAddress -> getNumber(postalAddress)); System.out.println(newStreetNumber); // print "Optional[11]" !! }"><pre><span class="pl-k">import</span> <span class="pl-s1">java</span>.<span class="pl-s1">util</span>.<span class="pl-s1">Optional</span>; <span class="pl-k">import</span> <span class="pl-s1">java</span>.<span class="pl-s1">util</span>.<span class="pl-s1">function</span>.<span class="pl-s1">Function</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Addresss</span>.<span class="pl-s1">Address</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Addresss</span>.<span class="pl-s1">getNumber</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Addresss</span>.<span class="pl-s1">modNumber</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Contacts</span>.<span class="pl-s1">getPostalAddress</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Contacts</span>.<span class="pl-s1">modPostalAddress</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Persons</span>.<span class="pl-s1">Person</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Persons</span>.<span class="pl-s1">getContact</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Persons</span>.<span class="pl-s1">modContact</span>; <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">void</span> <span class="pl-en">main</span>(<span class="pl-smi">String</span>[] <span class="pl-s1">args</span>) { <span class="pl-smi">Person</span> <span class="pl-s1">joe</span> = <span class="pl-en">Person</span>(<span class="pl-s">"Joe"</span>, <span class="pl-smi">Contacts</span>.<span class="pl-en">byMail</span>(<span class="pl-en">Address</span>(<span class="pl-c1">10</span>, <span class="pl-s">"Main St"</span>))); <span class="pl-smi">Function</span><<span class="pl-smi">Person</span>, <span class="pl-smi">Person</span>> <span class="pl-s1">incrementStreetNumber</span> = <span class="pl-en">modContact</span>( <span class="pl-en">modPostalAddress</span>( <span class="pl-en">modNumber</span>(<span class="pl-s1">number</span> -> <span class="pl-s1">number</span> + <span class="pl-c1">1</span>))); <span class="pl-c">// correctedJoe is a copy of joe with the street number incremented:</span> <span class="pl-smi">Person</span> <span class="pl-s1">correctedJoe</span> = <span class="pl-s1">incrementStreetNumber</span>.<span class="pl-en">apply</span>(<span class="pl-s1">joe</span>); <span class="pl-smi">Optional</span><<span class="pl-smi">Integer</span>> <span class="pl-s1">newStreetNumber</span> = <span class="pl-en">getPostalAddress</span>(<span class="pl-en">getContact</span>(<span class="pl-s1">correctedJoe</span>)) .<span class="pl-en">map</span>(<span class="pl-s1">postalAddress</span> -> <span class="pl-en">getNumber</span>(<span class="pl-s1">postalAddress</span>)); <span class="pl-smi">System</span>.<span class="pl-s1">out</span>.<span class="pl-en">println</span>(<span class="pl-s1">newStreetNumber</span>); <span class="pl-c">// print "Optional[11]" !!</span> }</pre></div> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Popular use-case: domain specific languages</h1><a id="user-content-popular-use-case-domain-specific-languages" class="anchor" aria-label="Permalink: Popular use-case: domain specific languages" href="#popular-use-case-domain-specific-languages"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Algebraic data types are particulary well fitted for creating DSLs. A calculator for arithmetic expressions could be built like this:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import java.util.function.Function; import org.derive4j.Data; import static org.derive4j.example.Expressions.*; @Data public abstract class Expression { interface Cases<R> { R Const(Integer value); R Add(Expression left, Expression right); R Mult(Expression left, Expression right); R Neg(Expression expr); } public abstract <R> R match(Cases<R> cases); private static Function<Expression, Integer> eval = Expressions .cases() .Const(value -> value) .Add((left, right) -> eval(left) + eval(right)) .Mult((left, right) -> eval(left) * eval(right)) .Neg(expr -> -eval(expr)); public static Integer eval(Expression expression) { return eval.apply(expression); } public static void main(String[] args) { Expression expr = Add(Const(1), Mult(Const(2), Mult(Const(3), Const(3)))); System.out.println(eval(expr)); // (1+(2*(3*3))) = 19 } }"><pre><span class="pl-k">import</span> <span class="pl-s1">java</span>.<span class="pl-s1">util</span>.<span class="pl-s1">function</span>.<span class="pl-s1">Function</span>; <span class="pl-k">import</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">Data</span>; <span class="pl-k">import</span> <span class="pl-k">static</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">example</span>.<span class="pl-s1">Expressions</span>.*; <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Expression</span> { <span class="pl-k">interface</span> <span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> { <span class="pl-smi">R</span> <span class="pl-en">Const</span>(<span class="pl-smi">Integer</span> <span class="pl-s1">value</span>); <span class="pl-smi">R</span> <span class="pl-en">Add</span>(<span class="pl-smi">Expression</span> <span class="pl-s1">left</span>, <span class="pl-smi">Expression</span> <span class="pl-s1">right</span>); <span class="pl-smi">R</span> <span class="pl-en">Mult</span>(<span class="pl-smi">Expression</span> <span class="pl-s1">left</span>, <span class="pl-smi">Expression</span> <span class="pl-s1">right</span>); <span class="pl-smi">R</span> <span class="pl-en">Neg</span>(<span class="pl-smi">Expression</span> <span class="pl-s1">expr</span>); } <span class="pl-k">public</span> <span class="pl-k">abstract</span> <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> <span class="pl-s1">cases</span>); <span class="pl-k">private</span> <span class="pl-k">static</span> <span class="pl-smi">Function</span><<span class="pl-smi">Expression</span>, <span class="pl-smi">Integer</span>> <span class="pl-s1">eval</span> = <span class="pl-smi">Expressions</span> .<span class="pl-en">cases</span>() .<span class="pl-en">Const</span>(<span class="pl-s1">value</span> -> <span class="pl-s1">value</span>) .<span class="pl-en">Add</span>((<span class="pl-s1">left</span>, <span class="pl-s1">right</span>) -> <span class="pl-en">eval</span>(<span class="pl-s1">left</span>) + <span class="pl-en">eval</span>(<span class="pl-s1">right</span>)) .<span class="pl-en">Mult</span>((<span class="pl-s1">left</span>, <span class="pl-s1">right</span>) -> <span class="pl-en">eval</span>(<span class="pl-s1">left</span>) * <span class="pl-en">eval</span>(<span class="pl-s1">right</span>)) .<span class="pl-en">Neg</span>(<span class="pl-s1">expr</span> -> -<span class="pl-en">eval</span>(<span class="pl-s1">expr</span>)); <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Integer</span> <span class="pl-en">eval</span>(<span class="pl-smi">Expression</span> <span class="pl-s1">expression</span>) { <span class="pl-k">return</span> <span class="pl-s1">eval</span>.<span class="pl-en">apply</span>(<span class="pl-s1">expression</span>); } <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">void</span> <span class="pl-en">main</span>(<span class="pl-smi">String</span>[] <span class="pl-s1">args</span>) { <span class="pl-smi">Expression</span> <span class="pl-s1">expr</span> = <span class="pl-en">Add</span>(<span class="pl-en">Const</span>(<span class="pl-c1">1</span>), <span class="pl-en">Mult</span>(<span class="pl-en">Const</span>(<span class="pl-c1">2</span>), <span class="pl-en">Mult</span>(<span class="pl-en">Const</span>(<span class="pl-c1">3</span>), <span class="pl-en">Const</span>(<span class="pl-c1">3</span>)))); <span class="pl-smi">System</span>.<span class="pl-s1">out</span>.<span class="pl-en">println</span>(<span class="pl-en">eval</span>(<span class="pl-s1">expr</span>)); <span class="pl-c">// (1+(2*(3*3))) = 19</span> } }</pre></div> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Catamorphisms</h1><a id="user-content-catamorphisms" class="anchor" aria-label="Permalink: Catamorphisms" href="#catamorphisms"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">are generated for recursively defined datatypes. So that you can rewrite the above <code>eval</code> method into:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" public static Integer eval(Expression expression) { Expressions .cata( value -> value, (left, right) -> left + right, (left, right) -> left * right, expr -> -expr, Supplier::get ) .apply(expression) }"><pre> <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Integer</span> <span class="pl-en">eval</span>(<span class="pl-smi">Expression</span> <span class="pl-s1">expression</span>) { <span class="pl-smi">Expressions</span> .<span class="pl-en">cata</span>( <span class="pl-s1">value</span> -> <span class="pl-s1">value</span>, (<span class="pl-s1">left</span>, <span class="pl-s1">right</span>) -> <span class="pl-s1">left</span> + <span class="pl-s1">right</span>, (<span class="pl-s1">left</span>, <span class="pl-s1">right</span>) -> <span class="pl-s1">left</span> * <span class="pl-s1">right</span>, <span class="pl-s1">expr</span> -> -<span class="pl-s1">expr</span>, <span class="pl-smi">Supplier</span>::<span class="pl-s1">get</span> ) .<span class="pl-en">apply</span>(<span class="pl-s1">expression</span>) }</pre></div> <p dir="auto">The last parameter (<code>Supplier::get</code> above) specify how recursive calls are suspended. Using <code>Supplier::get</code> means that the computation is not suspended: for deep structures it may blow the stack!</p> <p dir="auto">To be safe, use the <code>lazy</code>, (or <code>delay</code> or <code>suspend</code> or <code>defer</code>...) constructor of your result type, such as the lazy constructor generated by Derive4J.</p> <p dir="auto">If no such constructor is available then your safe option is to use a <code>Trampoline</code>, such as the one provided by FunctionalJava:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="public static Integer stackSafeEval(Expression expression) { Expressions.cata( value -> Trampoline.pure(value), (left, right) -> left.zipWith(right, (l, r) -> l + r), (left, right) -> left.zipWith(right, (l, r) -> l * r), expr -> expr.map(i -> -i), Trampoline::suspend ).f(expression).run(); }"><pre><span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">Integer</span> <span class="pl-en">stackSafeEval</span>(<span class="pl-smi">Expression</span> <span class="pl-s1">expression</span>) { <span class="pl-smi">Expressions</span>.<span class="pl-en">cata</span>( <span class="pl-s1">value</span> -> <span class="pl-smi">Trampoline</span>.<span class="pl-en">pure</span>(<span class="pl-s1">value</span>), (<span class="pl-s1">left</span>, <span class="pl-s1">right</span>) -> <span class="pl-s1">left</span>.<span class="pl-en">zipWith</span>(<span class="pl-s1">right</span>, (<span class="pl-s1">l</span>, <span class="pl-s1">r</span>) -> <span class="pl-s1">l</span> + <span class="pl-s1">r</span>), (<span class="pl-s1">left</span>, <span class="pl-s1">right</span>) -> <span class="pl-s1">left</span>.<span class="pl-en">zipWith</span>(<span class="pl-s1">right</span>, (<span class="pl-s1">l</span>, <span class="pl-s1">r</span>) -> <span class="pl-s1">l</span> * <span class="pl-s1">r</span>), <span class="pl-s1">expr</span> -> <span class="pl-s1">expr</span>.<span class="pl-en">map</span>(<span class="pl-s1">i</span> -> -<span class="pl-s1">i</span>), <span class="pl-smi">Trampoline</span>::<span class="pl-s1">suspend</span> ).<span class="pl-en">f</span>(<span class="pl-s1">expression</span>).<span class="pl-en">run</span>(); }</pre></div> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Extensible algebraic data types</h1><a id="user-content-extensible-algebraic-data-types" class="anchor" aria-label="Permalink: Extensible algebraic data types" href="#extensible-algebraic-data-types"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Algebraic data types defined as fix-point (aka initial algebra) of an object algebras can enjoy <a href="https://www.cs.utexas.edu/~wcook/Drafts/2012/ecoop2012.pdf" rel="nofollow">their extensibility properties</a>.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">When the data type is not inductive the extensibility property comes directly from covariance</h2><a id="user-content-when-the-data-type-is-not-inductive-the-extensibility-property-comes-directly-from-covariance" class="anchor" aria-label="Permalink: When the data type is not inductive the extensibility property comes directly from covariance" href="#when-the-data-type-is-not-inductive-the-extensibility-property-comes-directly-from-covariance"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Eg. an event type for an inventory service:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" @Data interface EventV1 { interface Cases<R> { R newItem(Long ref, String itemName); R itemRemoved(Long ref); } <R> R match(Cases<R> cases); }"><pre> <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">interface</span> <span class="pl-smi">EventV1</span> { <span class="pl-k">interface</span> <span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> { <span class="pl-smi">R</span> <span class="pl-en">newItem</span>(<span class="pl-smi">Long</span> <span class="pl-s1">ref</span>, <span class="pl-smi">String</span> <span class="pl-s1">itemName</span>); <span class="pl-smi">R</span> <span class="pl-en">itemRemoved</span>(<span class="pl-smi">Long</span> <span class="pl-s1">ref</span>); } <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> <span class="pl-s1">cases</span>); }</pre></div> <p dir="auto">Then comes a new version of the service, with enriched events and new cases. If the visitor for the new event type extend the old visitor interface then old events can be easily converted to new events, without change to the old classes:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content=" @Data interface EventV2 { interface Cases<R> extends EventV1.Cases<R> { // extends V1 with: // new `initialStock` field in `newItem` event: R newItem(Long ref, String itemName, int initialStock); // default to 0 for old events: @Override default R newItem(Long ref, String itemName) { return newItem(ref, itemName, 0); } // new event: R itemRenamed(Long ref, String newName); } <R> R match(Cases<R> cases); static EventV2 fromV1(EventV1 v1Event) { // Events are (polymorphic) functions! // And functions are contra-variant in type argument, // thus we can use method reference to convert from V1 to V2: return v1Event::match; } }"><pre> <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">interface</span> <span class="pl-smi">EventV2</span> { <span class="pl-k">interface</span> <span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> <span class="pl-k">extends</span> <span class="pl-smi">EventV1</span>.<span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> { <span class="pl-c">// extends V1 with:</span> <span class="pl-c">// new `initialStock` field in `newItem` event:</span> <span class="pl-smi">R</span> <span class="pl-en">newItem</span>(<span class="pl-smi">Long</span> <span class="pl-s1">ref</span>, <span class="pl-smi">String</span> <span class="pl-s1">itemName</span>, <span class="pl-smi">int</span> <span class="pl-s1">initialStock</span>); <span class="pl-c">// default to 0 for old events:</span> <span class="pl-c1">@</span><span class="pl-c1">Override</span> <span class="pl-k">default</span> <span class="pl-smi">R</span> <span class="pl-en">newItem</span>(<span class="pl-smi">Long</span> <span class="pl-s1">ref</span>, <span class="pl-smi">String</span> <span class="pl-s1">itemName</span>) { <span class="pl-k">return</span> <span class="pl-en">newItem</span>(<span class="pl-s1">ref</span>, <span class="pl-s1">itemName</span>, <span class="pl-c1">0</span>); } <span class="pl-c">// new event:</span> <span class="pl-smi">R</span> <span class="pl-en">itemRenamed</span>(<span class="pl-smi">Long</span> <span class="pl-s1">ref</span>, <span class="pl-smi">String</span> <span class="pl-s1">newName</span>); } <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">match</span>(<span class="pl-smi">Cases</span><<span class="pl-smi">R</span>> <span class="pl-s1">cases</span>); <span class="pl-k">static</span> <span class="pl-smi">EventV2</span> <span class="pl-en">fromV1</span>(<span class="pl-smi">EventV1</span> <span class="pl-s1">v1Event</span>) { <span class="pl-c">// Events are (polymorphic) functions!</span> <span class="pl-c">// And functions are contra-variant in type argument,</span> <span class="pl-c">// thus we can use method reference to convert from V1 to V2:</span> <span class="pl-k">return</span> <span class="pl-s1">v1Event</span>::<span class="pl-s1">match</span>; } }</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Extensible inductive data types via hylomorphisms</h2><a id="user-content-extensible-inductive-data-types-via-hylomorphisms" class="anchor" aria-label="Permalink: Extensible inductive data types via hylomorphisms" href="#extensible-inductive-data-types-via-hylomorphisms"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Aka solving the expression problem via object-algebras used as visitor. For this, we need to slightly change the visitor of the above <code>Expression</code> so that a type variable (<code>E</code>) is used instead of the self-reference:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="@Data interface Exp { interface ExpAlg<E, R> { R Lit(int lit); R Add(E e1, E e2); } <R> R accept(ExpAlg<Exp, R> alg); }"><pre><span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">interface</span> <span class="pl-smi">Exp</span> { <span class="pl-k">interface</span> <span class="pl-smi">ExpAlg</span><<span class="pl-smi">E</span>, <span class="pl-smi">R</span>> { <span class="pl-smi">R</span> <span class="pl-en">Lit</span>(<span class="pl-smi">int</span> <span class="pl-s1">lit</span>); <span class="pl-smi">R</span> <span class="pl-en">Add</span>(<span class="pl-smi">E</span> <span class="pl-s1">e1</span>, <span class="pl-smi">E</span> <span class="pl-s1">e2</span>); } <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">accept</span>(<span class="pl-smi">ExpAlg</span><<span class="pl-smi">Exp</span>, <span class="pl-smi">R</span>> <span class="pl-s1">alg</span>); }</pre></div> <p dir="auto">When data types are defined is such a way (as a fix-point of the algebra), Derive4J generate (by default) an instance of the visitor/algebra that can serve as factory (aka. anamorphism). Using this factory as an argument to compatible catamorphism (thus creating a hylomorphism) we obtain a conversion function from one ADT to another.</p> <p dir="auto">Eg. we can create a new data type that add a multiplication case to the above data type, and still be able to maximally reuse the existing code without modification:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="@Data interface ExpMul { interface ExpMulAlg<E, R> extends Exp.ExpAlg<E, R> { R Mul(E e1, E e2); } <R> R accept(ExpMulAlg<ExpMul, R> alg); static Function<Exp, ExpMul> fromExp() { ExpMulAlg<ExpMul, ExpMul> factory = ExpMuls.factory(); return Exps.cata(factory, ExpMuls::lazy); } }"><pre><span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">interface</span> <span class="pl-smi">ExpMul</span> { <span class="pl-k">interface</span> <span class="pl-smi">ExpMulAlg</span><<span class="pl-smi">E</span>, <span class="pl-smi">R</span>> <span class="pl-k">extends</span> <span class="pl-smi">Exp</span>.<span class="pl-smi">ExpAlg</span><<span class="pl-smi">E</span>, <span class="pl-smi">R</span>> { <span class="pl-smi">R</span> <span class="pl-en">Mul</span>(<span class="pl-smi">E</span> <span class="pl-s1">e1</span>, <span class="pl-smi">E</span> <span class="pl-s1">e2</span>); } <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">accept</span>(<span class="pl-smi">ExpMulAlg</span><<span class="pl-smi">ExpMul</span>, <span class="pl-smi">R</span>> <span class="pl-s1">alg</span>); <span class="pl-k">static</span> <span class="pl-smi">Function</span><<span class="pl-smi">Exp</span>, <span class="pl-smi">ExpMul</span>> <span class="pl-en">fromExp</span>() { <span class="pl-smi">ExpMulAlg</span><<span class="pl-smi">ExpMul</span>, <span class="pl-smi">ExpMul</span>> <span class="pl-s1">factory</span> = <span class="pl-smi">ExpMuls</span>.<span class="pl-en">factory</span>(); <span class="pl-k">return</span> <span class="pl-smi">Exps</span>.<span class="pl-en">cata</span>(<span class="pl-s1">factory</span>, <span class="pl-smi">ExpMuls</span>::<span class="pl-s1">lazy</span>); } }</pre></div> <p dir="auto">To ensure smooth extensibility across compilation unit (or even during incremental compilation), it is best to <strong>use the <code>-parameters</code> option of javac</strong>.</p> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">But what exactly is generated?</h1><a id="user-content-but-what-exactly-is-generated" class="anchor" aria-label="Permalink: But what exactly is generated?" href="#but-what-exactly-is-generated"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This is a very legitimate question. Here is the <a href="https://gist.github.com/jbgi/31b891f00566feb301f8100762ee8511#file-expmuls-java"><code>ExpMuls.java</code></a> file that is generated for the above <code>@Data ExpMul</code> type.</p> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Parametric polymorphism</h1><a id="user-content-parametric-polymorphism" class="anchor" aria-label="Permalink: Parametric polymorphism" href="#parametric-polymorphism"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">... works as expected. For example, you can write the following:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import java.util.function.Function; import java.util.function.Supplier; import org.derive4j.Data; @Data public abstract class Option<A> { public abstract <R> R cata(Supplier<R> none, Function<A, R> some); public final <B> Option<B> map(final Function<A, B> mapper) { return Options.modSome(mapper).apply(this); } }"><pre><span class="pl-k">import</span> <span class="pl-s1">java</span>.<span class="pl-s1">util</span>.<span class="pl-s1">function</span>.<span class="pl-s1">Function</span>; <span class="pl-k">import</span> <span class="pl-s1">java</span>.<span class="pl-s1">util</span>.<span class="pl-s1">function</span>.<span class="pl-s1">Supplier</span>; <span class="pl-k">import</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">Data</span>; <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Option</span><<span class="pl-smi">A</span>> { <span class="pl-k">public</span> <span class="pl-k">abstract</span> <<span class="pl-smi">R</span>> <span class="pl-smi">R</span> <span class="pl-en">cata</span>(<span class="pl-smi">Supplier</span><<span class="pl-smi">R</span>> <span class="pl-s1">none</span>, <span class="pl-smi">Function</span><<span class="pl-smi">A</span>, <span class="pl-smi">R</span>> <span class="pl-s1">some</span>); <span class="pl-k">public</span> <span class="pl-k">final</span> <<span class="pl-smi">B</span>> <span class="pl-smi">Option</span><<span class="pl-smi">B</span>> <span class="pl-en">map</span>(<span class="pl-k">final</span> <span class="pl-smi">Function</span><<span class="pl-smi">A</span>, <span class="pl-smi">B</span>> <span class="pl-s1">mapper</span>) { <span class="pl-k">return</span> <span class="pl-smi">Options</span>.<span class="pl-en">modSome</span>(<span class="pl-s1">mapper</span>).<span class="pl-en">apply</span>(<span class="pl-smi">this</span>); } }</pre></div> <p dir="auto">The generated modifier method <code>modSome</code> allows polymorphic update and is incidentaly the functor for our <code>Option</code>!</p> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Generalized Algebraic Data Types</h1><a id="user-content-generalized-algebraic-data-types" class="anchor" aria-label="Permalink: Generalized Algebraic Data Types" href="#generalized-algebraic-data-types"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">GADTs are also supported out of the box by Derive4J (within the limitations of the Java type system). Here is how you can translate the example from <a href="http://www.cs.ox.ac.uk/ralf.hinze/publications/With.pdf" rel="nofollow">Fun with phantom types</a>:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="import org.derive4j.hkt.TypeEq; @Data public abstract class Term<T> { interface Cases<A, R> { R Zero(TypeEq<Integer, A> id); R Succ(Term<Integer> pred, TypeEq<Integer, A> id); R Pred(Term<Integer> succ, TypeEq<Integer, A> id); R IsZero(Term<Integer> a, TypeEq<Boolean, A> id); R If(Term<Boolean> cond, Term<A> then, Term<A> otherwise); } public abstract <X> X match(Cases<T, X> cases); public static <T> T eval(final Term<T> term) { return Terms.caseOf(term). Zero(id -> id.coerce(0)). Succ((t, id) -> id.coerce(eval(t) + 1)). Pred((t, id) -> id.coerce(eval(t) - 1)). IsZero((t, id) -> id.coerce(eval(t) == 0)). If((cond, then, otherwise) -> eval(cond) ? eval(then) : eval(otherwise)); } public static void main(final String[] args) { Term<Integer> one = Succ(Zero()); out.println(eval(one)); // "1" out.println(eval(IsZero(one))); // "false" // IsZero(IsZero(one)); // does not compile: // "The method IsZero(Term<Integer>) in the type Term<T> is not // applicable for the arguments (Term<Boolean>)" out.println(eval(If(IsZero(one), Zero(), one))); // "1" Term<Boolean> True = IsZero(Zero()); Term<Boolean> False = IsZero(one); out.println(eval(If(True, True, False))); // "true" // out.println(prettyPrint(If(True, True, False), 0)); // "if IsZero(0) // then IsZero(0) // else IsZero(Succ(0))" } } "><pre><span class="pl-k">import</span> <span class="pl-s1">org</span>.<span class="pl-s1">derive4j</span>.<span class="pl-s1">hkt</span>.<span class="pl-s1">TypeEq</span>; <span class="pl-c1">@</span><span class="pl-c1">Data</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Term</span><<span class="pl-smi">T</span>> { <span class="pl-k">interface</span> <span class="pl-smi">Cases</span><<span class="pl-smi">A</span>, <span class="pl-smi">R</span>> { <span class="pl-smi">R</span> <span class="pl-en">Zero</span>(<span class="pl-smi">TypeEq</span><<span class="pl-smi">Integer</span>, <span class="pl-smi">A</span>> <span class="pl-s1">id</span>); <span class="pl-smi">R</span> <span class="pl-en">Succ</span>(<span class="pl-smi">Term</span><<span class="pl-smi">Integer</span>> <span class="pl-s1">pred</span>, <span class="pl-smi">TypeEq</span><<span class="pl-smi">Integer</span>, <span class="pl-smi">A</span>> <span class="pl-s1">id</span>); <span class="pl-smi">R</span> <span class="pl-en">Pred</span>(<span class="pl-smi">Term</span><<span class="pl-smi">Integer</span>> <span class="pl-s1">succ</span>, <span class="pl-smi">TypeEq</span><<span class="pl-smi">Integer</span>, <span class="pl-smi">A</span>> <span class="pl-s1">id</span>); <span class="pl-smi">R</span> <span class="pl-en">IsZero</span>(<span class="pl-smi">Term</span><<span class="pl-smi">Integer</span>> <span class="pl-s1">a</span>, <span class="pl-smi">TypeEq</span><<span class="pl-smi">Boolean</span>, <span class="pl-smi">A</span>> <span class="pl-s1">id</span>); <span class="pl-smi">R</span> <span class="pl-en">If</span>(<span class="pl-smi">Term</span><<span class="pl-smi">Boolean</span>> <span class="pl-s1">cond</span>, <span class="pl-smi">Term</span><<span class="pl-smi">A</span>> <span class="pl-s1">then</span>, <span class="pl-smi">Term</span><<span class="pl-smi">A</span>> <span class="pl-s1">otherwise</span>); } <span class="pl-k">public</span> <span class="pl-k">abstract</span> <<span class="pl-smi">X</span>> <span class="pl-smi">X</span> <span class="pl-en">match</span>(<span class="pl-smi">Cases</span><<span class="pl-smi">T</span>, <span class="pl-smi">X</span>> <span class="pl-s1">cases</span>); <span class="pl-k">public</span> <span class="pl-k">static</span> <<span class="pl-smi">T</span>> <span class="pl-smi">T</span> <span class="pl-en">eval</span>(<span class="pl-k">final</span> <span class="pl-smi">Term</span><<span class="pl-smi">T</span>> <span class="pl-s1">term</span>) { <span class="pl-k">return</span> <span class="pl-smi">Terms</span>.<span class="pl-en">caseOf</span>(<span class="pl-s1">term</span>). <span class="pl-en">Zero</span>(<span class="pl-s1">id</span> -> <span class="pl-s1">id</span>.<span class="pl-en">coerce</span>(<span class="pl-c1">0</span>)). <span class="pl-en">Succ</span>((<span class="pl-s1">t</span>, <span class="pl-s1">id</span>) -> <span class="pl-s1">id</span>.<span class="pl-en">coerce</span>(<span class="pl-en">eval</span>(<span class="pl-s1">t</span>) + <span class="pl-c1">1</span>)). <span class="pl-en">Pred</span>((<span class="pl-s1">t</span>, <span class="pl-s1">id</span>) -> <span class="pl-s1">id</span>.<span class="pl-en">coerce</span>(<span class="pl-en">eval</span>(<span class="pl-s1">t</span>) - <span class="pl-c1">1</span>)). <span class="pl-en">IsZero</span>((<span class="pl-s1">t</span>, <span class="pl-s1">id</span>) -> <span class="pl-s1">id</span>.<span class="pl-en">coerce</span>(<span class="pl-en">eval</span>(<span class="pl-s1">t</span>) == <span class="pl-c1">0</span>)). <span class="pl-en">If</span>((<span class="pl-s1">cond</span>, <span class="pl-s1">then</span>, <span class="pl-s1">otherwise</span>) -> <span class="pl-en">eval</span>(<span class="pl-s1">cond</span>) ? <span class="pl-en">eval</span>(<span class="pl-s1">then</span>) : <span class="pl-en">eval</span>(<span class="pl-s1">otherwise</span>)); } <span class="pl-k">public</span> <span class="pl-k">static</span> <span class="pl-smi">void</span> <span class="pl-en">main</span>(<span class="pl-k">final</span> <span class="pl-smi">String</span>[] <span class="pl-s1">args</span>) { <span class="pl-smi">Term</span><<span class="pl-smi">Integer</span>> <span class="pl-s1">one</span> = <span class="pl-en">Succ</span>(<span class="pl-en">Zero</span>()); <span class="pl-s1">out</span>.<span class="pl-en">println</span>(<span class="pl-en">eval</span>(<span class="pl-s1">one</span>)); <span class="pl-c">// "1"</span> <span class="pl-s1">out</span>.<span class="pl-en">println</span>(<span class="pl-en">eval</span>(<span class="pl-en">IsZero</span>(<span class="pl-s1">one</span>))); <span class="pl-c">// "false"</span> <span class="pl-c">// IsZero(IsZero(one)); // does not compile:</span> <span class="pl-c">// "The method IsZero(Term<Integer>) in the type Term<T> is not</span> <span class="pl-c">// applicable for the arguments (Term<Boolean>)"</span> <span class="pl-s1">out</span>.<span class="pl-en">println</span>(<span class="pl-en">eval</span>(<span class="pl-en">If</span>(<span class="pl-en">IsZero</span>(<span class="pl-s1">one</span>), <span class="pl-en">Zero</span>(), <span class="pl-s1">one</span>))); <span class="pl-c">// "1"</span> <span class="pl-smi">Term</span><<span class="pl-smi">Boolean</span>> <span class="pl-s1">True</span> = <span class="pl-en">IsZero</span>(<span class="pl-en">Zero</span>()); <span class="pl-smi">Term</span><<span class="pl-smi">Boolean</span>> <span class="pl-s1">False</span> = <span class="pl-en">IsZero</span>(<span class="pl-s1">one</span>); <span class="pl-s1">out</span>.<span class="pl-en">println</span>(<span class="pl-en">eval</span>(<span class="pl-en">If</span>(<span class="pl-s1">True</span>, <span class="pl-s1">True</span>, <span class="pl-s1">False</span>))); <span class="pl-c">// "true"</span> <span class="pl-c">// out.println(prettyPrint(If(True, True, False), 0)); // "if IsZero(0)</span> <span class="pl-c">// then IsZero(0)</span> <span class="pl-c">// else IsZero(Succ(0))"</span> } }</pre></div> <p dir="auto">For GADT you will need to add a dependency on <a href="https://github.com/derive4j/hkt">derive4j/hkt</a> which provides <code>TypeEq<A, B></code>: a witness of the equality of two types, <code>A</code> and <code>B</code>.</p> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">DRY annotation configuration</h1><a id="user-content-dry-annotation-configuration" class="anchor" aria-label="Permalink: DRY annotation configuration" href="#dry-annotation-configuration"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">By default the <code>@Data</code> annotation triggers the generation of <a href="/derive4j/derive4j/blob/master/annotation/src/main/java/org/derive4j/Make.java#L22">everything which is available</a>, in a file whose name is the English plural of the annotated class. But you may want to restrict the scope of what is generated, or change the name of the file, and you usually want all you ADTs to use the same flavour. You may even dislike the name of the annotation because it clashes with another framework...</p> <p dir="auto">For example, let's say that you want to always use the <code>FJ</code> flavour (FunctionalJava), make the generated code package private in a class suffixed by <code>Impl</code> and only generate the pattern matching syntax and the constructors. Then all you have to do is to create the following annotation:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="@Data(flavour = Flavour.FJ, value = @Derive( inClass = "{ClassName}Impl", withVisibility = Visibility.Package, make = { Make.constructors, Make.caseOfMatching } )) public @interface myADT {}"><pre><span class="pl-c1">@</span><span class="pl-c1">Data</span>(<span class="pl-s1">flavour</span> = <span class="pl-smi">Flavour</span>.<span class="pl-c1">FJ</span>, <span class="pl-s1">value</span> = <span class="pl-c1">@</span><span class="pl-c1">Derive</span>( <span class="pl-s1">inClass</span> = <span class="pl-s">"{ClassName}Impl"</span>, <span class="pl-s1">withVisibility</span> = <span class="pl-smi">Visibility</span>.<span class="pl-s1">Package</span>, <span class="pl-s1">make</span> = { <span class="pl-smi">Make</span>.<span class="pl-s1">constructors</span>, <span class="pl-smi">Make</span>.<span class="pl-s1">caseOfMatching</span> } )) <span class="pl-k">public</span> @interface <span class="pl-s1">myADT</span> {}</pre></div> <p dir="auto">And you annotate your classes with <code>@myADT</code> instead of <code>@Data</code>, saving on that configuration every time.</p> <p dir="auto">But now for some of your ADTs you may want to also generate getters and functional setters. In order to not lose the benefits of your <code>@myADT</code>, derive4j allows you to do this:</p> <div class="highlight highlight-source-java notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="@myADT @Derive(make = { Make.getters, Make.modifiers }) // add-up to the @myADT configuration public abstract class Adt {...} "><pre><span class="pl-c1">@</span><span class="pl-c1">myADT</span> <span class="pl-c1">@</span><span class="pl-c1">Derive</span>(<span class="pl-s1">make</span> = { <span class="pl-smi">Make</span>.<span class="pl-s1">getters</span>, <span class="pl-smi">Make</span>.<span class="pl-s1">modifiers</span> }) <span class="pl-c">// add-up to the @myADT configuration</span> <span class="pl-k">public</span> <span class="pl-k">abstract</span> <span class="pl-k">class</span> <span class="pl-smi">Adt</span> {...}</pre></div> <div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Use it in your project</h1><a id="user-content-use-it-in-your-project" class="anchor" aria-label="Permalink: Use it in your project" href="#use-it-in-your-project"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Derive4J should be declared as a compile-time only dependency (not needed at runtime). So while derive4j is (L)GPL-licensed, the generated code is not linked to derive4j, and thus <strong>derive4j can be used in any project (proprietary or not)</strong>.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Maven:</h2><a id="user-content-maven" class="anchor" aria-label="Permalink: Maven:" href="#maven"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <div class="highlight highlight-text-xml notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="<dependency> <groupId>org.derive4j</groupId> <artifactId>derive4j</artifactId> <version>1.1.1</version> <optional>true</optional> </dependency>"><pre><<span class="pl-ent">dependency</span>> <<span class="pl-ent">groupId</span>>org.derive4j</<span class="pl-ent">groupId</span>> <<span class="pl-ent">artifactId</span>>derive4j</<span class="pl-ent">artifactId</span>> <<span class="pl-ent">version</span>>1.1.1</<span class="pl-ent">version</span>> <<span class="pl-ent">optional</span>>true</<span class="pl-ent">optional</span>> </<span class="pl-ent">dependency</span>></pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Gradle</h2><a id="user-content-gradle" class="anchor" aria-label="Permalink: Gradle" href="#gradle"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="compile(group: 'org.derive4j', name: 'derive4j', version: '1.1.1', ext: 'jar')"><pre class="notranslate"><code>compile(group: 'org.derive4j', name: 'derive4j', version: '1.1.1', ext: 'jar') </code></pre></div> <p dir="auto">or better using the <a href="https://github.com/tbroyer/gradle-apt-plugin">gradle-apt-plugin</a>:</p> <div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="compileOnly "org.derive4j:derive4j-annotation:1.1.1" apt "org.derive4j:derive4j:1.1.1""><pre class="notranslate"><code>compileOnly "org.derive4j:derive4j-annotation:1.1.1" apt "org.derive4j:derive4j:1.1.1" </code></pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Contributing</h2><a id="user-content-contributing" class="anchor" aria-label="Permalink: Contributing" href="#contributing"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Bug reports and feature requests are welcome, as well as contributions to improve documentation.</p> <p dir="auto">Right now the codebase is not ready for external contribution (many blocks of code are more complicated than they should be). So you might be better off waiting for the resolution of <a href="https://github.com/derive4j/derive4j/issues/2" data-hovercard-type="issue" data-hovercard-url="/derive4j/derive4j/issues/2/hovercard">#2</a> before trying to dig into the codebase.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Contact</h2><a id="user-content-contact" class="anchor" aria-label="Permalink: Contact" href="#contact"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto"><a href="mailto:jb@giraudeau.info">jb@giraudeau.info</a>, <a href="https://twitter.com/jb9i" rel="nofollow">@jb9i</a> or use the project GitHub issues.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Further reading</h2><a id="user-content-further-reading" class="anchor" aria-label="Permalink: Further reading" href="#further-reading"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <ul dir="auto"> <li><a href="https://gracious-hypatia-aac58b.netlify.com/" rel="nofollow">Encoding FP in Java</a> (and links)</li> <li><a href="http://eng.wealthfront.com/2015/02/pattern-matching-in-java-with-visitor.html" rel="nofollow">http://eng.wealthfront.com/2015/02/pattern-matching-in-java-with-visitor.html</a></li> <li><a href="https://en.wikipedia.org/wiki/Algebraic_data_type" rel="nofollow">https://en.wikipedia.org/wiki/Algebraic_data_type</a></li> <li><a href="https://en.wikipedia.org/wiki/Tagged_union" rel="nofollow">https://en.wikipedia.org/wiki/Tagged_union</a></li> <li><a href="http://blog.higher-order.com/blog/2009/08/21/structural-pattern-matching-in-java/" rel="nofollow">http://blog.higher-order.com/blog/2009/08/21/structural-pattern-matching-in-java/</a></li> <li><a href="http://tomasp.net/blog/types-and-math.aspx/" rel="nofollow">http://tomasp.net/blog/types-and-math.aspx/</a></li> <li><a href="http://fsharpforfunandprofit.com/posts/type-size-and-design/" rel="nofollow">http://fsharpforfunandprofit.com/posts/type-size-and-design/</a></li> <li><a href="https://codewords.recurse.com/issues/three/algebra-and-calculus-of-algebraic-data-types" rel="nofollow">https://codewords.recurse.com/issues/three/algebra-and-calculus-of-algebraic-data-types</a></li> <li><a href="http://chris-taylor.github.io/blog/2013/02/10/the-algebra-of-algebraic-data-types/" rel="nofollow">http://chris-taylor.github.io/blog/2013/02/10/the-algebra-of-algebraic-data-types/</a></li> </ul> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Thanks</h2><a id="user-content-thanks" class="anchor" aria-label="Permalink: Thanks" href="#thanks"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This project has a special dedication to Tony Morris' blog post <a href="http://blog.tmorris.net/posts/debut-with-a-catamorphism/index.html" rel="nofollow">Debut with a catamorphism</a>. I'm also very thankful to <a href="https://github.com/sviperll">@sviperll</a> and his <a href="https://github.com/sviperll/adt4j/">adt4j</a> project which was the initial inspiration for Derive4J.</p> </article></div></div></div></div></div> <!-- --> <!-- --> <script type="application/json" id="__PRIMER_DATA_:R0:__">{"resolvedServerColorMode":"day"}</script></div> </react-partial> <input type="hidden" data-csrf="true" value="tGjwO9a2PKRf8wqjixz+y3Y1ARmauIc4c38o7eciV88S5Lby1RbsiEez3b88upIZXszJNTupIe8ryfgFICK3yg==" /> </div> <div data-view-component="true" class="Layout-sidebar"> <div class="BorderGrid about-margin" data-pjax> <div class="BorderGrid-row"> <div class="BorderGrid-cell"> <div class="hide-sm hide-md"> <h2 class="mb-3 h4">About</h2> <p class="f4 my-3"> Java 8 annotation processor and framework for deriving algebraic data types constructors, pattern-matching, folds, optics and typeclasses. </p> <h3 class="sr-only">Topics</h3> <div class="my-3"> <div class="f6"> <a href="/topics/java" title="Topic: java" data-view-component="true" class="topic-tag topic-tag-link"> java </a> <a href="/topics/algebra" title="Topic: algebra" data-view-component="true" class="topic-tag topic-tag-link"> algebra </a> <a href="/topics/functional-programming" title="Topic: functional-programming" data-view-component="true" class="topic-tag topic-tag-link"> functional-programming </a> <a href="/topics/fold" title="Topic: fold" data-view-component="true" class="topic-tag topic-tag-link"> fold </a> <a href="/topics/annotation-processor" title="Topic: annotation-processor" data-view-component="true" class="topic-tag topic-tag-link"> annotation-processor </a> <a href="/topics/algebraic-data-types" title="Topic: algebraic-data-types" data-view-component="true" class="topic-tag topic-tag-link"> algebraic-data-types </a> <a href="/topics/visitor" title="Topic: visitor" data-view-component="true" class="topic-tag topic-tag-link"> visitor </a> <a href="/topics/java-8" title="Topic: java-8" data-view-component="true" class="topic-tag topic-tag-link"> java-8 </a> <a href="/topics/optics" title="Topic: optics" data-view-component="true" class="topic-tag topic-tag-link"> optics </a> <a href="/topics/visitor-pattern" title="Topic: visitor-pattern" data-view-component="true" class="topic-tag topic-tag-link"> visitor-pattern </a> <a href="/topics/discriminated-unions" title="Topic: discriminated-unions" data-view-component="true" class="topic-tag topic-tag-link"> discriminated-unions </a> <a href="/topics/sum-types" title="Topic: sum-types" data-view-component="true" class="topic-tag topic-tag-link"> sum-types </a> <a href="/topics/laziness" title="Topic: laziness" data-view-component="true" class="topic-tag topic-tag-link"> laziness </a> <a href="/topics/derive4j" title="Topic: derive4j" data-view-component="true" class="topic-tag topic-tag-link"> derive4j </a> <a href="/topics/catamorphisms" title="Topic: catamorphisms" data-view-component="true" class="topic-tag topic-tag-link"> catamorphisms </a> <a href="/topics/exhaustiveness-checking" title="Topic: exhaustiveness-checking" data-view-component="true" class="topic-tag topic-tag-link"> exhaustiveness-checking </a> <a href="/topics/tagged-unions" title="Topic: tagged-unions" data-view-component="true" class="topic-tag topic-tag-link"> tagged-unions </a> </div> </div> <h3 class="sr-only">Resources</h3> <div class="mt-2"> <a class="Link--muted" data-analytics-event="{"category":"Repository Overview","action":"click","label":"location:sidebar;file:readme"}" href="#readme-ov-file"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-book mr-2"> <path d="M0 1.75A.75.75 0 0 1 .75 1h4.253c1.227 0 2.317.59 3 1.501A3.743 3.743 0 0 1 11.006 1h4.245a.75.75 0 0 1 .75.75v10.5a.75.75 0 0 1-.75.75h-4.507a2.25 2.25 0 0 0-1.591.659l-.622.621a.75.75 0 0 1-1.06 0l-.622-.621A2.25 2.25 0 0 0 5.258 13H.75a.75.75 0 0 1-.75-.75Zm7.251 10.324.004-5.073-.002-2.253A2.25 2.25 0 0 0 5.003 2.5H1.5v9h3.757a3.75 3.75 0 0 1 1.994.574ZM8.755 4.75l-.004 7.322a3.752 3.752 0 0 1 1.992-.572H14.5v-9h-3.495a2.25 2.25 0 0 0-2.25 2.25Z"></path> </svg> Readme </a> </div> <include-fragment src="/derive4j/derive4j/hovercards/citation/sidebar_partial?tree_name=master"> </include-fragment> <div class="mt-2"> <a href="/derive4j/derive4j/activity" data-view-component="true" class="Link Link--muted"><svg text="gray" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-pulse mr-2"> <path d="M6 2c.306 0 .582.187.696.471L10 10.731l1.304-3.26A.751.751 0 0 1 12 7h3.25a.75.75 0 0 1 0 1.5h-2.742l-1.812 4.528a.751.751 0 0 1-1.392 0L6 4.77 4.696 8.03A.75.75 0 0 1 4 8.5H.75a.75.75 0 0 1 0-1.5h2.742l1.812-4.529A.751.751 0 0 1 6 2Z"></path> </svg> <span class="color-fg-muted">Activity</span></a> </div> <div class="mt-2"> <a href="/derive4j/derive4j/custom-properties" data-view-component="true" class="Link Link--muted"><svg text="gray" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-note mr-2"> <path d="M0 3.75C0 2.784.784 2 1.75 2h12.5c.966 0 1.75.784 1.75 1.75v8.5A1.75 1.75 0 0 1 14.25 14H1.75A1.75 1.75 0 0 1 0 12.25Zm1.75-.25a.25.25 0 0 0-.25.25v8.5c0 .138.112.25.25.25h12.5a.25.25 0 0 0 .25-.25v-8.5a.25.25 0 0 0-.25-.25ZM3.5 6.25a.75.75 0 0 1 .75-.75h7a.75.75 0 0 1 0 1.5h-7a.75.75 0 0 1-.75-.75Zm.75 2.25h4a.75.75 0 0 1 0 1.5h-4a.75.75 0 0 1 0-1.5Z"></path> </svg> <span class="color-fg-muted">Custom properties</span></a> </div> <h3 class="sr-only">Stars</h3> <div class="mt-2"> <a href="/derive4j/derive4j/stargazers" data-view-component="true" class="Link Link--muted"><svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-star mr-2"> <path d="M8 .25a.75.75 0 0 1 .673.418l1.882 3.815 4.21.612a.75.75 0 0 1 .416 1.279l-3.046 2.97.719 4.192a.751.751 0 0 1-1.088.791L8 12.347l-3.766 1.98a.75.75 0 0 1-1.088-.79l.72-4.194L.818 6.374a.75.75 0 0 1 .416-1.28l4.21-.611L7.327.668A.75.75 0 0 1 8 .25Zm0 2.445L6.615 5.5a.75.75 0 0 1-.564.41l-3.097.45 2.24 2.184a.75.75 0 0 1 .216.664l-.528 3.084 2.769-1.456a.75.75 0 0 1 .698 0l2.77 1.456-.53-3.084a.75.75 0 0 1 .216-.664l2.24-2.183-3.096-.45a.75.75 0 0 1-.564-.41L8 2.694Z"></path> </svg> <strong>570</strong> stars</a> </div> <h3 class="sr-only">Watchers</h3> <div class="mt-2"> <a href="/derive4j/derive4j/watchers" data-view-component="true" class="Link Link--muted"><svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-eye mr-2"> <path d="M8 2c1.981 0 3.671.992 4.933 2.078 1.27 1.091 2.187 2.345 2.637 3.023a1.62 1.62 0 0 1 0 1.798c-.45.678-1.367 1.932-2.637 3.023C11.67 13.008 9.981 14 8 14c-1.981 0-3.671-.992-4.933-2.078C1.797 10.83.88 9.576.43 8.898a1.62 1.62 0 0 1 0-1.798c.45-.677 1.367-1.931 2.637-3.022C4.33 2.992 6.019 2 8 2ZM1.679 7.932a.12.12 0 0 0 0 .136c.411.622 1.241 1.75 2.366 2.717C5.176 11.758 6.527 12.5 8 12.5c1.473 0 2.825-.742 3.955-1.715 1.124-.967 1.954-2.096 2.366-2.717a.12.12 0 0 0 0-.136c-.412-.621-1.242-1.75-2.366-2.717C10.824 4.242 9.473 3.5 8 3.5c-1.473 0-2.825.742-3.955 1.715-1.124.967-1.954 2.096-2.366 2.717ZM8 10a2 2 0 1 1-.001-3.999A2 2 0 0 1 8 10Z"></path> </svg> <strong>29</strong> watching</a> </div> <h3 class="sr-only">Forks</h3> <div class="mt-2"> <a href="/derive4j/derive4j/forks" data-view-component="true" class="Link Link--muted"><svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-repo-forked mr-2"> <path d="M5 5.372v.878c0 .414.336.75.75.75h4.5a.75.75 0 0 0 .75-.75v-.878a2.25 2.25 0 1 1 1.5 0v.878a2.25 2.25 0 0 1-2.25 2.25h-1.5v2.128a2.251 2.251 0 1 1-1.5 0V8.5h-1.5A2.25 2.25 0 0 1 3.5 6.25v-.878a2.25 2.25 0 1 1 1.5 0ZM5 3.25a.75.75 0 1 0-1.5 0 .75.75 0 0 0 1.5 0Zm6.75.75a.75.75 0 1 0 0-1.5.75.75 0 0 0 0 1.5Zm-3 8.75a.75.75 0 1 0-1.5 0 .75.75 0 0 0 1.5 0Z"></path> </svg> <strong>49</strong> forks</a> </div> <div class="mt-2"> <a class="Link--muted" href="/contact/report-content?content_url=https%3A%2F%2Fgithub.com%2Fderive4j%2Fderive4j&report=derive4j+%28user%29"> Report repository </a> </div> </div> </div> </div> <div class="BorderGrid-row"> <div class="BorderGrid-cell"> <h2 class="h4 mb-3" data-pjax="#repo-content-pjax-container" data-turbo-frame="repo-content-turbo-frame"> <a href="/derive4j/derive4j/releases" data-view-component="true" class="Link--primary no-underline Link">Releases</a></h2> <a class="Link--primary no-underline" data-pjax="#repo-content-pjax-container" data-turbo-frame="repo-content-turbo-frame" href="/derive4j/derive4j/tags"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" 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