CINXE.COM

<!doctype html><html lang="en"><head><meta charset="utf-8"><meta http-equiv="x-ua-compatible" content="ie=edge"><meta name="viewport" content="width=device-width,initial-scale=1"><script>!function(e,t,a,n){e[n]=e[n]||[],e[n].push({"gtm.start":(new Date).getTime(),event:"gtm.js"});var g=t.getElementsByTagName(a)[0],m=t.createElement(a);m.async=!0,m.src="https://www.googletagmanager.com/gtm.js?id=GTM-5VSZM5J",g.parentNode.insertBefore(m,g)}(window,document,"script","dataLayer")</script><meta name="description" content="A glossary reference for terminology used across dart.dev."><title>Glossary | Dart</title><link rel="icon" sizes="64x64" href="/assets/img/logo/dart-64.png" eleventy:ignore><link href="/assets/img/touch-icon-iphone.png" rel="apple-touch-icon" eleventy:ignore><link href="/assets/img/touch-icon-ipad.png" rel="apple-touch-icon" sizes="152x152" eleventy:ignore><link href="/assets/img/touch-icon-iphone-retina.png" rel="apple-touch-icon" sizes="180x180" eleventy:ignore><link href="/assets/img/touch-icon-ipad-retina.png" rel="apple-touch-icon" sizes="167x167" eleventy:ignore><meta name="twitter:card" content="summary"><meta name="twitter:site" content="@dart_lang"><meta name="twitter:title" content="Glossary"><meta name="twitter:description" content="A glossary reference for terminology used across dart.dev."><meta property="og:title" content="Glossary"><meta property="og:description" content="A glossary reference for terminology used across dart.dev."><meta property="og:url" content="/resources/glossary/"><meta property="og:image" content="/assets/img/logo/dart-logo-for-shares.png?2" eleventy:ignore><link rel="preconnect" href="https://fonts.googleapis.com"><link rel="preconnect" href="https://fonts.gstatic.com" crossorigin><link href="https://fonts.googleapis.com/css2?family=Google+Sans:wght@400;500;700&display=swap" rel="stylesheet"><link href="https://fonts.googleapis.com/css2?family=Google+Sans+Display:wght@400&display=swap" rel="stylesheet"><link href="https://fonts.googleapis.com/css2?family=Google+Sans+Mono:wght@400;500;700&display=swap" rel="stylesheet"><link href="https://fonts.googleapis.com/css2?family=Google+Sans+Text:wght@400;500;700&display=swap" rel="stylesheet"><link href="https://fonts.googleapis.com/css2?family=Material+Symbols+Outlined:opsz,wght,FILL,GRAD@24,400,0..1,0" rel="stylesheet"><link rel="stylesheet" href="/assets/css/main.css?v=3"><script src="/assets/js/os-tabs.js?v=3"></script><script src="/assets/js/main.js?v=3"></script><script defer="defer" src="/assets/js/glossary.js"></script><script>!function(e,a,t,n,c,o,s){e.GoogleAnalyticsObject=c,e[c]=e[c]||function(){(e[c].q=e[c].q||[]).push(arguments)},e[c].l=1*new Date,o=a.createElement(t),s=a.getElementsByTagName(t)[0],o.async=1,o.src="//www.google-analytics.com/analytics.js",s.parentNode.insertBefore(o,s)}(window,document,"script",0,"ga"),ga("create","UA-26406144-4","auto"),ga("send","pageview")</script></head><body class="default.html glossary-page"><a id="skip-to-main" class="filled-button" href="#site-content-title" tabindex="1">Skip to main content</a><section id="cookie-notice"><div class="container"><p>dart.dev uses cookies from Google to deliver and enhance the quality of its services and to analyze traffic.</p><div class="button-group"><a class="text-button" href="https://policies.google.com/technologies/cookies" target="_blank" rel="noopener">Learn more</a> <button id="cookie-consent" class="filled-button">OK, got it</button></div></div></section><noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-5VSZM5J" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript><div id="site-banner" role="alert"><p>Announcing Dart 3.7! Find out about updates to the language, analyzer, pub.dev, and more, in the <a href="https://medium.com/dartlang/announcing-dart-3-7-bf864a1b195c" target="_blank">blog post</a>.</p></div><header id="site-header"><nav id="mainnav"><div id="menu-toggle"><span class="material-symbols" title="Toggle side navigation menu." aria-label="Toggle side navigation menu." type="button">menu</span></div><a href="/" class="brand" title="Dart"><img src="/assets/img/logo/logo-white-text.svg" alt="Dart"></a><ul class="navbar"><li><a href="/overview" class="nav-link">Overview</a></li><li class="mainnav__get-started"><a href="/docs" class="nav-link active"><span>Docs</span></a></li><li><a href="/community" class="nav-link">Community</a></li><li><a href="/#try-dart" class="nav-link">Try Dart</a></li><li><a href="/get-dart" class="nav-link">Get Dart</a></li><li class="searchfield"><form action="/search" class="site-header-search form-inline" id="cse-search-box"><input type="hidden" name="cx" value="011220921317074318178:_yy-tmb5t_i"> <input type="hidden" name="ie" value="UTF-8"> <input type="hidden" name="hl" value="en"> <input class="site-header-searchfield form-control search-field" type="search" name="q" id="search-main" autocomplete="off" placeholder="Search" aria-label="Search"></form></li></ul></nav></header><div id="site-below-header"><div id="site-main-row"><div id="sidenav"><form action="/search/" class="site-header-search form-inline"><input class="site-header-searchfield form-control search-field" type="search" name="q" id="search-side" autocomplete="off" placeholder="Search" aria-label="Search"></form><ul class="navbar-nav"><li aria-hidden="true"><div class="sidenav-divider"></div></li><li class="nav-item"><a href="/overview" class="nav-link">Overview</a></li><li class="nav-item"><a href="/community" class="nav-link">Community</a></li><li class="nav-item"><a href="https://dartpad.dev" class="nav-link">Try Dart</a></li><li class="nav-item"><a href="/get-dart" class="nav-link">Get Dart</a></li><li class="nav-item"><a href="/docs" class="nav-link">Docs</a></li><li aria-hidden="true"><div class="sidenav-divider"></div></li></ul><ul class="nav"><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-1" role="button" aria-expanded="false" aria-controls="-sidenav-1"><span>Language</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1"><li class="nav-item"><a class="nav-link" href="/language"><div><span>Introduction</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-2" role="button" aria-expanded="false" aria-controls="-sidenav-1-2"><span>Syntax basics</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-2"><li class="nav-item"><a class="nav-link" href="/language/variables"><div><span>Variables</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/operators"><div><span>Operators</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/comments"><div><span>Comments</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/metadata"><div><span>Metadata</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/libraries"><div><span>Libraries & imports</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/keywords"><div><span>Keywords</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-3" role="button" aria-expanded="false" aria-controls="-sidenav-1-3"><span>Types</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-3"><li class="nav-item"><a class="nav-link" href="/language/built-in-types"><div><span>Built-in types</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/records"><div><span>Records</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/collections"><div><span>Collections</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/generics"><div><span>Generics</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/typedefs"><div><span>Typedefs</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/type-system"><div><span>Type system</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-4" role="button" aria-expanded="false" aria-controls="-sidenav-1-4"><span>Patterns</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-4"><li class="nav-item"><a class="nav-link" href="/language/patterns"><div><span>Overview & usage</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/pattern-types"><div><span>Pattern types</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://codelabs.developers.google.com/codelabs/dart-patterns-records" target="_blank" rel="noopener"><div><span>Applied tutorial</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li></ul></li><li class="nav-item"><a class="nav-link" href="/language/functions"><div><span>Functions</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-6" role="button" aria-expanded="false" aria-controls="-sidenav-1-6"><span>Control flow</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-6"><li class="nav-item"><a class="nav-link" href="/language/loops"><div><span>Loops</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/branches"><div><span>Branches</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/error-handling"><div><span>Error handling</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-7" role="button" aria-expanded="false" aria-controls="-sidenav-1-7"><span>Classes & objects</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-7"><li class="nav-item"><a class="nav-link" href="/language/classes"><div><span>Classes</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/constructors"><div><span>Constructors</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/methods"><div><span>Methods</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/extend"><div><span>Extend a class</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/mixins"><div><span>Mixins</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/enums"><div><span>Enums</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/extension-methods"><div><span>Extension methods</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/extension-types"><div><span>Extension types</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/callable-objects"><div><span>Callable objects</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-8" role="button" aria-expanded="false" aria-controls="-sidenav-1-8"><span>Class modifiers</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-8"><li class="nav-item"><a class="nav-link" href="/language/class-modifiers"><div><span>Overview & usage</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/class-modifiers-for-apis"><div><span>Class modifiers for API maintainers</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/modifier-reference"><div><span>Reference</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-9" role="button" aria-expanded="false" aria-controls="-sidenav-1-9"><span>Concurrency</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-9"><li class="nav-item"><a class="nav-link" href="/language/concurrency"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/async"><div><span>Asynchronous support</span></div></a></li><li class="nav-item"><a class="nav-link" href="/language/isolates"><div><span>Isolates</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-1-10" role="button" aria-expanded="false" aria-controls="-sidenav-1-10"><span>Null safety</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-1-10"><li class="nav-item"><a class="nav-link" href="/null-safety"><div><span>Sound null safety</span></div></a></li><li class="nav-item"><a class="nav-link" href="/null-safety/migration-guide"><div><span>Migrating to null safety</span></div></a></li><li class="nav-item"><a class="nav-link" href="/null-safety/understanding-null-safety"><div><span>Understanding null safety</span></div></a></li><li class="nav-item"><a class="nav-link" href="/null-safety/unsound-null-safety"><div><span>Unsound null safety</span></div></a></li><li class="nav-item"><a class="nav-link" href="/null-safety/faq"><div><span>FAQ</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-2" role="button" aria-expanded="false" aria-controls="-sidenav-2"><span>Core libraries</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-2"><li class="nav-item"><a class="nav-link" href="/libraries"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-core"><div><span>dart:core</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-async"><div><span>dart:async</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-math"><div><span>dart:math</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-convert"><div><span>dart:convert</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/dart-io"><div><span>dart:io</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/js-interop"><div><span>dart:js_interop</span></div></a></li><div class="sidenav-divider"></div><li class="nav-item"><a class="nav-link" href="/libraries/collections/iterables"><div><span>Iterable collections</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-2-10" role="button" aria-expanded="false" aria-controls="-sidenav-2-10"><span>Asynchronous programming</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-2-10"><li class="nav-item"><a class="nav-link" href="/libraries/async/async-await"><div><span>Tutorial</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/futures-error-handling"><div><span>Futures and error handling</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/using-streams"><div><span>Using streams</span></div></a></li><li class="nav-item"><a class="nav-link" href="/libraries/async/creating-streams"><div><span>Creating streams</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-3" role="button" aria-expanded="false" aria-controls="-sidenav-3"><span>Effective Dart</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-3"><li class="nav-item"><a class="nav-link" href="/effective-dart"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/style"><div><span>Style</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/documentation"><div><span>Documentation</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/usage"><div><span>Usage</span></div></a></li><li class="nav-item"><a class="nav-link" href="/effective-dart/design"><div><span>Design</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-4" role="button" aria-expanded="false" aria-controls="-sidenav-4"><span>Packages</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-4"><li class="nav-item"><a class="nav-link" href="/tools/pub/packages"><div><span>How to use packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/useful-packages"><div><span>Commonly used packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/create-packages"><div><span>Creating packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/publishing"><div><span>Publishing packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/writing-package-pages"><div><span>Writing package pages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/workspaces"><div><span>Workspaces (monorepo support)</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-4-7" role="button" aria-expanded="false" aria-controls="-sidenav-4-7"><span>Package reference</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-4-7"><li class="nav-item"><a class="nav-link" href="/tools/pub/dependencies"><div><span>Dependencies</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/glossary"><div><span>Glossary</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/package-layout"><div><span>Package layout conventions</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/environment-variables"><div><span>Pub environment variables</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/pubspec"><div><span>Pubspec file</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/troubleshoot"><div><span>Troubleshooting pub</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/verified-publishers"><div><span>Verified publishers</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/security-advisories"><div><span>Security advisories</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/versioning"><div><span>Versioning</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/custom-package-repositories"><div><span>Custom package repositories</span></div></a></li></ul></li><li class="nav-item"><a class="nav-link" href="/tools/pub/private-files"><div><span>What not to commit</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-5" role="button" aria-expanded="false" aria-controls="-sidenav-5"><span>Development</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-5"><li class="nav-item"><a class="nav-link" href="/libraries/serialization/json"><div><span>JSON serialization</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/language/number-representation"><div><span>Number representation</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/google-apis"><div><span>Google APIs</span></div></a></li><li class="nav-item"><a class="nav-link" href="/multiplatform-apps"><div><span>Multi-platform apps</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-5-5" role="button" aria-expanded="false" aria-controls="-sidenav-5-5"><span>Command-line & server apps</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-5-5"><li class="nav-item"><a class="nav-link" href="/server"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/get-started"><div><span>Get started</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/cmdline"><div><span>Write command-line apps</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/fetch-data"><div><span>Fetch data from the internet</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials/server/httpserver"><div><span>Write HTTP servers</span></div></a></li><li class="nav-item"><a class="nav-link" href="/server/libraries"><div><span>Libraries & packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/server/google-cloud"><div><span>Google Cloud</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-5-6" role="button" aria-expanded="false" aria-controls="-sidenav-5-6"><span>Web apps</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-5-6"><li class="nav-item"><a class="nav-link" href="/web"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/web/get-started"><div><span>Get started</span></div></a></li><li class="nav-item"><a class="nav-link" href="/web/deployment"><div><span>Deployment</span></div></a></li><li class="nav-item"><a class="nav-link" href="/web/libraries"><div><span>Libraries & packages</span></div></a></li><li class="nav-item"><a class="nav-link" href="/web/wasm"><div><span>Wasm compilation</span></div></a></li></ul></li><li class="nav-item"><a class="nav-link" href="/libraries/core/environment-declarations"><div><span>Environment declarations</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-6" role="button" aria-expanded="false" aria-controls="-sidenav-6"><span>Interoperability</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-6"><li class="nav-item"><a class="nav-link" href="/interop/c-interop"><div><span>C interop</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/objective-c-interop"><div><span>Objective-C & Swift interop</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/java-interop"><div><span>Java & Kotlin interop</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-6-4" role="button" aria-expanded="false" aria-controls="-sidenav-6-4"><span>JavaScript interop</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-6-4"><li class="nav-item"><a class="nav-link" href="/interop/js-interop"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/js-interop/usage"><div><span>Usage</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/js-interop/js-types"><div><span>JS types</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/js-interop/tutorials"><div><span>Tutorials</span></div></a></li><li class="nav-item"><a class="nav-link" href="/interop/js-interop/past-js-interop"><div><span>Past JS interop</span></div></a></li><div class="sidenav-divider"></div><li class="nav-item"><a class="nav-link" href="/interop/js-interop/package-web"><div><span>Web interop</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsed collapsible" data-toggle="collapse" data-target="#-sidenav-7" role="button" aria-expanded="false" aria-controls="-sidenav-7"><span>Tools & techniques</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-7"><li class="nav-item"><a class="nav-link" href="/tools"><div><span>Overview</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-7-2" role="button" aria-expanded="false" aria-controls="-sidenav-7-2"><span>Editors & debuggers</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-7-2"><li class="nav-item"><a class="nav-link" href="/tools/jetbrains-plugin"><div><span>IntelliJ & Android Studio</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/vs-code"><div><span>VS Code</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-devtools"><div><span>Dart DevTools</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-7-2-4" role="button" aria-expanded="false" aria-controls="-sidenav-7-2-4"><span>DartPad</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-7-2-4"><li class="nav-item"><a class="nav-link" href="/tools/dartpad"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dartpad/troubleshoot"><div><span>Troubleshooting DartPad</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-7-3" role="button" aria-expanded="false" aria-controls="-sidenav-7-3"><span>Command-line tools</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-7-3"><li class="nav-item"><button class="nav-link collapsible" data-toggle="collapse" data-target="#-sidenav-7-3-1" role="button" aria-expanded="true" aria-controls="-sidenav-7-3-1"><span>Dart SDK</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse show" id="-sidenav-7-3-1"><li class="nav-item"><a class="nav-link" href="/tools/sdk"><div><span>Overview</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-tool"><div><span>dart</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-analyze"><div><span>dart analyze</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-compile"><div><span>dart compile</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-create"><div><span>dart create</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-doc"><div><span>dart doc</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-fix"><div><span>dart fix</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-format"><div><span>dart format</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-info"><div><span>dart info</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/pub/cmd"><div><span>dart pub</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-run"><div><span>dart run</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dart-test"><div><span>dart test</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/dartaotruntime"><div><span>dartaotruntime</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/experiment-flags"><div><span>Experiment flags</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible" data-toggle="collapse" data-target="#-sidenav-7-3-2" role="button" aria-expanded="true" aria-controls="-sidenav-7-3-2"><span>Other command-line tools</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse show" id="-sidenav-7-3-2"><li class="nav-item"><a class="nav-link" href="/tools/build_runner"><div><span>build_runner</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/webdev"><div><span>webdev</span></div></a></li></ul></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-7-4" role="button" aria-expanded="false" aria-controls="-sidenav-7-4"><span>Static analysis</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-7-4"><li class="nav-item"><a class="nav-link" href="/tools/analysis"><div><span>Customizing static analysis</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/non-promotion-reasons"><div><span>Fixing type promotion failures</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/linter-rules"><div><span>Linter rules</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tools/diagnostic-messages"><div><span>Diagnostic messages</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-7-5" role="button" aria-expanded="false" aria-controls="-sidenav-7-5"><span>Testing & optimization</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-7-5"><li class="nav-item"><a class="nav-link" href="/tools/testing"><div><span>Testing</span></div></a></li><li class="nav-item"><a class="nav-link" href="/web/debugging"><div><span>Debugging web apps</span></div></a></li></ul></li></ul></li><li aria-hidden="true"><div class="sidenav-divider"></div></li><li class="nav-item"><button class="nav-link active collapsible" data-toggle="collapse" data-target="#-sidenav-9" role="button" aria-expanded="true" aria-controls="-sidenav-9"><span>Resources</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse show" id="-sidenav-9"><li class="nav-item"><a class="nav-link" href="/resources/dart-cheatsheet"><div><span>Language cheatsheet</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/breaking-changes"><div><span>Breaking changes</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/language/evolution"><div><span>Language evolution</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/language/spec"><div><span>Language specification</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/dart-3-migration"><div><span>Dart 3 migration guide</span></div></a></li><li class="nav-item"><button class="nav-link collapsible collapsed" data-toggle="collapse" data-target="#-sidenav-9-6" role="button" aria-expanded="false" aria-controls="-sidenav-9-6"><span>Coming from ...</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse" id="-sidenav-9-6"><li class="nav-item"><a class="nav-link" href="/resources/coming-from/js-to-dart"><div><span>JavaScript to Dart</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/coming-from/swift-to-dart"><div><span>Swift to Dart</span></div></a></li></ul></li><div class="sidenav-divider"></div><li class="nav-item"><a class="nav-link" href="/resources/faq"><div><span>FAQ</span></div></a></li><li class="nav-item"><a class="nav-link active" href="/resources/glossary"><div><span>Glossary</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/books"><div><span>Books</span></div></a></li><li class="nav-item"><a class="nav-link" href="/resources/videos"><div><span>Videos</span></div></a></li><li class="nav-item"><a class="nav-link" href="/tutorials"><div><span>Tutorials</span></div></a></li></ul></li><li class="nav-item"><button class="nav-link collapsible" data-toggle="collapse" data-target="#-sidenav-10" role="button" aria-expanded="true" aria-controls="-sidenav-10"><span>Related sites</span> <span class="material-symbols expander" aria-hidden="true">expand_more</span></button><ul class="nav collapse show" id="-sidenav-10"><li class="nav-item"><a class="nav-link" href="https://api.dart.dev" target="_blank" rel="noopener"><div><span>API reference</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://medium.com/dartlang" target="_blank" rel="noopener"><div><span>Blog</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://dartpad.dev" target="_blank" rel="noopener"><div><span>DartPad (online editor)</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://flutter.dev" target="_blank" rel="noopener"><div><span>Flutter</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li><li class="nav-item"><a class="nav-link" href="https://pub.dev" target="_blank" rel="noopener"><div><span>Package site</span><span class="material-symbols" aria-hidden="true">open_in_new</span></div></a></li></ul></li></ul></div><main id="page-content"><article><div class="content"><div id="site-content-title"><h1>Glossary</h1></div><p>The following are definitions of terms used across the Dart documentation.</p><section id="filter-and-search" class="hidden"><div class="search-row"><div class="search-wrapper"><span class="material-symbols leading-icon" aria-hidden="true">search</span> <input type="search" placeholder="Search terms..." aria-label="Search terms by name..."></div></div></section><section id="content-search-results"><div class="card-list"><div class="card outlined-card glossary-card expandable-card" id="assist" data-partial-matches="assist" data-full-matches="quick assist"><div class="card-header"><h2 class="card-title">Assist</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#assist" title="Link to card" aria-label="Link to Assist card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="assist-content" title="Expand or collapse card" aria-label="Expand or collapse Assist card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>An automated, local code edit targeted at making common improvements to code.</p></div><div id="assist-content" class="expandable-content"><p>An assist is an automated, local code edit targeted at making common improvements to code. Examples of assists include converting <code>switch</code> statements to <code>switch</code> expressions, reversing the <code>then</code> and <code>else</code> blocks in an <code>if</code> statement, and inserting widgets into a widget structure.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="#refactor" class="filled-button"><span class="material-symbols" aria-hidden="true">dictionary </span><span>Refactor</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="constant-context" data-partial-matches="constant context" data-full-matches="implicit const"><div class="card-header"><h2 class="card-title">Constant context</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#constant-context" title="Link to card" aria-label="Link to Constant context card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="constant-context-content" title="Expand or collapse card" aria-label="Expand or collapse Constant context card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A region of code where the const keyword is implied and everything within that region must be a constant.</p></div><div id="constant-context-content" class="expandable-content"><p>A <em>constant context</em> is a region of code in which it isn't necessary to include the <code>const</code> keyword because it's implied by the fact that everything in that region is required to be a constant. The following locations are constant contexts:</p><ul><li><p>Everything inside a list, map or set literal that's prefixed by the <code>const</code> keyword. For example:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> l = </span><span style="color:#D43324">const</span><span style="color:#222222"> [</span><span style="color:#6E6E70">/*constant context*/</span><span style="color:#222222">];</span></span></code></pre></div></div></li><li><p>The arguments inside an invocation of a constant constructor. For example:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> p = </span><span style="color:#D43324">const</span><span style="color:#0468D7"> Point</span><span style="color:#222222">(</span><span style="color:#6E6E70">/*constant context*/</span><span style="color:#222222">);</span></span></code></pre></div></div></li><li><p>The initializer for a variable that's prefixed by the <code>const</code> keyword. For example:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">const</span><span style="color:#222222"> v = </span><span style="color:#6E6E70">/*constant context*/</span><span style="color:#222222">;</span></span></code></pre></div></div></li><li><p>Annotations.</p></li><li><p>The expression in a <code>case</code> clause. For example:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> f</span><span style="color:#222222">(</span><span style="color:#0468D7">int</span><span style="color:#222222"> e) {</span></span> <span class="line"><span style="color:#D43324"> switch</span><span style="color:#222222"> (e) {</span></span> <span class="line"><span style="color:#D43324"> case</span><span style="color:#6E6E70"> /*constant context*/</span><span style="color:#222222">:</span></span> <span class="line"><span style="color:#D43324"> break</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div></li></ul><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/language/variables#final-and-const" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Final and const variables</span></a></li><li><a href="/effective-dart/usage#dont-use-const-redundantly" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Don't use const redundantly</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="definite-assignment" data-partial-matches="definite assignment" data-full-matches=""><div class="card-header"><h2 class="card-title">Definite assignment</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#definite-assignment" title="Link to card" aria-label="Link to Definite assignment card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="definite-assignment-content" title="Expand or collapse card" aria-label="Expand or collapse Definite assignment card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>The determination of whether a variable has definitely been assigned a value before it's used.</p></div><div id="definite-assignment-content" class="expandable-content"><p>Definite assignment analysis is the process of determining, for each local variable at each point in the code, which of the following is true:</p><ul><li>The variable has definitely been assigned a value (<em>definitely assigned</em>).</li><li>The variable has definitely not been assigned a value (<em>definitely unassigned</em>).</li><li>The variable might or might not have been assigned a value, depending on the execution path taken to arrive at that point.</li></ul><p>Definite assignment analysis helps find problems in code, such as places where a variable that might not have been assigned a value is being referenced, or places where a variable that can only be assigned a value one time is being assigned after it might already have been assigned a value.</p><p>For example, in the following code the variable <code>s</code> is definitely unassigned when it's passed as an argument to <code>print</code>:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> f</span><span style="color:#222222">() {</span></span> <span class="line"><span style="color:#0468D7"> String</span><span style="color:#222222"> s;</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(s);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>But in the following code, the variable <code>s</code> is definitely assigned:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> f</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> name) {</span></span> <span class="line"><span style="color:#0468D7"> String</span><span style="color:#222222"> s = </span><span style="color:#11796D">'Hello </span><span style="color:#11796D">$</span><span style="color:#222222">name</span><span style="color:#11796D">!'</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(s);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>Definite assignment analysis can even tell whether a variable is definitely assigned (or unassigned) when there are multiple possible execution paths. In the following code the <code>print</code> function is called if execution goes through either the true or the false branch of the <code>if</code> statement, but because <code>s</code> is assigned no matter which branch is taken, it's definitely assigned before it's passed to <code>print</code>:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> f</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> name, </span><span style="color:#0468D7">bool</span><span style="color:#222222"> casual) {</span></span> <span class="line"><span style="color:#0468D7"> String</span><span style="color:#222222"> s;</span></span> <span class="line"><span style="color:#D43324"> if</span><span style="color:#222222"> (casual) {</span></span> <span class="line"><span style="color:#222222"> s = </span><span style="color:#11796D">'Hi </span><span style="color:#11796D">$</span><span style="color:#222222">name</span><span style="color:#11796D">!'</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222"> } </span><span style="color:#D43324">else</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#222222"> s = </span><span style="color:#11796D">'Hello </span><span style="color:#11796D">$</span><span style="color:#222222">name</span><span style="color:#11796D">!'</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(s);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>In flow analysis, the end of the <code>if</code> statement is referred to as a <em>join</em>鈥攁 place where two or more execution paths merge back together. Where there's a join, the analysis says that a variable is definitely assigned if it's definitely assigned along all of the paths that are merging, and definitely unassigned if it's definitely unassigned along all of the paths.</p><p>Sometimes a variable is assigned a value on one path but not on another, in which case the variable might or might not have been assigned a value. In the following example, the true branch of the <code>if</code> statement might or might not be executed, so the variable might or might be assigned a value:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> f</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> name, </span><span style="color:#0468D7">bool</span><span style="color:#222222"> casual) {</span></span> <span class="line"><span style="color:#0468D7"> String</span><span style="color:#222222"> s;</span></span> <span class="line"><span style="color:#D43324"> if</span><span style="color:#222222"> (casual) {</span></span> <span class="line"><span style="color:#222222"> s = </span><span style="color:#11796D">'Hi </span><span style="color:#11796D">$</span><span style="color:#222222">name</span><span style="color:#11796D">!'</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(s);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>The same is true if there is a false branch that doesn't assign a value to <code>s</code>.</p><p>The analysis of loops is a little more complicated, but it follows the same basic reasoning. For example, the condition in a <code>while</code> loop is always executed, but the body might or might not be. So just like an <code>if</code> statement, there's a join at the end of the <code>while</code> statement between the path in which the condition is <code>true</code> and the path in which the condition is <code>false</code>.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="https://github.com/dart-lang/language/blob/main/resources/type-system/flow-analysis.md" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Definite assignment specification</span></a></li><li><a href="/null-safety/understanding-null-safety#definite-assignment-analysis" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Understanding definite assignment analysis</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="function" data-partial-matches="function" data-full-matches="procedure"><div class="card-header"><h2 class="card-title">Function</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#function" title="Link to card" aria-label="Link to Function card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="function-content" title="Expand or collapse card" aria-label="Expand or collapse Function card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>An umbrella term to refer to top-level functions, local functions, static methods, and instance methods.</p></div><div id="function-content" class="expandable-content"><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/language/functions" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Functions overview</span></a></li><li><a href="/language/methods" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Instance methods</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="irrefutable-pattern" data-partial-matches="irrefutable pattern" data-full-matches="irrefutable context"><div class="card-header"><h2 class="card-title">Irrefutable pattern</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#irrefutable-pattern" title="Link to card" aria-label="Link to Irrefutable pattern card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="irrefutable-pattern-content" title="Expand or collapse card" aria-label="Expand or collapse Irrefutable pattern card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A pattern that always matches.</p></div><div id="irrefutable-pattern-content" class="expandable-content"><p><em>Irrefutable patterns</em> are patterns that always match. Irrefutable patterns are the only patterns that can appear in <em>irrefutable contexts</em>: the <a href="/language/patterns#variable-declaration"><em>declaration</em></a> and <a href="/language/patterns#variable-assignment"><em>assignment</em></a> pattern contexts.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/language/patterns#places-patterns-can-appear" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Places patterns can appear</span></a></li><li><a href="https://codelabs.developers.google.com/codelabs/dart-patterns-records" class="filled-button"><span class="material-symbols" aria-hidden="true">school </span><span>Dive into Dart patterns and records</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="mixin-application" data-partial-matches="mixin application" data-full-matches="with mixin"><div class="card-header"><h2 class="card-title">Mixin application</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#mixin-application" title="Link to card" aria-label="Link to Mixin application card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="mixin-application-content" title="Expand or collapse card" aria-label="Expand or collapse Mixin application card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A class created when a mixin is applied to a class.</p></div><div id="mixin-application-content" class="expandable-content"><p>A <em>mixin application</em> is the class created when a mixin is applied to a class. For example, consider the following declarations:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#222222"> {}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">mixin</span><span style="color:#0468D7"> M</span><span style="color:#222222"> {}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> B</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> A</span><span style="color:#D43324"> with</span><span style="color:#0468D7"> M</span><span style="color:#222222"> {}</span></span></code></pre></div></div><p>The class <code>B</code> is a subclass of the mixin application of <code>M</code> to <code>A</code>, sometimes nomenclated as <code>A+M</code>. The class <code>A+M</code> is a subclass of <code>A</code> and has members that are copied from <code>M</code>.</p><p>You can give an actual name to a mixin application by defining it as:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#222222"> {}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">mixin</span><span style="color:#0468D7"> M</span><span style="color:#222222"> {}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A_M</span><span style="color:#222222"> = </span><span style="color:#0468D7">A</span><span style="color:#D43324"> with</span><span style="color:#0468D7"> M</span><span style="color:#222222">;</span></span></code></pre></div></div><p>Given this declaration of <code>A_M</code>, the following declaration of <code>B</code> is equivalent to the declaration of <code>B</code> in the original example:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> B</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> A_M</span><span style="color:#222222"> {}</span></span></code></pre></div></div><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/language/mixins" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Mixins in Dart</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="override-inference" data-partial-matches="override inference" data-full-matches=""><div class="card-header"><h2 class="card-title">Override inference</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#override-inference" title="Link to card" aria-label="Link to Override inference card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="override-inference-content" title="Expand or collapse card" aria-label="Expand or collapse Override inference card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>How missing types in a method declaration are inferred.</p></div><div id="override-inference-content" class="expandable-content"><p>Override inference is the process by which any missing types in a method declaration are inferred based on the corresponding types from the method or methods that it overrides.</p><p>If a candidate method (the method that's missing type information) overrides a single inherited method, then the corresponding types from the overridden method are inferred. For example, consider the following code:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#0468D7"> int</span><span style="color:#6200EE"> m</span><span style="color:#222222">(</span><span style="color:#0468D7">String</span><span style="color:#222222"> s) => </span><span style="color:#11796D">0</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> B</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> A</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> @override</span></span> <span class="line"><span style="color:#6200EE"> m</span><span style="color:#222222">(s) => </span><span style="color:#11796D">1</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>The declaration of <code>m</code> in <code>B</code> is a candidate because it's missing both the return type and the parameter type. Because it overrides a single method (the method <code>m</code> in <code>A</code>), the types from the overridden method will be used to infer the missing types and it will be as if the method in <code>B</code> had been declared as <code>int m(String s) => 1;</code>.</p><p>If a candidate method overrides multiple methods, and the function type one of those overridden methods, M<sub>s</sub>, is a supertype of the function types of all of the other overridden methods, then M<sub>s</sub> is used to infer the missing types. For example, consider the following code:</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#0468D7"> int</span><span style="color:#6200EE"> m</span><span style="color:#222222">(</span><span style="color:#0468D7">num</span><span style="color:#222222"> n) => </span><span style="color:#11796D">0</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> B</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#0468D7"> num</span><span style="color:#6200EE"> m</span><span style="color:#222222">(</span><span style="color:#0468D7">int</span><span style="color:#222222"> i) => </span><span style="color:#11796D">0</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222">}</span></span> <span class="line"></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> C</span><span style="color:#D43324"> implements</span><span style="color:#0468D7"> A</span><span style="color:#222222">, </span><span style="color:#0468D7">B</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> @override</span></span> <span class="line"><span style="color:#6200EE"> m</span><span style="color:#222222">(n) => </span><span style="color:#11796D">1</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>The declaration of <code>m</code> in <code>C</code> is a candidate for override inference because it's missing both the return type and the parameter type. It overrides both <code>m</code> in <code>A</code> and <code>m</code> in <code>B</code>, so the compiler needs to choose one of them from which the missing types can be inferred. But because the function type of <code>m</code> in <code>A</code> (<code>int Function(num)</code>) is a supertype of the function type of <code>m</code> in <code>B</code> (<code>num Function(int)</code>), the function in <code>A</code> is used to infer the missing types. The result is the same as declaring the method in <code>C</code> as <code>int m(num n) => 1;</code>.</p><p>It is an error if none of the overridden methods have a function type that is a supertype of all the other overridden methods.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/language/extend" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Inheritance in Dart</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="part-file" data-partial-matches="part file" data-full-matches="part"><div class="card-header"><h2 class="card-title">Part file</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#part-file" title="Link to card" aria-label="Link to Part file card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="part-file-content" title="Expand or collapse card" aria-label="Expand or collapse Part file card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A Dart source file that contains a <code>part of</code> directive.</p></div><div id="part-file-content" class="expandable-content"><p>A part file is a Dart source file that contains a <code>part of</code> directive and is included in a library using the <code>part</code> directive.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/tools/pub/create-packages#organizing-a-package" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Guidance on libraries and parts</span></a></li><li><a href="/effective-dart/usage#do-use-strings-in-part-of-directives" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Use library URIs in `part of` directives</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="potentially-non-nullable" data-partial-matches="potentially non-nullable" data-full-matches="potentially nullable"><div class="card-header"><h2 class="card-title">Potentially non-nullable</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#potentially-non-nullable" title="Link to card" aria-label="Link to Potentially non-nullable card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="potentially-non-nullable-content" title="Expand or collapse card" aria-label="Expand or collapse Potentially non-nullable card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A type that is either non-nullable explicitly or due to being a type parameter.</p></div><div id="potentially-non-nullable-content" class="expandable-content"><p>A type is <em>potentially non-nullable</em> if it's either explicitly non-nullable or if it's a type parameter.</p><p>A type is explicitly non-nullable if it is a type name that isn't followed by a question mark (<code>?</code>). Note that there are a few types that are always nullable, such as <code>Null</code> and <code>dynamic</code>, and that <code>FutureOr</code> is only non-nullable if it isn't followed by a question mark <em>and</em> the type argument is non-nullable (such as <code>FutureOr<String></code>).</p><p>Type parameters are potentially non-nullable because the actual runtime type (the type specified as a type argument) might be non-nullable. For example, given a declaration of <code>class C<T> {}</code>, the type <code>C</code> could be used with a non-nullable type argument as in <code>C<int></code>.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/null-safety/understanding-null-safety#nullability-and-generics" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Nullability and generics</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="public-library" data-partial-matches="public library" data-full-matches=""><div class="card-header"><h2 class="card-title">Public library</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#public-library" title="Link to card" aria-label="Link to Public library card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="public-library-content" title="Expand or collapse card" aria-label="Expand or collapse Public library card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A library that is located in a package's <code>lib</code> directory but not inside the <code>lib/src</code> directory.</p></div><div id="public-library-content" class="expandable-content"><p>A public library is a library that is located inside the package's <code>lib</code> directory but not inside the <code>lib/src</code> directory.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/tools/pub/create-packages#organizing-a-package" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Organizing a package's libraries</span></a></li><li><a href="/tools/pub/package-layout#public-directories" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Public package directories</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="quick-fix" data-partial-matches="quick fix" data-full-matches=""><div class="card-header"><h2 class="card-title">Quick fix</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#quick-fix" title="Link to card" aria-label="Link to Quick fix card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="quick-fix-content" title="Expand or collapse card" aria-label="Expand or collapse Quick fix card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>An automated, local code edit targeted at fixing the issue reported by a specific diagnostic.</p></div><div id="quick-fix-content" class="expandable-content"><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="https://medium.com/dartlang/quick-fixes-for-analysis-issues-c10df084971a" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Quick fixes for analysis issues</span></a></li><li><a href="/tools/diagnostic-messages" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Diagnostic messages</span></a></li><li><a href="/tools/linter-rules" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Linter rules</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="refactor" data-partial-matches="refactor" data-full-matches=""><div class="card-header"><h2 class="card-title">Refactor</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#refactor" title="Link to card" aria-label="Link to Refactor card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="refactor-content" title="Expand or collapse card" aria-label="Expand or collapse Refactor card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A code edit targeted at modifications that are either non-local or that require user interaction.</p></div><div id="refactor-content" class="expandable-content"><p>A refactor is a code edit targeted at modifications that are either non-local or that require user interaction. Examples of refactors include renaming, removing, or extracting code.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="#assist" class="filled-button"><span class="material-symbols" aria-hidden="true">dictionary </span><span>Assist</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="refutable-pattern" data-partial-matches="refutable pattern" data-full-matches="matching contexts"><div class="card-header"><h2 class="card-title">Refutable pattern</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#refutable-pattern" title="Link to card" aria-label="Link to Refutable pattern card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="refutable-pattern-content" title="Expand or collapse card" aria-label="Expand or collapse Refutable pattern card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A pattern that can be tested against a value.</p></div><div id="refutable-pattern-content" class="expandable-content"><p>A <em>refutable pattern</em> is a pattern that can be tested against a value to determine if the pattern matches the value. If not, the pattern <em>refutes</em>, or denies, the match. Refutable patterns appear in <a href="/language/patterns#matching"><em>matching contexts</em></a>.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/language/patterns#places-patterns-can-appear" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Places patterns can appear</span></a></li><li><a href="https://codelabs.developers.google.com/codelabs/dart-patterns-records" class="filled-button"><span class="material-symbols" aria-hidden="true">school </span><span>Dive into Dart patterns and records</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="subclass" data-partial-matches="subclass" data-full-matches="child class"><div class="card-header"><h2 class="card-title">Subclass</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#subclass" title="Link to card" aria-label="Link to Subclass card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="subclass-content" title="Expand or collapse card" aria-label="Expand or collapse Subclass card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A class that inherits the implementation of another class.</p></div><div id="subclass-content" class="expandable-content"><p>A <em>subclass</em> is a class that inherits the implementation of another class by using the <a href="/language/extend"><code>extends</code></a> keyword, or by <a href="#mixin-application">mixin application</a>.</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#6E6E70">// A is a subclass of B; B is the superclass of A.</span></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> B</span><span style="color:#222222"> {}</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// B1 has the superclass `A with M`, which has the superclass A.</span></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> B1</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> A</span><span style="color:#D43324"> with</span><span style="color:#0468D7"> M</span><span style="color:#222222"> {}</span></span></code></pre></div></div><p>A subclass relation also implies an associated <a href="#subtype">subtype</a> relation. For example, <code>class A</code> implicitly defines an associated type <code>A</code> which instances of the class <code>A</code> inhabit. So, <code>class A extends B</code> declares not just that the class <code>A</code> is a subclass of <code>B</code>, but also establishes that the <em>type</em> <code>A</code> is a <em>subtype</em> of the type <code>B</code>.</p><p>Subclass relations are a subset of subtype relations. When the documentation says "<code>S</code> must be a subtype of <code>T</code>", it's fine for <code>S</code> to be a subclass of <code>T</code>. However, the converse is not true: not all subtypes are subclasses.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="/language/extend" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Inheritance in Dart</span></a></li><li><a href="#subtype" class="filled-button"><span class="material-symbols" aria-hidden="true">dictionary </span><span>Subtype relationship</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="subtype" data-partial-matches="subtype" data-full-matches="subtyping,subtype polymorphism"><div class="card-header"><h2 class="card-title">Subtype</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#subtype" title="Link to card" aria-label="Link to Subtype card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="subtype-content" title="Expand or collapse card" aria-label="Expand or collapse Subtype card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>A type that can be used wherever a value of its supertype is expected.</p></div><div id="subtype-content" class="expandable-content"><p>A <em>subtype</em> relation is where a value of a certain type is substitutable where the value of another type, the supertype, is expected. For example, if <code>S</code> is a subtype of <code>T</code>, then you can substitute a value of type <code>S</code> where a value of type <code>T</code> is expected.</p><p>A subtype supports all of the operations of its supertype (and possibly some extra operations). In practice, this means you can assign the value of a subtype to any location expecting the supertype, and all of the methods of the supertype are available on the subtype.</p><p>This is true at least statically. A specific API might not allow the substitution at run time, depending on its operations.</p><p>Some subtype relations are based on the structure of the type, like with nullable types (for example, <code>int</code> is a subtype of <code>int?</code>) and function types (for example, <code>String Function()</code> is a subtype of <code>void Function()</code>).</p><p>Subtypes can also be introduced for classes by <a href="/language/classes#implicit-interfaces">implementation</a> or <a href="/language/extend">inheritance</a> (direct or indirect):</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#6E6E70">// A is a subtype of B, but NOT a subclass of B.</span></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> A</span><span style="color:#D43324"> implements</span><span style="color:#0468D7"> B</span><span style="color:#222222"> {}</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// C is a subtype AND a subclass of D.</span></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> C</span><span style="color:#D43324"> extends</span><span style="color:#0468D7"> D</span><span style="color:#222222"> {}</span></span></code></pre></div></div><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="#subclass" class="filled-button"><span class="material-symbols" aria-hidden="true">dictionary </span><span>Subclass</span></a></li><li><a href="/language/type-system#substituting-types" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Substituting types</span></a></li></ul></div></div></div><div class="card outlined-card glossary-card expandable-card" id="variance" data-partial-matches="variance and variance positions" data-full-matches="variance positions,covariance,covariant,contravariance,contravariant"><div class="card-header"><h2 class="card-title">Variance and variance positions</h2><div class="card-header-buttons"><a class="share-button icon-button" href="#variance" title="Link to card" aria-label="Link to Variance and variance positions card"><span class="material-symbols" aria-hidden="true">tag</span> </a><button class="expand-button icon-button" aria-expanded="true" aria-controls="variance-content" title="Expand or collapse card" aria-label="Expand or collapse Variance and variance positions card"><span class="material-symbols" aria-hidden="true">keyboard_arrow_up</span></button></div></div><div class="initial-content"><p>How changing a type argument of a type affects the relationship between the original type and the resulting one.</p><p>In Dart, changing the type argument of a type declaration (like a class) or function return type, changes the overall type relationship in the same direction (covariant).</p><p>However, changing the type of a function's parameter types, changes the overall type relationship in the opposite direction (contravariant).</p></div><div id="variance-content" class="expandable-content"><p>A type parameter of a class (or other type declaration, like a mixin) is said to be <em>covariant</em> when the type as a whole "co-varies" with the actual type argument. In other words, if the type argument is replaced by a subtype then the type as a whole is also a subtype.</p><p>For example, the type parameter of the class <code>List</code> is covariant because list types co-vary with their type argument: <code>List<int></code> is a subtype of <code>List<Object></code> because <code>int</code> is a subtype of <code>Object</code>.</p><p>In Dart, all type parameters of all class, mixin, mixin class, and enum declarations are covariant.</p><p>However, function types are different: A function type is covariant in its return type, but the opposite (known as <em>contravariant</em>) in its parameter types. For example, the type <code>int Function(int)</code> is a subtype of the type <code>Object Function(int)</code>, but it is a supertype of <code>int Function(Object)</code>.</p><p>This makes sense if you consider their <a href="#subtype">substitutability</a>. If you call a function with a static type of <code>int Function(int)</code>, that function can actually be of type <code>int Function(Object)</code> at runtime. Based on the static type, you expect to be able to pass an <code>int</code> to it. That will be fine since the function actually accepts any <code>Object</code>, and this includes every object of type <code>int</code>. Similarly, the returned result will be of type <code>int</code>, which is also what you expect based on the static type.</p><p>Hence, <code>int Function(Object)</code> is a subtype of <code>int Function(int)</code>.</p><p>Note that everything is turned upside-down for parameter types. In particular, this subtype relation among function types requires that the <em>opposite</em> subtype relation exists for the parameter type. For example, <code>void Function(Object)</code> is a subtype of <code>void Function(int)</code> because <code>int</code> is a subtype of <code>Object</code>.</p><p>With a more complex type like <code>List<void Function(int)></code>, you have to consider the <em>positions</em> in the type. To accomplish this, turn one of the parts of the type into a placeholder, and then consider what happens to the type when different types are placed in that position.</p><p>For example, consider <code>List<void Function(_)></code> as a template for a type where you can put different types in place of the placeholder <code>_</code>. This type is contravariant in the position where that placeholder occurs.</p><p>The following illustrates this by substituting <code>Object</code> and <code>int</code> for <code>_</code>. <code>List<void Function(Object)></code> is a subtype of <code>List<void Function(int)></code> because <code>void Function(Object)</code> is a subtype of <code>void Function(int)</code> because <code>void</code> is a subtype of <code>void</code> (the return types) and <code>int</code> is a subtype of <code>Object</code> (the parameter types, in the opposite order). Hence, the type at <code>_</code> varies in the opposite direction of the type <code>List<void Function(_)></code> as a whole, and this 'opposite direction' by definition makes it a <em>contravariant position</em>.</p><p>A <em>covariant position</em> is defined similarly. For example, <code>_</code> is at a covariant position in the type <code>List<_></code>, and <code>_</code> is also at a covariant position in the type <code>_ Function(int)</code>.</p><p>There is yet another kind of position known as <em>invariant</em>, but it occurs much more rarely so the details are omitted here.</p><p>In practice, it's often sufficient to know that the type arguments of a class, mixin, etc. are in a covariant position, and so is the return type of a function type, but the parameter types are in a contravariant position.</p><div><h3 class="no_toc details-header">Related docs and resources</h3><ul class="resources-list"><li><a href="#subtype" class="filled-button"><span class="material-symbols" aria-hidden="true">dictionary </span><span>Subtype relationship</span></a></li><li><a href="https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>Covariance and contravariance</span></a></li><li><a href="/language/type-system#covariant-keyword" class="filled-button"><span class="material-symbols" aria-hidden="true">article </span><span>The covariant keyword</span></a></li></ul></div></div></div></div></section><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.7.1. Page last updated on 2025-03-07.</span> <a href="https://github.com/dart-lang/site-www/tree/main/src/content/resources/glossary.md" target="_blank" rel="noopener">View source</a> <span>or </span><a href="https://github.com/dart-lang/site-www/issues/new?template=1_page_issue.yml&page-url=https://dart.dev/resources/glossary/&page-source=https://github.com/dart-lang/site-www/tree/main/src/content/resources/glossary.md" title="Report an issue with this page" target="_blank" rel="noopener">report an issue</a>.</p></div></article></main></div><footer id="page-footer"><div class="footer-section footer-main"><a href="/" class="brand" title="Dart"><img src="/assets/img/logo/logo-white-text.svg" alt="Dart" width="164"></a><div class="footer-social-links"><a href="https://medium.com/dartlang" target="_blank" rel="noopener" title="Dart's Medium publication"><svg><use href="/assets/img/social/medium.svg#medium"></use></svg> </a><a href="https://github.com/dart-lang" target="_blank" rel="noopener" title="Dart's GitHub organization"><svg><use href="/assets/img/social/github.svg#github"></use></svg> </a><a href="https://bsky.app/profile/dart.dev" target="_blank" rel="noopener" title="Dart's Bluesky profile"><svg><use href="/assets/img/social/bluesky.svg#bluesky"></use></svg> </a><a href="https://twitter.com/dart_lang" target="_blank" rel="noopener" title="Dart's X (Twitter) profile"><svg><use href="/assets/img/social/x.svg#x"></use></svg></a></div></div><div class="footer-section footer-tray"><div class="footer-licenses">Except as otherwise noted, this site is licensed under a <a href="https://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>, and code samples are licensed under the <a href="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</a>.</div><div class="footer-utility-links"><ul><li><a href="/terms" title="Terms of use">Terms</a></li><li><a href="https://policies.google.com/privacy" target="_blank" rel="noopener" title="Privacy policy">Privacy</a></li><li><a href="/security" title="Security philosophy and practices">Security</a></li></ul></div></div></footer></div></body></html>