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KB","deleteTooltip":"You must be signed in to make or propose changes","editTooltip":"You must be signed in to make or propose changes","ghDesktopPath":"https://desktop.github.com","isGitLfs":false,"onBranch":true,"shortPath":"376a5ab","siteNavLoginPath":"/login?return_to=https%3A%2F%2Fgithub.com%2Fdart-lang%2Fsite-www%2Fblob%2Fmain%2Fsrc%2Fcontent%2Flanguage%2Ffunctions.md","isCSV":false,"isRichtext":true,"toc":[{"level":2,"text":"Parameters","anchor":"parameters","htmlText":"Parameters"},{"level":3,"text":"Named parameters","anchor":"named-parameters","htmlText":"Named parameters"},{"level":3,"text":"Optional positional parameters","anchor":"optional-positional-parameters","htmlText":"Optional positional parameters"},{"level":2,"text":"The main() function {:#main}","anchor":"the-main-function-main","htmlText":"The main() function {:#main}"},{"level":2,"text":"Functions as first-class objects","anchor":"functions-as-first-class-objects","htmlText":"Functions as first-class objects"},{"level":2,"text":"Function types","anchor":"function-types","htmlText":"Function types"},{"level":2,"text":"Anonymous functions","anchor":"anonymous-functions","htmlText":"Anonymous functions"},{"level":2,"text":"Lexical scope","anchor":"lexical-scope","htmlText":"Lexical scope"},{"level":2,"text":"Lexical closures","anchor":"lexical-closures","htmlText":"Lexical closures"},{"level":2,"text":"Tear-offs","anchor":"tear-offs","htmlText":"Tear-offs"},{"level":2,"text":"Testing functions for equality","anchor":"testing-functions-for-equality","htmlText":"Testing functions for equality"},{"level":2,"text":"Return values","anchor":"return-values","htmlText":"Return values"},{"level":2,"text":"Generators","anchor":"generators","htmlText":"Generators"},{"level":2,"text":"External functions {:#external}","anchor":"external-functions-external","htmlText":"External functions {:#external}"}],"lineInfo":{"truncatedLoc":"624","truncatedSloc":"485"},"mode":"file"},"image":false,"isCodeownersFile":null,"isPlain":false,"isValidLegacyIssueTemplate":false,"issueTemplate":null,"discussionTemplate":null,"language":"Markdown","languageID":222,"large":false,"planSupportInfo":{"repoIsFork":null,"repoOwnedByCurrentUser":null,"requestFullPath":"/dart-lang/site-www/blob/main/src/content/language/functions.md","showFreeOrgGatedFeatureMessage":null,"showPlanSupportBanner":null,"upgradeDataAttributes":null,"upgradePath":null},"publishBannersInfo":{"dismissActionNoticePath":"/settings/dismiss-notice/publish_action_from_dockerfile","releasePath":"/dart-lang/site-www/releases/new?marketplace=true","showPublishActionBanner":false},"rawBlobUrl":"https://github.com/dart-lang/site-www/raw/refs/heads/main/src/content/language/functions.md","renderImageOrRaw":false,"richText":"\u003carticle class=\"markdown-body entry-content container-lg\" itemprop=\"text\"\u003e\u003cmarkdown-accessiblity-table\u003e\u003ctable\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth\u003etitle\u003c/th\u003e\n \u003cth\u003edescription\u003c/th\u003e\n \u003cth\u003ejs\u003c/th\u003e\n \u003cth\u003eprevpage\u003c/th\u003e\n \u003cth\u003enextpage\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003eFunctions\u003c/div\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003eEverything about functions in Dart.\u003c/div\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003e\u003ctable\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003e\u003ctable\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth\u003eurl\u003c/th\u003e\n \u003cth\u003edefer\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003e/assets/js/inject_dartpad.js\u003c/div\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003etrue\u003c/div\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003e\u003ctable\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth\u003eurl\u003c/th\u003e\n \u003cth\u003etitle\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003e/language/pattern-types\u003c/div\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003ePattern types\u003c/div\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003e\u003ctable\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth\u003eurl\u003c/th\u003e\n \u003cth\u003etitle\u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003e/language/loops\u003c/div\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cdiv dir=\"auto\"\u003eLoops\u003c/div\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\u003c/markdown-accessiblity-table\u003e\n\n\u003cp dir=\"auto\"\u003eDart is a true object-oriented language, so even functions are objects\nand have a type, \u003ca href=\"/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.dart-api%7D%7D/dart-core/Function-class.html\"\u003eFunction.\u003c/a\u003e\nThis means that functions can be assigned to variables or passed as arguments\nto other functions. You can also call an instance of a Dart class as if\nit were a function. For details, see \u003ca href=\"/dart-lang/site-www/blob/main/language/callable-objects\"\u003eCallable objects\u003c/a\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eHere's an example of implementing a function:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"bool isNoble(int atomicNumber) {\n return _nobleGases[atomicNumber] != null;\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003ebool\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eisNoble\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e atomicNumber) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e _nobleGases[atomicNumber] \u003cspan class=\"pl-k\"\u003e!=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003enull\u003c/span\u003e;\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAlthough Effective Dart recommends\n\u003ca href=\"/dart-lang/site-www/blob/main/effective-dart/design#do-type-annotate-fields-and-top-level-variables-if-the-type-isnt-obvious\"\u003etype annotations for public APIs\u003c/a\u003e,\nthe function still works if you omit the types:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"isNoble(atomicNumber) {\n return _nobleGases[atomicNumber] != null;\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-en\"\u003eisNoble\u003c/span\u003e(atomicNumber) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e _nobleGases[atomicNumber] \u003cspan class=\"pl-k\"\u003e!=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003enull\u003c/span\u003e;\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eFor functions that contain just one expression, you can use a shorthand\nsyntax:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"bool isNoble(int atomicNumber) =\u0026gt; _nobleGases[atomicNumber] != null;\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003ebool\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eisNoble\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e atomicNumber) \u003cspan class=\"pl-k\"\u003e=\u0026gt;\u003c/span\u003e _nobleGases[atomicNumber] \u003cspan class=\"pl-k\"\u003e!=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003enull\u003c/span\u003e;\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe \u003ccode\u003e=\u0026gt; \u003cem\u003eexpr\u003c/em\u003e\u003c/code\u003e syntax is a shorthand for\n\u003ccode\u003e{ return \u003cem\u003eexpr\u003c/em\u003e; }\u003c/code\u003e. The \u003ccode\u003e=\u0026gt;\u003c/code\u003e notation\nis sometimes referred to as \u003cem\u003earrow\u003c/em\u003e syntax.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003e:::note\nOnly \u003cem\u003eexpressions\u003c/em\u003e can appear between the arrow (\u003ccode\u003e=\u0026gt;\u003c/code\u003e) and the semicolon (\u003ccode\u003e;\u003c/code\u003e).\nExpressions evaluate to values.\nThis means that you can't write a statement where Dart expects a value.\nFor example,\nyou could use a \u003ca href=\"/dart-lang/site-www/blob/main/language/operators#conditional-expressions\"\u003econditional expression\u003c/a\u003e but not an \u003ca href=\"/dart-lang/site-www/blob/main/language/branches#if\"\u003eif statement\u003c/a\u003e.\nIn the previous example,\n\u003ccode\u003e_nobleGases[atomicNumber] != null;\u003c/code\u003e returns a boolean value.\nThe function then returns a boolean value\nthat indicates whether the \u003ccode\u003eatomicNumber\u003c/code\u003e falls into the noble gas range.\n:::\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eParameters\u003c/h2\u003e\u003ca id=\"user-content-parameters\" class=\"anchor\" aria-label=\"Permalink: Parameters\" href=\"#parameters\"\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\"\u003eA function can have any number of \u003cem\u003erequired positional\u003c/em\u003e parameters. These can be\nfollowed either by \u003cem\u003enamed\u003c/em\u003e parameters or by \u003cem\u003eoptional positional\u003c/em\u003e parameters\n(but not both).\u003c/p\u003e\n\u003cp dir=\"auto\"\u003e:::note\nSome APIs—notably \u003ca href=\"/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.flutter%7D%7D\"\u003eFlutter\u003c/a\u003e widget constructors—use only named\nparameters, even for parameters that are mandatory. See the next section for\ndetails.\n:::\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eYou can use \u003ca href=\"/dart-lang/site-www/blob/main/language/collections#lists\"\u003etrailing commas\u003c/a\u003e when you pass arguments to a function\nor when you define function parameters.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eNamed parameters\u003c/h3\u003e\u003ca id=\"user-content-named-parameters\" class=\"anchor\" aria-label=\"Permalink: Named parameters\" href=\"#named-parameters\"\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\"\u003eNamed parameters are optional\nunless they're explicitly marked as \u003ccode\u003erequired\u003c/code\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eWhen defining a function, use\n\u003ccode\u003e{\u003cem\u003eparam1\u003c/em\u003e, \u003cem\u003eparam2\u003c/em\u003e, …}\u003c/code\u003e\nto specify named parameters.\nIf you don't provide a default value\nor mark a named parameter as \u003ccode\u003erequired\u003c/code\u003e,\ntheir types must be nullable\nas their default value will be \u003ccode\u003enull\u003c/code\u003e:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"/// Sets the [bold] and [hidden] flags ...\nvoid enableFlags({bool? bold, bool? hidden}) {\n ...\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e/// Sets the \u003cspan class=\"pl-v\"\u003e[bold]\u003c/span\u003e and \u003cspan class=\"pl-v\"\u003e[hidden]\u003c/span\u003e flags ...\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e\u003c/span\u003e\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eenableFlags\u003c/span\u003e({\u003cspan class=\"pl-c1\"\u003ebool\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e?\u003c/span\u003e bold, \u003cspan class=\"pl-c1\"\u003ebool\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e?\u003c/span\u003e hidden}) {\n ...\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWhen calling a function,\nyou can specify named arguments using\n\u003ccode\u003e\u003cem\u003eparamName\u003c/em\u003e: \u003cem\u003evalue\u003c/em\u003e\u003c/code\u003e.\nFor example:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"enableFlags(bold: true, hidden: false);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-en\"\u003eenableFlags\u003c/span\u003e(bold\u003cspan class=\"pl-k\"\u003e:\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003etrue\u003c/span\u003e, hidden\u003cspan class=\"pl-k\"\u003e:\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003efalse\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e\u003ca id=\"user-content-default-parameters\"\u003e\u003c/a\u003e\nTo define a default value for a named parameter besides \u003ccode\u003enull\u003c/code\u003e,\nuse \u003ccode\u003e=\u003c/code\u003e to specify a default value.\nThe specified value must be a compile-time constant.\nFor example:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"/// Sets the [bold] and [hidden] flags ...\nvoid enableFlags({bool bold = false, bool hidden = false}) {\n ...\n}\n\n// bold will be true; hidden will be false.\nenableFlags(bold: true);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e/// Sets the \u003cspan class=\"pl-v\"\u003e[bold]\u003c/span\u003e and \u003cspan class=\"pl-v\"\u003e[hidden]\u003c/span\u003e flags ...\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e\u003c/span\u003e\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eenableFlags\u003c/span\u003e({\u003cspan class=\"pl-c1\"\u003ebool\u003c/span\u003e bold \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003efalse\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003ebool\u003c/span\u003e hidden \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003efalse\u003c/span\u003e}) {\n ...\n}\n\n\u003cspan class=\"pl-c\"\u003e// bold will be true; hidden will be false.\u003c/span\u003e\n\u003cspan class=\"pl-en\"\u003eenableFlags\u003c/span\u003e(bold\u003cspan class=\"pl-k\"\u003e:\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003etrue\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIf you instead want a named parameter to be mandatory,\nrequiring callers to provide a value for the parameter,\nannotate them with \u003ccode\u003erequired\u003c/code\u003e:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"const Scrollbar({super.key, [!required!] Widget child});\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003econst\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eScrollbar\u003c/span\u003e({\u003cspan class=\"pl-c1\"\u003esuper\u003c/span\u003e.key, [\u003cspan class=\"pl-k\"\u003e!\u003c/span\u003e\u003cspan class=\"pl-k\"\u003erequired\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e!\u003c/span\u003e] \u003cspan class=\"pl-c1\"\u003eWidget\u003c/span\u003e child});\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIf someone tries to create a \u003ccode\u003eScrollbar\u003c/code\u003e\nwithout specifying the \u003ccode\u003echild\u003c/code\u003e argument,\nthen the analyzer reports an issue.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003e:::note\nA parameter marked as \u003ccode\u003erequired\u003c/code\u003e can still be nullable:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"const Scrollbar({super.key, required [!Widget?!] child});\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003econst\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eScrollbar\u003c/span\u003e({\u003cspan class=\"pl-c1\"\u003esuper\u003c/span\u003e.key, \u003cspan class=\"pl-k\"\u003erequired\u003c/span\u003e [\u003cspan class=\"pl-k\"\u003e!\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eWidget\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e?\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e!\u003c/span\u003e] child});\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e:::\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eYou might want to place positional arguments first,\nbut Dart doesn't require it.\nDart allows named arguments to be placed anywhere in the\nargument list when it suits your API:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"repeat(times: 2, () {\n ...\n});\"\u003e\u003cpre\u003e\u003cspan class=\"pl-en\"\u003erepeat\u003c/span\u003e(times\u003cspan class=\"pl-k\"\u003e:\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e2\u003c/span\u003e, () {\n ...\n});\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eOptional positional parameters\u003c/h3\u003e\u003ca id=\"user-content-optional-positional-parameters\" class=\"anchor\" aria-label=\"Permalink: Optional positional parameters\" href=\"#optional-positional-parameters\"\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\"\u003eWrapping a set of function parameters in \u003ccode\u003e[]\u003c/code\u003e\nmarks them as optional positional parameters.\nIf you don't provide a default value,\ntheir types must be nullable\nas their default value will be \u003ccode\u003enull\u003c/code\u003e:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"String say(String from, String msg, [String? device]) {\n var result = '$from says $msg';\n if (device != null) {\n result = '$result with a $device';\n }\n return result;\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-en\"\u003esay\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e from, \u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e msg, [\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e?\u003c/span\u003e device]) {\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e result \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'$\u003cspan class=\"pl-v\"\u003efrom\u003c/span\u003e says $\u003cspan class=\"pl-v\"\u003emsg\u003c/span\u003e'\u003c/span\u003e;\n \u003cspan class=\"pl-k\"\u003eif\u003c/span\u003e (device \u003cspan class=\"pl-k\"\u003e!=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003enull\u003c/span\u003e) {\n result \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'$\u003cspan class=\"pl-v\"\u003eresult\u003c/span\u003e with a $\u003cspan class=\"pl-v\"\u003edevice\u003c/span\u003e'\u003c/span\u003e;\n }\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e result;\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eHere's an example of calling this function\nwithout the optional parameter:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"assert(say('Bob', 'Howdy') == 'Bob says Howdy');\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003esay\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'Bob'\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e'Howdy'\u003c/span\u003e) \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'Bob says Howdy'\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAnd here's an example of calling this function with the third parameter:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"assert(\n say('Bob', 'Howdy', 'smoke signal') ==\n 'Bob says Howdy with a smoke signal',\n);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\n \u003cspan class=\"pl-en\"\u003esay\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'Bob'\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e'Howdy'\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e'smoke signal'\u003c/span\u003e) \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e\n \u003cspan class=\"pl-s\"\u003e'Bob says Howdy with a smoke signal'\u003c/span\u003e,\n);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eTo define a default value for an optional positional parameter besides \u003ccode\u003enull\u003c/code\u003e,\nuse \u003ccode\u003e=\u003c/code\u003e to specify a default value.\nThe specified value must be a compile-time constant.\nFor example:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"String say(String from, String msg, [String device = 'carrier pigeon']) {\n var result = '$from says $msg with a $device';\n return result;\n}\n\nassert(say('Bob', 'Howdy') == 'Bob says Howdy with a carrier pigeon');\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e \u003cspan class=\"pl-en\"\u003esay\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e from, \u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e msg, [\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e device \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'carrier pigeon'\u003c/span\u003e]) {\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e result \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'$\u003cspan class=\"pl-v\"\u003efrom\u003c/span\u003e says $\u003cspan class=\"pl-v\"\u003emsg\u003c/span\u003e with a $\u003cspan class=\"pl-v\"\u003edevice\u003c/span\u003e'\u003c/span\u003e;\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e result;\n}\n\n\u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003esay\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'Bob'\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e'Howdy'\u003c/span\u003e) \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'Bob says Howdy with a carrier pigeon'\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e\u003ca id=\"user-content-the-main-function\"\u003e\u003c/a\u003e\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eThe main() function {:#main}\u003c/h2\u003e\u003ca id=\"user-content-the-main-function-main\" class=\"anchor\" aria-label=\"Permalink: The main() function {:#main}\" href=\"#the-main-function-main\"\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\"\u003eEvery app must have a top-level \u003ccode\u003emain()\u003c/code\u003e function, which serves as the\nentrypoint to the app. The \u003ccode\u003emain()\u003c/code\u003e function returns \u003ccode\u003evoid\u003c/code\u003e and has an\noptional \u003ccode\u003eList\u0026lt;String\u0026gt;\u003c/code\u003e parameter for arguments.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eHere's a simple \u003ccode\u003emain()\u003c/code\u003e function:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"void main() {\n print('Hello, World!');\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e() {\n \u003cspan class=\"pl-en\"\u003eprint\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'Hello, World!'\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eHere's an example of the \u003ccode\u003emain()\u003c/code\u003e function for a command-line app that\ntakes arguments:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"// Run the app like this: dart run args.dart 1 test\nvoid main(List\u0026lt;String\u0026gt; arguments) {\n print(arguments);\n\n assert(arguments.length == 2);\n assert(int.parse(arguments[0]) == 1);\n assert(arguments[1] == 'test');\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e// Run the app like this: dart run args.dart 1 test\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eList\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e\u0026gt; arguments) {\n \u003cspan class=\"pl-en\"\u003eprint\u003c/span\u003e(arguments);\n\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(arguments.length \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e2\u003c/span\u003e);\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003eparse\u003c/span\u003e(arguments[\u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e]) \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e1\u003c/span\u003e);\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(arguments[\u003cspan class=\"pl-c1\"\u003e1\u003c/span\u003e] \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'test'\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eYou can use the \u003ca href=\"/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.pub-pkg%7D%7D/args\"\u003eargs library\u003c/a\u003e to\ndefine and parse command-line arguments.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFunctions as first-class objects\u003c/h2\u003e\u003ca id=\"user-content-functions-as-first-class-objects\" class=\"anchor\" aria-label=\"Permalink: Functions as first-class objects\" href=\"#functions-as-first-class-objects\"\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\"\u003eYou can pass a function as a parameter to another function. For example:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"void printElement(int element) {\n print(element);\n}\n\nvar list = [1, 2, 3];\n\n// Pass printElement as a parameter.\nlist.forEach(printElement);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eprintElement\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e element) {\n \u003cspan class=\"pl-en\"\u003eprint\u003c/span\u003e(element);\n}\n\n\u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e list \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e [\u003cspan class=\"pl-c1\"\u003e1\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003e2\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003e3\u003c/span\u003e];\n\n\u003cspan class=\"pl-c\"\u003e// Pass printElement as a parameter.\u003c/span\u003e\nlist.\u003cspan class=\"pl-en\"\u003eforEach\u003c/span\u003e(printElement);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eYou can also assign a function to a variable, such as:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"var loudify = (msg) =\u0026gt; '!!! ${msg.toUpperCase()} !!!';\nassert(loudify('hello') == '!!! HELLO !!!');\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e loudify \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e (msg) \u003cspan class=\"pl-k\"\u003e=\u0026gt;\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'!!! ${\u003cspan class=\"pl-v\"\u003emsg\u003c/span\u003e.\u003cspan class=\"pl-en\"\u003etoUpperCase\u003c/span\u003e()} !!!'\u003c/span\u003e;\n\u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eloudify\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'hello'\u003c/span\u003e) \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'!!! HELLO !!!'\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis example uses an anonymous function.\nMore about those in the next section.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFunction types\u003c/h2\u003e\u003ca id=\"user-content-function-types\" class=\"anchor\" aria-label=\"Permalink: Function types\" href=\"#function-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\"\u003eYou can specify the type of a function, which is known as a \u003cem\u003efunction type\u003c/em\u003e.\nA function type is obtained from a function declaration header by\nreplacing the function name by the keyword \u003ccode\u003eFunction\u003c/code\u003e.\nMoreover, you are allowed to omit the names of positional parameters, but\nthe names of named parameters can't be omitted. For example:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"void greet(String name, {String greeting = 'Hello'}) =\u0026gt;\n print('$greeting $name!');\n\n// Store `greet` in a variable and call it.\nvoid Function(String, {String greeting}) g = greet;\ng('Dash', greeting: 'Howdy');\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003egreet\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e name, {\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e greeting \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'Hello'\u003c/span\u003e}) \u003cspan class=\"pl-k\"\u003e=\u0026gt;\u003c/span\u003e\n \u003cspan class=\"pl-en\"\u003eprint\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'$\u003cspan class=\"pl-v\"\u003egreeting\u003c/span\u003e $\u003cspan class=\"pl-v\"\u003ename\u003c/span\u003e!'\u003c/span\u003e);\n\n\u003cspan class=\"pl-c\"\u003e// Store `greet` in a variable and call it.\u003c/span\u003e\n\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eFunction\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e, {\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e greeting}) g \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e greet;\n\u003cspan class=\"pl-en\"\u003eg\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'Dash'\u003c/span\u003e, greeting\u003cspan class=\"pl-k\"\u003e:\u003c/span\u003e \u003cspan class=\"pl-s\"\u003e'Howdy'\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003e:::note\nIn Dart, functions are first-class objects,\nmeaning they can be assigned to variables,\npassed as arguments, and returned from other functions.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eYou can use a \u003ca href=\"/dart-lang/site-www/blob/main/language/typedefs\"\u003e\u003ccode\u003etypedef\u003c/code\u003e\u003c/a\u003e declaration to explicitly name function types,\nwhich can be useful for clarity and reusability.\n:::\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAnonymous functions\u003c/h2\u003e\u003ca id=\"user-content-anonymous-functions\" class=\"anchor\" aria-label=\"Permalink: Anonymous functions\" href=\"#anonymous-functions\"\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\"\u003eThough you name most functions, such as \u003ccode\u003emain()\u003c/code\u003e or \u003ccode\u003eprintElement()\u003c/code\u003e.\nyou can also create functions without names.\nThese functions are called \u003cem\u003eanonymous functions\u003c/em\u003e, \u003cem\u003elambdas\u003c/em\u003e, or \u003cem\u003eclosures\u003c/em\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAn anonymous function resembles a named function as it has:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003eZero or more parameters, comma-separated\u003c/li\u003e\n\u003cli\u003eOptional type annotations between parentheses.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eThe following code block contains the function's body:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"([[Type] param1[, ...]]) {\n codeBlock;\n}\"\u003e\u003cpre\u003e([[\u003cspan class=\"pl-c1\"\u003eType\u003c/span\u003e] param1[, ...]]) {\n codeBlock;\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe following example defines an anonymous function\nwith an untyped parameter, \u003ccode\u003eitem\u003c/code\u003e.\nThe anonymous function passes it to the \u003ccode\u003emap\u003c/code\u003e function.\nThe \u003ccode\u003emap\u003c/code\u003e function, invoked for each item in the list,\nconverts each string to uppercase.\nThen, the anonymous function passed to \u003ccode\u003eforEach\u003c/code\u003e,\nprints each converted string with its length.\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"const list = ['apples', 'bananas', 'oranges'];\n\nvar uppercaseList =\n list.map((item) {\n return item.toUpperCase();\n }).toList();\n// Convert to list after mapping\n\nfor (var item in uppercaseList) {\n print('$item: ${item.length}');\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003econst\u003c/span\u003e list \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e [\u003cspan class=\"pl-s\"\u003e'apples'\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e'bananas'\u003c/span\u003e, \u003cspan class=\"pl-s\"\u003e'oranges'\u003c/span\u003e];\n\n\u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e uppercaseList \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e\n list.\u003cspan class=\"pl-en\"\u003emap\u003c/span\u003e((item) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e item.\u003cspan class=\"pl-en\"\u003etoUpperCase\u003c/span\u003e();\n }).\u003cspan class=\"pl-en\"\u003etoList\u003c/span\u003e();\n\u003cspan class=\"pl-c\"\u003e// Convert to list after mapping\u003c/span\u003e\n\n\u003cspan class=\"pl-k\"\u003efor\u003c/span\u003e (\u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e item \u003cspan class=\"pl-k\"\u003ein\u003c/span\u003e uppercaseList) {\n \u003cspan class=\"pl-en\"\u003eprint\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'$\u003cspan class=\"pl-v\"\u003eitem\u003c/span\u003e: ${\u003cspan class=\"pl-v\"\u003eitem\u003c/span\u003e.\u003cspan class=\"pl-v\"\u003elength\u003c/span\u003e}'\u003c/span\u003e);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eClick \u003cstrong\u003eRun\u003c/strong\u003e to execute the code.\u003c/p\u003e\n\n\u003cdiv class=\"snippet-clipboard-content notranslate position-relative overflow-auto\" data-snippet-clipboard-copy-content=\"void main() {\n const list = ['apples', 'bananas', 'oranges'];\n\n var uppercaseList =\n list.map((item) {\n return item.toUpperCase();\n }).toList();\n // Convert to list after mapping\n\n for (var item in uppercaseList) {\n print('$item: ${item.length}');\n }\n}\"\u003e\u003cpre lang=\"dartpad\" class=\"notranslate\"\u003e\u003ccode\u003evoid main() {\n const list = ['apples', 'bananas', 'oranges'];\n\n var uppercaseList =\n list.map((item) {\n return item.toUpperCase();\n }).toList();\n // Convert to list after mapping\n\n for (var item in uppercaseList) {\n print('$item: ${item.length}');\n }\n}\n\u003c/code\u003e\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIf the function contains only a single expression or return statement,\nyou can shorten it using arrow notation.\nPaste the following line into DartPad and click \u003cstrong\u003eRun\u003c/strong\u003e\nto verify that it is functionally equivalent.\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"var uppercaseList = list.map((item) =\u0026gt; item.toUpperCase()).toList();\nuppercaseList.forEach((item) =\u0026gt; print('$item: ${item.length}'));\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e uppercaseList \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e list.\u003cspan class=\"pl-en\"\u003emap\u003c/span\u003e((item) \u003cspan class=\"pl-k\"\u003e=\u0026gt;\u003c/span\u003e item.\u003cspan class=\"pl-en\"\u003etoUpperCase\u003c/span\u003e()).\u003cspan class=\"pl-en\"\u003etoList\u003c/span\u003e();\nuppercaseList.\u003cspan class=\"pl-en\"\u003eforEach\u003c/span\u003e((item) \u003cspan class=\"pl-k\"\u003e=\u0026gt;\u003c/span\u003e \u003cspan class=\"pl-en\"\u003eprint\u003c/span\u003e(\u003cspan class=\"pl-s\"\u003e'$\u003cspan class=\"pl-v\"\u003eitem\u003c/span\u003e: ${\u003cspan class=\"pl-v\"\u003eitem\u003c/span\u003e.\u003cspan class=\"pl-v\"\u003elength\u003c/span\u003e}'\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\"\u003eLexical scope\u003c/h2\u003e\u003ca id=\"user-content-lexical-scope\" class=\"anchor\" aria-label=\"Permalink: Lexical scope\" href=\"#lexical-scope\"\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\"\u003eDart determines the scope of variables based on the layout of its code.\nA programming language with this feature is termed a lexically scoped language.\nYou can \"follow the curly braces outwards\" to see if a variable is in scope.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003e\u003cstrong\u003eExample:\u003c/strong\u003e A series of nested functions with variables at each scope level:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"bool topLevel = true;\n\nvoid main() {\n var insideMain = true;\n\n void myFunction() {\n var insideFunction = true;\n\n void nestedFunction() {\n var insideNestedFunction = true;\n\n assert(topLevel);\n assert(insideMain);\n assert(insideFunction);\n assert(insideNestedFunction);\n }\n }\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003ebool\u003c/span\u003e topLevel \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003etrue\u003c/span\u003e;\n\n\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e() {\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e insideMain \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003etrue\u003c/span\u003e;\n\n \u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emyFunction\u003c/span\u003e() {\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e insideFunction \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003etrue\u003c/span\u003e;\n\n \u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003enestedFunction\u003c/span\u003e() {\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e insideNestedFunction \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003etrue\u003c/span\u003e;\n\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(topLevel);\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(insideMain);\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(insideFunction);\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(insideNestedFunction);\n }\n }\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe \u003ccode\u003enestedFunction()\u003c/code\u003e method can use variables from every level,\nall the way up to the top level.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eLexical closures\u003c/h2\u003e\u003ca id=\"user-content-lexical-closures\" class=\"anchor\" aria-label=\"Permalink: Lexical closures\" href=\"#lexical-closures\"\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\"\u003eA function object that can access variables in its lexical scope\nwhen the function sits outside that scope is called a \u003cem\u003eclosure\u003c/em\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFunctions can close over variables defined in surrounding scopes. In the\nfollowing example, \u003ccode\u003emakeAdder()\u003c/code\u003e captures the variable \u003ccode\u003eaddBy\u003c/code\u003e. Wherever the\nreturned function goes, it remembers \u003ccode\u003eaddBy\u003c/code\u003e.\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"/// Returns a function that adds [addBy] to the\n/// function's argument.\nFunction makeAdder(int addBy) {\n return (int i) =\u0026gt; addBy + i;\n}\n\nvoid main() {\n // Create a function that adds 2.\n var add2 = makeAdder(2);\n\n // Create a function that adds 4.\n var add4 = makeAdder(4);\n\n assert(add2(3) == 5);\n assert(add4(3) == 7);\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e/// Returns a function that adds \u003cspan class=\"pl-v\"\u003e[addBy]\u003c/span\u003e to the\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e/// function's argument.\u003c/span\u003e\n\u003cspan class=\"pl-c\"\u003e\u003c/span\u003e\u003cspan class=\"pl-c1\"\u003eFunction\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emakeAdder\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e addBy) {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e (\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e i) \u003cspan class=\"pl-k\"\u003e=\u0026gt;\u003c/span\u003e addBy \u003cspan class=\"pl-k\"\u003e+\u003c/span\u003e i;\n}\n\n\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e() {\n \u003cspan class=\"pl-c\"\u003e// Create a function that adds 2.\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e add2 \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emakeAdder\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e2\u003c/span\u003e);\n\n \u003cspan class=\"pl-c\"\u003e// Create a function that adds 4.\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e add4 \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emakeAdder\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e4\u003c/span\u003e);\n\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eadd2\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e3\u003c/span\u003e) \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e5\u003c/span\u003e);\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003eadd4\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003e3\u003c/span\u003e) \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e7\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\"\u003eTear-offs\u003c/h2\u003e\u003ca id=\"user-content-tear-offs\" class=\"anchor\" aria-label=\"Permalink: Tear-offs\" href=\"#tear-offs\"\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\"\u003eWhen you refer to a function, method, or named constructor without parentheses,\nDart creates a \u003cem\u003etear-off\u003c/em\u003e. This is a closure that takes the same\nparameters as the function and invokes the underlying function when you call it.\nIf your code needs a closure that invokes a named function with the same\nparameters as the closure accepts, don't wrap the call in a lambda.\nUse a tear-off.\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"var charCodes = [68, 97, 114, 116];\nvar buffer = StringBuffer();\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e charCodes \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e [\u003cspan class=\"pl-c1\"\u003e68\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003e97\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003e114\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003e116\u003c/span\u003e];\n\u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e buffer \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eStringBuffer\u003c/span\u003e();\u003c/pre\u003e\u003c/div\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"// Function tear-off\ncharCodes.forEach(print);\n\n// Method tear-off\ncharCodes.forEach(buffer.write);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e// Function tear-off\u003c/span\u003e\ncharCodes.\u003cspan class=\"pl-en\"\u003eforEach\u003c/span\u003e(print);\n\n\u003cspan class=\"pl-c\"\u003e// Method tear-off\u003c/span\u003e\ncharCodes.\u003cspan class=\"pl-en\"\u003eforEach\u003c/span\u003e(buffer.write);\u003c/pre\u003e\u003c/div\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"// Function lambda\ncharCodes.forEach((code) {\n print(code);\n});\n\n// Method lambda\ncharCodes.forEach((code) {\n buffer.write(code);\n});\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e// Function lambda\u003c/span\u003e\ncharCodes.\u003cspan class=\"pl-en\"\u003eforEach\u003c/span\u003e((code) {\n \u003cspan class=\"pl-en\"\u003eprint\u003c/span\u003e(code);\n});\n\n\u003cspan class=\"pl-c\"\u003e// Method lambda\u003c/span\u003e\ncharCodes.\u003cspan class=\"pl-en\"\u003eforEach\u003c/span\u003e((code) {\n buffer.\u003cspan class=\"pl-en\"\u003ewrite\u003c/span\u003e(code);\n});\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTesting functions for equality\u003c/h2\u003e\u003ca id=\"user-content-testing-functions-for-equality\" class=\"anchor\" aria-label=\"Permalink: Testing functions for equality\" href=\"#testing-functions-for-equality\"\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\"\u003eHere's an example of testing top-level functions, static methods, and\ninstance methods for equality:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"void foo() {} // A top-level function\n\nclass A {\n static void bar() {} // A static method\n void baz() {} // An instance method\n}\n\nvoid main() {\n Function x;\n\n // Comparing top-level functions.\n x = foo;\n assert(foo == x);\n\n // Comparing static methods.\n x = A.bar;\n assert(A.bar == x);\n\n // Comparing instance methods.\n var v = A(); // Instance #1 of A\n var w = A(); // Instance #2 of A\n var y = w;\n x = w.baz;\n\n // These closures refer to the same instance (#2),\n // so they're equal.\n assert(y.baz == x);\n\n // These closures refer to different instances,\n // so they're unequal.\n assert(v.baz != w.baz);\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003efoo\u003c/span\u003e() {} \u003cspan class=\"pl-c\"\u003e// A top-level function\u003c/span\u003e\n\n\u003cspan class=\"pl-k\"\u003eclass\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eA\u003c/span\u003e {\n \u003cspan class=\"pl-k\"\u003estatic\u003c/span\u003e \u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ebar\u003c/span\u003e() {} \u003cspan class=\"pl-c\"\u003e// A static method\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003ebaz\u003c/span\u003e() {} \u003cspan class=\"pl-c\"\u003e// An instance method\u003c/span\u003e\n}\n\n\u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003emain\u003c/span\u003e() {\n \u003cspan class=\"pl-c1\"\u003eFunction\u003c/span\u003e x;\n\n \u003cspan class=\"pl-c\"\u003e// Comparing top-level functions.\u003c/span\u003e\n x \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e foo;\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(foo \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e x);\n\n \u003cspan class=\"pl-c\"\u003e// Comparing static methods.\u003c/span\u003e\n x \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eA\u003c/span\u003e.bar;\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eA\u003c/span\u003e.bar \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e x);\n\n \u003cspan class=\"pl-c\"\u003e// Comparing instance methods.\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e v \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eA\u003c/span\u003e(); \u003cspan class=\"pl-c\"\u003e// Instance #1 of A\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e w \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003eA\u003c/span\u003e(); \u003cspan class=\"pl-c\"\u003e// Instance #2 of A\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003evar\u003c/span\u003e y \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e w;\n x \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e w.baz;\n\n \u003cspan class=\"pl-c\"\u003e// These closures refer to the same instance (#2),\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// so they're equal.\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(y.baz \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e x);\n\n \u003cspan class=\"pl-c\"\u003e// These closures refer to different instances,\u003c/span\u003e\n \u003cspan class=\"pl-c\"\u003e// so they're unequal.\u003c/span\u003e\n \u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(v.baz \u003cspan class=\"pl-k\"\u003e!=\u003c/span\u003e w.baz);\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eReturn values\u003c/h2\u003e\u003ca id=\"user-content-return-values\" class=\"anchor\" aria-label=\"Permalink: Return values\" href=\"#return-values\"\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\"\u003eAll functions return a value. If no return value is specified, the\nstatement \u003ccode\u003ereturn null;\u003c/code\u003e is implicitly appended to the function body.\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"foo() {}\n\nassert(foo() == null);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-en\"\u003efoo\u003c/span\u003e() {}\n\n\u003cspan class=\"pl-k\"\u003eassert\u003c/span\u003e(\u003cspan class=\"pl-en\"\u003efoo\u003c/span\u003e() \u003cspan class=\"pl-k\"\u003e==\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003enull\u003c/span\u003e);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eTo return multiple values in a function, aggregate the values in a \u003ca href=\"/dart-lang/site-www/blob/main/language/records#multiple-returns\"\u003erecord\u003c/a\u003e.\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"(String, int) foo() {\n return ('something', 42);\n}\"\u003e\u003cpre\u003e(\u003cspan class=\"pl-c1\"\u003eString\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e) \u003cspan class=\"pl-en\"\u003efoo\u003c/span\u003e() {\n \u003cspan class=\"pl-k\"\u003ereturn\u003c/span\u003e (\u003cspan class=\"pl-s\"\u003e'something'\u003c/span\u003e, \u003cspan class=\"pl-c1\"\u003e42\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\"\u003eGenerators\u003c/h2\u003e\u003ca id=\"user-content-generators\" class=\"anchor\" aria-label=\"Permalink: Generators\" href=\"#generators\"\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\"\u003eWhen you need to lazily produce a sequence of values,\nconsider using a \u003cem\u003egenerator function\u003c/em\u003e.\nDart has built-in support for two kinds of generator functions:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003e\u003cstrong\u003eSynchronous\u003c/strong\u003e generator: Returns an \u003ca href=\"/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.dart-api%7D%7D/dart-core/Iterable-class.html\"\u003e\u003ccode\u003eIterable\u003c/code\u003e\u003c/a\u003e object.\u003c/li\u003e\n\u003cli\u003e\u003cstrong\u003eAsynchronous\u003c/strong\u003e generator: Returns a \u003ca href=\"/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.dart-api%7D%7D/dart-async/Stream-class.html\"\u003e\u003ccode\u003eStream\u003c/code\u003e\u003c/a\u003e object.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eTo implement a \u003cstrong\u003esynchronous\u003c/strong\u003e generator function,\nmark the function body as \u003ccode\u003esync*\u003c/code\u003e,\nand use \u003ccode\u003eyield\u003c/code\u003e statements to deliver values:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"Iterable\u0026lt;int\u0026gt; naturalsTo(int n) sync* {\n int k = 0;\n while (k \u0026lt; n) yield k++;\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003eIterable\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003enaturalsTo\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e n) \u003cspan class=\"pl-k\"\u003esync\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e*\u003c/span\u003e {\n \u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e k \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e;\n \u003cspan class=\"pl-k\"\u003ewhile\u003c/span\u003e (k \u003cspan class=\"pl-k\"\u003e\u0026lt;\u003c/span\u003e n) \u003cspan class=\"pl-k\"\u003eyield\u003c/span\u003e k\u003cspan class=\"pl-k\"\u003e++\u003c/span\u003e;\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eTo implement an \u003cstrong\u003easynchronous\u003c/strong\u003e generator function,\nmark the function body as \u003ccode\u003easync*\u003c/code\u003e,\nand use \u003ccode\u003eyield\u003c/code\u003e statements to deliver values:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"Stream\u0026lt;int\u0026gt; asynchronousNaturalsTo(int n) async* {\n int k = 0;\n while (k \u0026lt; n) yield k++;\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003eStream\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003easynchronousNaturalsTo\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e n) \u003cspan class=\"pl-k\"\u003easync\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e*\u003c/span\u003e {\n \u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e k \u003cspan class=\"pl-k\"\u003e=\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e;\n \u003cspan class=\"pl-k\"\u003ewhile\u003c/span\u003e (k \u003cspan class=\"pl-k\"\u003e\u0026lt;\u003c/span\u003e n) \u003cspan class=\"pl-k\"\u003eyield\u003c/span\u003e k\u003cspan class=\"pl-k\"\u003e++\u003c/span\u003e;\n}\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIf your generator is recursive,\nyou can improve its performance by using \u003ccode\u003eyield*\u003c/code\u003e:\u003c/p\u003e\n\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"Iterable\u0026lt;int\u0026gt; naturalsDownFrom(int n) sync* {\n if (n \u0026gt; 0) {\n yield n;\n yield* naturalsDownFrom(n - 1);\n }\n}\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c1\"\u003eIterable\u003c/span\u003e\u0026lt;\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e\u0026gt; \u003cspan class=\"pl-en\"\u003enaturalsDownFrom\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e n) \u003cspan class=\"pl-k\"\u003esync\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e*\u003c/span\u003e {\n \u003cspan class=\"pl-k\"\u003eif\u003c/span\u003e (n \u003cspan class=\"pl-k\"\u003e\u0026gt;\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e0\u003c/span\u003e) {\n \u003cspan class=\"pl-k\"\u003eyield\u003c/span\u003e n;\n \u003cspan class=\"pl-k\"\u003eyield\u003c/span\u003e\u003cspan class=\"pl-k\"\u003e*\u003c/span\u003e \u003cspan class=\"pl-en\"\u003enaturalsDownFrom\u003c/span\u003e(n \u003cspan class=\"pl-k\"\u003e-\u003c/span\u003e \u003cspan class=\"pl-c1\"\u003e1\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\"\u003eExternal functions {:#external}\u003c/h2\u003e\u003ca id=\"user-content-external-functions-external\" class=\"anchor\" aria-label=\"Permalink: External functions {:#external}\" href=\"#external-functions-external\"\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\"\u003eAn external function is a function whose body is implemented separately from its\ndeclaration. Include the \u003ccode\u003eexternal\u003c/code\u003e keyword before a function declaration, like so:\u003c/p\u003e\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"external void someFunc(int i);\"\u003e\u003cpre\u003e\u003cspan class=\"pl-k\"\u003eexternal\u003c/span\u003e \u003cspan class=\"pl-k\"\u003evoid\u003c/span\u003e \u003cspan class=\"pl-en\"\u003esomeFunc\u003c/span\u003e(\u003cspan class=\"pl-c1\"\u003eint\u003c/span\u003e i);\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAn external function's implementation can come from another Dart library,\nor, more commonly, from another language. In interop contexts, \u003ccode\u003eexternal\u003c/code\u003e\nintroduces type information for foreign functions or values,\nmaking them usable in Dart. Implementation and usage is\nheavily platform specific, so check out the interop docs on, for example,\n\u003ca href=\"/dart-lang/site-www/blob/main/interop/c-interop\"\u003eC\u003c/a\u003e or \u003ca href=\"/dart-lang/site-www/blob/main/interop/js-interop\"\u003eJavaScript\u003c/a\u003e to learn more.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eExternal functions can be top-level functions, \u003ca href=\"/dart-lang/site-www/blob/main/language/methods#instance-methods\"\u003einstance methods\u003c/a\u003e,\n\u003ca href=\"/dart-lang/site-www/blob/main/language/methods#getters-and-setters\"\u003egetters or setters\u003c/a\u003e, or \u003ca href=\"/dart-lang/site-www/blob/main/language/constructors#redirecting-constructors\"\u003enon-redirecting constructors\u003c/a\u003e.\nAn \u003ca href=\"/dart-lang/site-www/blob/main/language/classes#instance-variables\"\u003einstance variable\u003c/a\u003e can be \u003ccode\u003eexternal\u003c/code\u003e too,\nwhich is equivalent to an external getter and (if the variable\nis not \u003ccode\u003efinal\u003c/code\u003e) an external setter.\u003c/p\u003e\n\u003c/article\u003e","renderedFileInfo":null,"shortPath":null,"symbolsEnabled":true,"tabSize":8,"topBannersInfo":{"overridingGlobalFundingFile":false,"globalPreferredFundingPath":null,"showInvalidCitationWarning":false,"citationHelpUrl":"https://docs.github.com/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-citation-files","actionsOnboardingTip":null},"truncated":false,"viewable":true,"workflowRedirectUrl":null,"symbols":{"timed_out":false,"not_analyzed":false,"symbols":[{"name":"Parameters","kind":"section_2","ident_start":1965,"ident_end":1975,"extent_start":1962,"extent_end":6358,"fully_qualified_name":"Parameters","ident_utf16":{"start":{"line_number":62,"utf16_col":3},"end":{"line_number":62,"utf16_col":13}},"extent_utf16":{"start":{"line_number":62,"utf16_col":0},"end":{"line_number":214,"utf16_col":0}}},{"name":"Named 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0 16 16" width="16" height="16" fill="currentColor" display="inline-block" overflow="visible" style="vertical-align:text-bottom"><path d="M8 9a1.5 1.5 0 1 0 0-3 1.5 1.5 0 0 0 0 3ZM1.5 9a1.5 1.5 0 1 0 0-3 1.5 1.5 0 0 0 0 3Zm13 0a1.5 1.5 0 1 0 0-3 1.5 1.5 0 0 0 0 3Z"></path></svg></button></div></div></div></div><div></div></div><div class="Box-sc-g0xbh4-0 hGyMdv"><section aria-labelledby="file-name-id-wide file-name-id-mobile" class="Box-sc-g0xbh4-0 fGqKFv"><div class="Box-sc-g0xbh4-0 eoaCFS js-snippet-clipboard-copy-unpositioned undefined" data-hpc="true"><article class="markdown-body entry-content container-lg" itemprop="text"><markdown-accessiblity-table><table> <thead> <tr> <th>title</th> <th>description</th> <th>js</th> <th>prevpage</th> <th>nextpage</th> </tr> </thead> <tbody> <tr> <td><div dir="auto">Functions</div></td> <td><div dir="auto">Everything about functions in Dart.</div></td> <td><div dir="auto"><table> <tbody> <tr> <td><div dir="auto"><table> <thead> <tr> <th>url</th> <th>defer</th> </tr> </thead> <tbody> <tr> <td><div dir="auto">/assets/js/inject_dartpad.js</div></td> <td><div dir="auto">true</div></td> </tr> </tbody> </table> </div></td> </tr> </tbody> </table> </div></td> <td><div dir="auto"><table> <thead> <tr> <th>url</th> <th>title</th> </tr> </thead> <tbody> <tr> <td><div dir="auto">/language/pattern-types</div></td> <td><div dir="auto">Pattern types</div></td> </tr> </tbody> </table> </div></td> <td><div dir="auto"><table> <thead> <tr> <th>url</th> <th>title</th> </tr> </thead> <tbody> <tr> <td><div dir="auto">/language/loops</div></td> <td><div dir="auto">Loops</div></td> </tr> </tbody> </table> </div></td> </tr> </tbody> </table></markdown-accessiblity-table> <p dir="auto">Dart is a true object-oriented language, so even functions are objects and have a type, <a href="/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.dart-api%7D%7D/dart-core/Function-class.html">Function.</a> This means that functions can be assigned to variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function. For details, see <a href="/dart-lang/site-www/blob/main/language/callable-objects">Callable objects</a>.</p> <p dir="auto">Here's an example of implementing a function:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="bool isNoble(int atomicNumber) { return _nobleGases[atomicNumber] != null; }"><pre><span class="pl-c1">bool</span> <span class="pl-en">isNoble</span>(<span class="pl-c1">int</span> atomicNumber) { <span class="pl-k">return</span> _nobleGases[atomicNumber] <span class="pl-k">!=</span> <span class="pl-c1">null</span>; }</pre></div> <p dir="auto">Although Effective Dart recommends <a href="/dart-lang/site-www/blob/main/effective-dart/design#do-type-annotate-fields-and-top-level-variables-if-the-type-isnt-obvious">type annotations for public APIs</a>, the function still works if you omit the types:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="isNoble(atomicNumber) { return _nobleGases[atomicNumber] != null; }"><pre><span class="pl-en">isNoble</span>(atomicNumber) { <span class="pl-k">return</span> _nobleGases[atomicNumber] <span class="pl-k">!=</span> <span class="pl-c1">null</span>; }</pre></div> <p dir="auto">For functions that contain just one expression, you can use a shorthand syntax:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;"><pre><span class="pl-c1">bool</span> <span class="pl-en">isNoble</span>(<span class="pl-c1">int</span> atomicNumber) <span class="pl-k">=></span> _nobleGases[atomicNumber] <span class="pl-k">!=</span> <span class="pl-c1">null</span>;</pre></div> <p dir="auto">The <code>=> <em>expr</em></code> syntax is a shorthand for <code>{ return <em>expr</em>; }</code>. The <code>=></code> notation is sometimes referred to as <em>arrow</em> syntax.</p> <p dir="auto">:::note Only <em>expressions</em> can appear between the arrow (<code>=></code>) and the semicolon (<code>;</code>). Expressions evaluate to values. This means that you can't write a statement where Dart expects a value. For example, you could use a <a href="/dart-lang/site-www/blob/main/language/operators#conditional-expressions">conditional expression</a> but not an <a href="/dart-lang/site-www/blob/main/language/branches#if">if statement</a>. In the previous example, <code>_nobleGases[atomicNumber] != null;</code> returns a boolean value. The function then returns a boolean value that indicates whether the <code>atomicNumber</code> falls into the noble gas range. :::</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Parameters</h2><a id="user-content-parameters" class="anchor" aria-label="Permalink: Parameters" href="#parameters"><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 function can have any number of <em>required positional</em> parameters. These can be followed either by <em>named</em> parameters or by <em>optional positional</em> parameters (but not both).</p> <p dir="auto">:::note Some APIs—notably <a href="/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.flutter%7D%7D">Flutter</a> widget constructors—use only named parameters, even for parameters that are mandatory. See the next section for details. :::</p> <p dir="auto">You can use <a href="/dart-lang/site-www/blob/main/language/collections#lists">trailing commas</a> when you pass arguments to a function or when you define function parameters.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Named parameters</h3><a id="user-content-named-parameters" class="anchor" aria-label="Permalink: Named parameters" href="#named-parameters"><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">Named parameters are optional unless they're explicitly marked as <code>required</code>.</p> <p dir="auto">When defining a function, use <code>{<em>param1</em>, <em>param2</em>, …}</code> to specify named parameters. If you don't provide a default value or mark a named parameter as <code>required</code>, their types must be nullable as their default value will be <code>null</code>:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="/// Sets the [bold] and [hidden] flags ... void enableFlags({bool? bold, bool? hidden}) { ... }"><pre><span class="pl-c">/// Sets the <span class="pl-v">[bold]</span> and <span class="pl-v">[hidden]</span> flags ...</span> <span class="pl-c"></span><span class="pl-k">void</span> <span class="pl-en">enableFlags</span>({<span class="pl-c1">bool</span><span class="pl-k">?</span> bold, <span class="pl-c1">bool</span><span class="pl-k">?</span> hidden}) { ... }</pre></div> <p dir="auto">When calling a function, you can specify named arguments using <code><em>paramName</em>: <em>value</em></code>. For example:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="enableFlags(bold: true, hidden: false);"><pre><span class="pl-en">enableFlags</span>(bold<span class="pl-k">:</span> <span class="pl-c1">true</span>, hidden<span class="pl-k">:</span> <span class="pl-c1">false</span>);</pre></div> <p dir="auto"><a id="user-content-default-parameters"></a> To define a default value for a named parameter besides <code>null</code>, use <code>=</code> to specify a default value. The specified value must be a compile-time constant. For example:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="/// Sets the [bold] and [hidden] flags ... void enableFlags({bool bold = false, bool hidden = false}) { ... } // bold will be true; hidden will be false. enableFlags(bold: true);"><pre><span class="pl-c">/// Sets the <span class="pl-v">[bold]</span> and <span class="pl-v">[hidden]</span> flags ...</span> <span class="pl-c"></span><span class="pl-k">void</span> <span class="pl-en">enableFlags</span>({<span class="pl-c1">bool</span> bold <span class="pl-k">=</span> <span class="pl-c1">false</span>, <span class="pl-c1">bool</span> hidden <span class="pl-k">=</span> <span class="pl-c1">false</span>}) { ... } <span class="pl-c">// bold will be true; hidden will be false.</span> <span class="pl-en">enableFlags</span>(bold<span class="pl-k">:</span> <span class="pl-c1">true</span>);</pre></div> <p dir="auto">If you instead want a named parameter to be mandatory, requiring callers to provide a value for the parameter, annotate them with <code>required</code>:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="const Scrollbar({super.key, [!required!] Widget child});"><pre><span class="pl-k">const</span> <span class="pl-c1">Scrollbar</span>({<span class="pl-c1">super</span>.key, [<span class="pl-k">!</span><span class="pl-k">required</span><span class="pl-k">!</span>] <span class="pl-c1">Widget</span> child});</pre></div> <p dir="auto">If someone tries to create a <code>Scrollbar</code> without specifying the <code>child</code> argument, then the analyzer reports an issue.</p> <p dir="auto">:::note A parameter marked as <code>required</code> can still be nullable:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="const Scrollbar({super.key, required [!Widget?!] child});"><pre><span class="pl-k">const</span> <span class="pl-c1">Scrollbar</span>({<span class="pl-c1">super</span>.key, <span class="pl-k">required</span> [<span class="pl-k">!</span><span class="pl-c1">Widget</span><span class="pl-k">?</span><span class="pl-k">!</span>] child});</pre></div> <p dir="auto">:::</p> <p dir="auto">You might want to place positional arguments first, but Dart doesn't require it. Dart allows named arguments to be placed anywhere in the argument list when it suits your API:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="repeat(times: 2, () { ... });"><pre><span class="pl-en">repeat</span>(times<span class="pl-k">:</span> <span class="pl-c1">2</span>, () { ... });</pre></div> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Optional positional parameters</h3><a id="user-content-optional-positional-parameters" class="anchor" aria-label="Permalink: Optional positional parameters" href="#optional-positional-parameters"><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">Wrapping a set of function parameters in <code>[]</code> marks them as optional positional parameters. If you don't provide a default value, their types must be nullable as their default value will be <code>null</code>:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="String say(String from, String msg, [String? device]) { var result = '$from says $msg'; if (device != null) { result = '$result with a $device'; } return result; }"><pre><span class="pl-c1">String</span> <span class="pl-en">say</span>(<span class="pl-c1">String</span> from, <span class="pl-c1">String</span> msg, [<span class="pl-c1">String</span><span class="pl-k">?</span> device]) { <span class="pl-k">var</span> result <span class="pl-k">=</span> <span class="pl-s">'$<span class="pl-v">from</span> says $<span class="pl-v">msg</span>'</span>; <span class="pl-k">if</span> (device <span class="pl-k">!=</span> <span class="pl-c1">null</span>) { result <span class="pl-k">=</span> <span class="pl-s">'$<span class="pl-v">result</span> with a $<span class="pl-v">device</span>'</span>; } <span class="pl-k">return</span> result; }</pre></div> <p dir="auto">Here's an example of calling this function without the optional parameter:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="assert(say('Bob', 'Howdy') == 'Bob says Howdy');"><pre><span class="pl-k">assert</span>(<span class="pl-en">say</span>(<span class="pl-s">'Bob'</span>, <span class="pl-s">'Howdy'</span>) <span class="pl-k">==</span> <span class="pl-s">'Bob says Howdy'</span>);</pre></div> <p dir="auto">And here's an example of calling this function with the third parameter:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="assert( say('Bob', 'Howdy', 'smoke signal') == 'Bob says Howdy with a smoke signal', );"><pre><span class="pl-k">assert</span>( <span class="pl-en">say</span>(<span class="pl-s">'Bob'</span>, <span class="pl-s">'Howdy'</span>, <span class="pl-s">'smoke signal'</span>) <span class="pl-k">==</span> <span class="pl-s">'Bob says Howdy with a smoke signal'</span>, );</pre></div> <p dir="auto">To define a default value for an optional positional parameter besides <code>null</code>, use <code>=</code> to specify a default value. The specified value must be a compile-time constant. For example:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="String say(String from, String msg, [String device = 'carrier pigeon']) { var result = '$from says $msg with a $device'; return result; } assert(say('Bob', 'Howdy') == 'Bob says Howdy with a carrier pigeon');"><pre><span class="pl-c1">String</span> <span class="pl-en">say</span>(<span class="pl-c1">String</span> from, <span class="pl-c1">String</span> msg, [<span class="pl-c1">String</span> device <span class="pl-k">=</span> <span class="pl-s">'carrier pigeon'</span>]) { <span class="pl-k">var</span> result <span class="pl-k">=</span> <span class="pl-s">'$<span class="pl-v">from</span> says $<span class="pl-v">msg</span> with a $<span class="pl-v">device</span>'</span>; <span class="pl-k">return</span> result; } <span class="pl-k">assert</span>(<span class="pl-en">say</span>(<span class="pl-s">'Bob'</span>, <span class="pl-s">'Howdy'</span>) <span class="pl-k">==</span> <span class="pl-s">'Bob says Howdy with a carrier pigeon'</span>);</pre></div> <p dir="auto"><a id="user-content-the-main-function"></a></p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">The main() function {:#main}</h2><a id="user-content-the-main-function-main" class="anchor" aria-label="Permalink: The main() function {:#main}" href="#the-main-function-main"><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">Every app must have a top-level <code>main()</code> function, which serves as the entrypoint to the app. The <code>main()</code> function returns <code>void</code> and has an optional <code>List<String></code> parameter for arguments.</p> <p dir="auto">Here's a simple <code>main()</code> function:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="void main() { print('Hello, World!'); }"><pre><span class="pl-k">void</span> <span class="pl-en">main</span>() { <span class="pl-en">print</span>(<span class="pl-s">'Hello, World!'</span>); }</pre></div> <p dir="auto">Here's an example of the <code>main()</code> function for a command-line app that takes arguments:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="// Run the app like this: dart run args.dart 1 test void main(List<String> arguments) { print(arguments); assert(arguments.length == 2); assert(int.parse(arguments[0]) == 1); assert(arguments[1] == 'test'); }"><pre><span class="pl-c">// Run the app like this: dart run args.dart 1 test</span> <span class="pl-k">void</span> <span class="pl-en">main</span>(<span class="pl-c1">List</span><<span class="pl-c1">String</span>> arguments) { <span class="pl-en">print</span>(arguments); <span class="pl-k">assert</span>(arguments.length <span class="pl-k">==</span> <span class="pl-c1">2</span>); <span class="pl-k">assert</span>(<span class="pl-c1">int</span>.<span class="pl-en">parse</span>(arguments[<span class="pl-c1">0</span>]) <span class="pl-k">==</span> <span class="pl-c1">1</span>); <span class="pl-k">assert</span>(arguments[<span class="pl-c1">1</span>] <span class="pl-k">==</span> <span class="pl-s">'test'</span>); }</pre></div> <p dir="auto">You can use the <a href="/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.pub-pkg%7D%7D/args">args library</a> to define and parse command-line arguments.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Functions as first-class objects</h2><a id="user-content-functions-as-first-class-objects" class="anchor" aria-label="Permalink: Functions as first-class objects" href="#functions-as-first-class-objects"><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">You can pass a function as a parameter to another function. For example:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="void printElement(int element) { print(element); } var list = [1, 2, 3]; // Pass printElement as a parameter. list.forEach(printElement);"><pre><span class="pl-k">void</span> <span class="pl-en">printElement</span>(<span class="pl-c1">int</span> element) { <span class="pl-en">print</span>(element); } <span class="pl-k">var</span> list <span class="pl-k">=</span> [<span class="pl-c1">1</span>, <span class="pl-c1">2</span>, <span class="pl-c1">3</span>]; <span class="pl-c">// Pass printElement as a parameter.</span> list.<span class="pl-en">forEach</span>(printElement);</pre></div> <p dir="auto">You can also assign a function to a variable, such as:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!'; assert(loudify('hello') == '!!! HELLO !!!');"><pre><span class="pl-k">var</span> loudify <span class="pl-k">=</span> (msg) <span class="pl-k">=></span> <span class="pl-s">'!!! ${<span class="pl-v">msg</span>.<span class="pl-en">toUpperCase</span>()} !!!'</span>; <span class="pl-k">assert</span>(<span class="pl-en">loudify</span>(<span class="pl-s">'hello'</span>) <span class="pl-k">==</span> <span class="pl-s">'!!! HELLO !!!'</span>);</pre></div> <p dir="auto">This example uses an anonymous function. More about those in the next section.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Function types</h2><a id="user-content-function-types" class="anchor" aria-label="Permalink: Function types" href="#function-types"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">You can specify the type of a function, which is known as a <em>function type</em>. A function type is obtained from a function declaration header by replacing the function name by the keyword <code>Function</code>. Moreover, you are allowed to omit the names of positional parameters, but the names of named parameters can't be omitted. For example:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="void greet(String name, {String greeting = 'Hello'}) => print('$greeting $name!'); // Store `greet` in a variable and call it. void Function(String, {String greeting}) g = greet; g('Dash', greeting: 'Howdy');"><pre><span class="pl-k">void</span> <span class="pl-en">greet</span>(<span class="pl-c1">String</span> name, {<span class="pl-c1">String</span> greeting <span class="pl-k">=</span> <span class="pl-s">'Hello'</span>}) <span class="pl-k">=></span> <span class="pl-en">print</span>(<span class="pl-s">'$<span class="pl-v">greeting</span> $<span class="pl-v">name</span>!'</span>); <span class="pl-c">// Store `greet` in a variable and call it.</span> <span class="pl-k">void</span> <span class="pl-c1">Function</span>(<span class="pl-c1">String</span>, {<span class="pl-c1">String</span> greeting}) g <span class="pl-k">=</span> greet; <span class="pl-en">g</span>(<span class="pl-s">'Dash'</span>, greeting<span class="pl-k">:</span> <span class="pl-s">'Howdy'</span>);</pre></div> <p dir="auto">:::note In Dart, functions are first-class objects, meaning they can be assigned to variables, passed as arguments, and returned from other functions.</p> <p dir="auto">You can use a <a href="/dart-lang/site-www/blob/main/language/typedefs"><code>typedef</code></a> declaration to explicitly name function types, which can be useful for clarity and reusability. :::</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Anonymous functions</h2><a id="user-content-anonymous-functions" class="anchor" aria-label="Permalink: Anonymous functions" href="#anonymous-functions"><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">Though you name most functions, such as <code>main()</code> or <code>printElement()</code>. you can also create functions without names. These functions are called <em>anonymous functions</em>, <em>lambdas</em>, or <em>closures</em>.</p> <p dir="auto">An anonymous function resembles a named function as it has:</p> <ul dir="auto"> <li>Zero or more parameters, comma-separated</li> <li>Optional type annotations between parentheses.</li> </ul> <p dir="auto">The following code block contains the function's body:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="([[Type] param1[, ...]]) { codeBlock; }"><pre>([[<span class="pl-c1">Type</span>] param1[, ...]]) { codeBlock; }</pre></div> <p dir="auto">The following example defines an anonymous function with an untyped parameter, <code>item</code>. The anonymous function passes it to the <code>map</code> function. The <code>map</code> function, invoked for each item in the list, converts each string to uppercase. Then, the anonymous function passed to <code>forEach</code>, prints each converted string with its length.</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="const list = ['apples', 'bananas', 'oranges']; var uppercaseList = list.map((item) { return item.toUpperCase(); }).toList(); // Convert to list after mapping for (var item in uppercaseList) { print('$item: ${item.length}'); }"><pre><span class="pl-k">const</span> list <span class="pl-k">=</span> [<span class="pl-s">'apples'</span>, <span class="pl-s">'bananas'</span>, <span class="pl-s">'oranges'</span>]; <span class="pl-k">var</span> uppercaseList <span class="pl-k">=</span> list.<span class="pl-en">map</span>((item) { <span class="pl-k">return</span> item.<span class="pl-en">toUpperCase</span>(); }).<span class="pl-en">toList</span>(); <span class="pl-c">// Convert to list after mapping</span> <span class="pl-k">for</span> (<span class="pl-k">var</span> item <span class="pl-k">in</span> uppercaseList) { <span class="pl-en">print</span>(<span class="pl-s">'$<span class="pl-v">item</span>: ${<span class="pl-v">item</span>.<span class="pl-v">length</span>}'</span>); }</pre></div> <p dir="auto">Click <strong>Run</strong> to execute the code.</p> <div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="void main() { const list = ['apples', 'bananas', 'oranges']; var uppercaseList = list.map((item) { return item.toUpperCase(); }).toList(); // Convert to list after mapping for (var item in uppercaseList) { print('$item: ${item.length}'); } }"><pre lang="dartpad" class="notranslate"><code>void main() { const list = ['apples', 'bananas', 'oranges']; var uppercaseList = list.map((item) { return item.toUpperCase(); }).toList(); // Convert to list after mapping for (var item in uppercaseList) { print('$item: ${item.length}'); } } </code></pre></div> <p dir="auto">If the function contains only a single expression or return statement, you can shorten it using arrow notation. Paste the following line into DartPad and click <strong>Run</strong> to verify that it is functionally equivalent.</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="var uppercaseList = list.map((item) => item.toUpperCase()).toList(); uppercaseList.forEach((item) => print('$item: ${item.length}'));"><pre><span class="pl-k">var</span> uppercaseList <span class="pl-k">=</span> list.<span class="pl-en">map</span>((item) <span class="pl-k">=></span> item.<span class="pl-en">toUpperCase</span>()).<span class="pl-en">toList</span>(); uppercaseList.<span class="pl-en">forEach</span>((item) <span class="pl-k">=></span> <span class="pl-en">print</span>(<span class="pl-s">'$<span class="pl-v">item</span>: ${<span class="pl-v">item</span>.<span class="pl-v">length</span>}'</span>));</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Lexical scope</h2><a id="user-content-lexical-scope" class="anchor" aria-label="Permalink: Lexical scope" href="#lexical-scope"><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">Dart determines the scope of variables based on the layout of its code. A programming language with this feature is termed a lexically scoped language. You can "follow the curly braces outwards" to see if a variable is in scope.</p> <p dir="auto"><strong>Example:</strong> A series of nested functions with variables at each scope level:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="bool topLevel = true; void main() { var insideMain = true; void myFunction() { var insideFunction = true; void nestedFunction() { var insideNestedFunction = true; assert(topLevel); assert(insideMain); assert(insideFunction); assert(insideNestedFunction); } } }"><pre><span class="pl-c1">bool</span> topLevel <span class="pl-k">=</span> <span class="pl-c1">true</span>; <span class="pl-k">void</span> <span class="pl-en">main</span>() { <span class="pl-k">var</span> insideMain <span class="pl-k">=</span> <span class="pl-c1">true</span>; <span class="pl-k">void</span> <span class="pl-en">myFunction</span>() { <span class="pl-k">var</span> insideFunction <span class="pl-k">=</span> <span class="pl-c1">true</span>; <span class="pl-k">void</span> <span class="pl-en">nestedFunction</span>() { <span class="pl-k">var</span> insideNestedFunction <span class="pl-k">=</span> <span class="pl-c1">true</span>; <span class="pl-k">assert</span>(topLevel); <span class="pl-k">assert</span>(insideMain); <span class="pl-k">assert</span>(insideFunction); <span class="pl-k">assert</span>(insideNestedFunction); } } }</pre></div> <p dir="auto">The <code>nestedFunction()</code> method can use variables from every level, all the way up to the top level.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Lexical closures</h2><a id="user-content-lexical-closures" class="anchor" aria-label="Permalink: Lexical closures" href="#lexical-closures"><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 function object that can access variables in its lexical scope when the function sits outside that scope is called a <em>closure</em>.</p> <p dir="auto">Functions can close over variables defined in surrounding scopes. In the following example, <code>makeAdder()</code> captures the variable <code>addBy</code>. Wherever the returned function goes, it remembers <code>addBy</code>.</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="/// Returns a function that adds [addBy] to the /// function's argument. Function makeAdder(int addBy) { return (int i) => addBy + i; } void main() { // Create a function that adds 2. var add2 = makeAdder(2); // Create a function that adds 4. var add4 = makeAdder(4); assert(add2(3) == 5); assert(add4(3) == 7); }"><pre><span class="pl-c">/// Returns a function that adds <span class="pl-v">[addBy]</span> to the</span> <span class="pl-c">/// function's argument.</span> <span class="pl-c"></span><span class="pl-c1">Function</span> <span class="pl-en">makeAdder</span>(<span class="pl-c1">int</span> addBy) { <span class="pl-k">return</span> (<span class="pl-c1">int</span> i) <span class="pl-k">=></span> addBy <span class="pl-k">+</span> i; } <span class="pl-k">void</span> <span class="pl-en">main</span>() { <span class="pl-c">// Create a function that adds 2.</span> <span class="pl-k">var</span> add2 <span class="pl-k">=</span> <span class="pl-en">makeAdder</span>(<span class="pl-c1">2</span>); <span class="pl-c">// Create a function that adds 4.</span> <span class="pl-k">var</span> add4 <span class="pl-k">=</span> <span class="pl-en">makeAdder</span>(<span class="pl-c1">4</span>); <span class="pl-k">assert</span>(<span class="pl-en">add2</span>(<span class="pl-c1">3</span>) <span class="pl-k">==</span> <span class="pl-c1">5</span>); <span class="pl-k">assert</span>(<span class="pl-en">add4</span>(<span class="pl-c1">3</span>) <span class="pl-k">==</span> <span class="pl-c1">7</span>); }</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Tear-offs</h2><a id="user-content-tear-offs" class="anchor" aria-label="Permalink: Tear-offs" href="#tear-offs"><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">When you refer to a function, method, or named constructor without parentheses, Dart creates a <em>tear-off</em>. This is a closure that takes the same parameters as the function and invokes the underlying function when you call it. If your code needs a closure that invokes a named function with the same parameters as the closure accepts, don't wrap the call in a lambda. Use a tear-off.</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="var charCodes = [68, 97, 114, 116]; var buffer = StringBuffer();"><pre><span class="pl-k">var</span> charCodes <span class="pl-k">=</span> [<span class="pl-c1">68</span>, <span class="pl-c1">97</span>, <span class="pl-c1">114</span>, <span class="pl-c1">116</span>]; <span class="pl-k">var</span> buffer <span class="pl-k">=</span> <span class="pl-c1">StringBuffer</span>();</pre></div> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="// Function tear-off charCodes.forEach(print); // Method tear-off charCodes.forEach(buffer.write);"><pre><span class="pl-c">// Function tear-off</span> charCodes.<span class="pl-en">forEach</span>(print); <span class="pl-c">// Method tear-off</span> charCodes.<span class="pl-en">forEach</span>(buffer.write);</pre></div> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="// Function lambda charCodes.forEach((code) { print(code); }); // Method lambda charCodes.forEach((code) { buffer.write(code); });"><pre><span class="pl-c">// Function lambda</span> charCodes.<span class="pl-en">forEach</span>((code) { <span class="pl-en">print</span>(code); }); <span class="pl-c">// Method lambda</span> charCodes.<span class="pl-en">forEach</span>((code) { buffer.<span class="pl-en">write</span>(code); });</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Testing functions for equality</h2><a id="user-content-testing-functions-for-equality" class="anchor" aria-label="Permalink: Testing functions for equality" href="#testing-functions-for-equality"><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">Here's an example of testing top-level functions, static methods, and instance methods for equality:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="void foo() {} // A top-level function class A { static void bar() {} // A static method void baz() {} // An instance method } void main() { Function x; // Comparing top-level functions. x = foo; assert(foo == x); // Comparing static methods. x = A.bar; assert(A.bar == x); // Comparing instance methods. var v = A(); // Instance #1 of A var w = A(); // Instance #2 of A var y = w; x = w.baz; // These closures refer to the same instance (#2), // so they're equal. assert(y.baz == x); // These closures refer to different instances, // so they're unequal. assert(v.baz != w.baz); }"><pre><span class="pl-k">void</span> <span class="pl-en">foo</span>() {} <span class="pl-c">// A top-level function</span> <span class="pl-k">class</span> <span class="pl-c1">A</span> { <span class="pl-k">static</span> <span class="pl-k">void</span> <span class="pl-en">bar</span>() {} <span class="pl-c">// A static method</span> <span class="pl-k">void</span> <span class="pl-en">baz</span>() {} <span class="pl-c">// An instance method</span> } <span class="pl-k">void</span> <span class="pl-en">main</span>() { <span class="pl-c1">Function</span> x; <span class="pl-c">// Comparing top-level functions.</span> x <span class="pl-k">=</span> foo; <span class="pl-k">assert</span>(foo <span class="pl-k">==</span> x); <span class="pl-c">// Comparing static methods.</span> x <span class="pl-k">=</span> <span class="pl-c1">A</span>.bar; <span class="pl-k">assert</span>(<span class="pl-c1">A</span>.bar <span class="pl-k">==</span> x); <span class="pl-c">// Comparing instance methods.</span> <span class="pl-k">var</span> v <span class="pl-k">=</span> <span class="pl-c1">A</span>(); <span class="pl-c">// Instance #1 of A</span> <span class="pl-k">var</span> w <span class="pl-k">=</span> <span class="pl-c1">A</span>(); <span class="pl-c">// Instance #2 of A</span> <span class="pl-k">var</span> y <span class="pl-k">=</span> w; x <span class="pl-k">=</span> w.baz; <span class="pl-c">// These closures refer to the same instance (#2),</span> <span class="pl-c">// so they're equal.</span> <span class="pl-k">assert</span>(y.baz <span class="pl-k">==</span> x); <span class="pl-c">// These closures refer to different instances,</span> <span class="pl-c">// so they're unequal.</span> <span class="pl-k">assert</span>(v.baz <span class="pl-k">!=</span> w.baz); }</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Return values</h2><a id="user-content-return-values" class="anchor" aria-label="Permalink: Return values" href="#return-values"><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">All functions return a value. If no return value is specified, the statement <code>return null;</code> is implicitly appended to the function body.</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="foo() {} assert(foo() == null);"><pre><span class="pl-en">foo</span>() {} <span class="pl-k">assert</span>(<span class="pl-en">foo</span>() <span class="pl-k">==</span> <span class="pl-c1">null</span>);</pre></div> <p dir="auto">To return multiple values in a function, aggregate the values in a <a href="/dart-lang/site-www/blob/main/language/records#multiple-returns">record</a>.</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="(String, int) foo() { return ('something', 42); }"><pre>(<span class="pl-c1">String</span>, <span class="pl-c1">int</span>) <span class="pl-en">foo</span>() { <span class="pl-k">return</span> (<span class="pl-s">'something'</span>, <span class="pl-c1">42</span>); }</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Generators</h2><a id="user-content-generators" class="anchor" aria-label="Permalink: Generators" href="#generators"><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">When you need to lazily produce a sequence of values, consider using a <em>generator function</em>. Dart has built-in support for two kinds of generator functions:</p> <ul dir="auto"> <li><strong>Synchronous</strong> generator: Returns an <a href="/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.dart-api%7D%7D/dart-core/Iterable-class.html"><code>Iterable</code></a> object.</li> <li><strong>Asynchronous</strong> generator: Returns a <a href="/dart-lang/site-www/blob/main/src/content/language/%7B%7Bsite.dart-api%7D%7D/dart-async/Stream-class.html"><code>Stream</code></a> object.</li> </ul> <p dir="auto">To implement a <strong>synchronous</strong> generator function, mark the function body as <code>sync*</code>, and use <code>yield</code> statements to deliver values:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="Iterable<int> naturalsTo(int n) sync* { int k = 0; while (k < n) yield k++; }"><pre><span class="pl-c1">Iterable</span><<span class="pl-c1">int</span>> <span class="pl-en">naturalsTo</span>(<span class="pl-c1">int</span> n) <span class="pl-k">sync</span><span class="pl-k">*</span> { <span class="pl-c1">int</span> k <span class="pl-k">=</span> <span class="pl-c1">0</span>; <span class="pl-k">while</span> (k <span class="pl-k"><</span> n) <span class="pl-k">yield</span> k<span class="pl-k">++</span>; }</pre></div> <p dir="auto">To implement an <strong>asynchronous</strong> generator function, mark the function body as <code>async*</code>, and use <code>yield</code> statements to deliver values:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="Stream<int> asynchronousNaturalsTo(int n) async* { int k = 0; while (k < n) yield k++; }"><pre><span class="pl-c1">Stream</span><<span class="pl-c1">int</span>> <span class="pl-en">asynchronousNaturalsTo</span>(<span class="pl-c1">int</span> n) <span class="pl-k">async</span><span class="pl-k">*</span> { <span class="pl-c1">int</span> k <span class="pl-k">=</span> <span class="pl-c1">0</span>; <span class="pl-k">while</span> (k <span class="pl-k"><</span> n) <span class="pl-k">yield</span> k<span class="pl-k">++</span>; }</pre></div> <p dir="auto">If your generator is recursive, you can improve its performance by using <code>yield*</code>:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="Iterable<int> naturalsDownFrom(int n) sync* { if (n > 0) { yield n; yield* naturalsDownFrom(n - 1); } }"><pre><span class="pl-c1">Iterable</span><<span class="pl-c1">int</span>> <span class="pl-en">naturalsDownFrom</span>(<span class="pl-c1">int</span> n) <span class="pl-k">sync</span><span class="pl-k">*</span> { <span class="pl-k">if</span> (n <span class="pl-k">></span> <span class="pl-c1">0</span>) { <span class="pl-k">yield</span> n; <span class="pl-k">yield</span><span class="pl-k">*</span> <span class="pl-en">naturalsDownFrom</span>(n <span class="pl-k">-</span> <span class="pl-c1">1</span>); } }</pre></div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">External functions {:#external}</h2><a id="user-content-external-functions-external" class="anchor" aria-label="Permalink: External functions {:#external}" href="#external-functions-external"><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">An external function is a function whose body is implemented separately from its declaration. Include the <code>external</code> keyword before a function declaration, like so:</p> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="external void someFunc(int i);"><pre><span class="pl-k">external</span> <span class="pl-k">void</span> <span class="pl-en">someFunc</span>(<span class="pl-c1">int</span> i);</pre></div> <p dir="auto">An external function's implementation can come from another Dart library, or, more commonly, from another language. In interop contexts, <code>external</code> introduces type information for foreign functions or values, making them usable in Dart. Implementation and usage is heavily platform specific, so check out the interop docs on, for example, <a href="/dart-lang/site-www/blob/main/interop/c-interop">C</a> or <a href="/dart-lang/site-www/blob/main/interop/js-interop">JavaScript</a> to learn more.</p> <p dir="auto">External functions can be top-level functions, <a href="/dart-lang/site-www/blob/main/language/methods#instance-methods">instance methods</a>, <a href="/dart-lang/site-www/blob/main/language/methods#getters-and-setters">getters or setters</a>, or <a href="/dart-lang/site-www/blob/main/language/constructors#redirecting-constructors">non-redirecting constructors</a>. An <a href="/dart-lang/site-www/blob/main/language/classes#instance-variables">instance variable</a> can be <code>external</code> too, which is equivalent to an external getter and (if the variable is not <code>final</code>) an external setter.</p> </article></div><button hidden=""></button></section></div></div></div> <!-- --> <!-- --> </div></div></div><div class="Box-sc-g0xbh4-0"></div></div></div></div></div><div id="find-result-marks-container" class="Box-sc-g0xbh4-0 cCoXib"></div><button hidden="" data-testid="" data-hotkey-scope="read-only-cursor-text-area"></button><button hidden=""></button></div> <!-- --> <!-- --> <script type="application/json" id="__PRIMER_DATA_:R0:__">{"resolvedServerColorMode":"day"}</script></div> </react-app> </turbo-frame> </div> </turbo-frame> </main> </div> </div> <footer class="footer pt-8 pb-6 f6 color-fg-muted p-responsive" role="contentinfo" > <h2 class='sr-only'>Footer</h2> <div class="d-flex flex-justify-center flex-items-center 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