docs/language/informal/generic-function-type-alias.md - Issue 2841483003: Added informal generic method syntax and generic function type specs.

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Issue 2841483003: Added informal generic method syntax and generic function type specs. (Closed)

Patch Set: Created 3 years, 8 months ago

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Index: docs/language/informal/generic-function-type-alias.md

diff --git a/docs/language/informal/generic-function-type-alias.md b/docs/language/informal/generic-function-type-alias.md

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Lasse Reichstein Nielsen 2017/04/24 13:02:05 The document has extension .md, but contains HTML.

eernst 2017/04/25 11:57:15 Fixed.

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+ INFORMAL SPECIFICATION: Generic Function Type Alias

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+ <div id="readme" class="readme blob instapaper_body">

+ <article class="markdown-body entry-content" itemprop="text"><h1><a id="user-content-feature-generic-function-type-alias" class="anchor" href="#feature-generic-function-type-alias" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Feature: Generic Function Type Alias</h1>

+<p><strong>Status</strong>: Under implementation.</p>

+<p><strong>This document</strong> is an informal specification of a feature supporting the

+definition of function type aliases using a more expressive syntax than the

+one available today, such that it also covers generic function types. The

+feature also introduces syntax for specifying function types directly, such

+that they can be used in type annotations etc. without going via a

+<code>typedef</code>. This feature is being introduced into Dart starting Q4, 2016.</p>

+<p><strong>This feature</strong> introduces a new syntactic form of typedef declaration

+which includes an identifier and a type, connecting the two with an equals

+sign, <code>=</code>. The effect of such a declaration is that the name is declared to

+be an alias for the type. Type parameterization may occur in the

+declaration itself, as well as in the declared type. This feature also

+introduces syntax for specifying function types directly, using a syntax

+which is similar to the header of a function declaration.</p>

+<p>The <strong>motivation</strong> for adding this feature is that it allows developers to

+specify generic function types everywhere a type is expected, including

+type annotations, return types, actual type arguments, and formal type

+parameter bounds. Currently there is no way to specify a generic function

+type in these situations. Even in the case where a generic function type

+<em>can</em> be specified (such as a type annotation for a formal parameter) it

+may be useful for readability to declare a name as an alias of a complex

+type, and use that name instead of the type.</p>

+<h2><a id="user-content-examples" class="anchor" href="#examples" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Examples</h2>

+<p>Using the new syntax, a function type alias may be declared as follows:</p>

+<div class="highlight highlight-source-dart"><pre><span class="pl-k">typedef</span> F = List<T> Function<T>(T);</pre></div>

+<p>This declares <code>F</code> to be the type of a function that accepts one type

+parameter <code>T</code> and one value parameter of type <code>T</code> whose name is

+unspecified, and returns a result of type <code>List<T></code>. It is possible to use

+the new syntax to declare function types that we can already declare using

+the existing typedef declaration. For instance, <code>G</code> and <code>H</code> both declare

+the same type:</p>

+<div class="highlight highlight-source-dart"><pre><span class="pl-k">typedef</span> G = List<int> Function(<span class="pl-k">int</span>); <span class="pl-c">// New form.</span>

+<span class="pl-k">typedef</span> List<int> H(<span class="pl-k">int</span> i); <span class="pl-c">// Old form.</span></pre></div>

+<p>Note that the name of the parameter is required in the old form, but the

+type may be omitted. In contrast, the type is required in the new form, but

+the name may be omitted.</p>

+<p>The reason for having two ways to express the same thing is that the new

+form seamlessly covers non-generic functions as well as generic ones, and

+developers might prefer to use the new form everywhere, for improved

+readability.</p>

+<p><em>We may deprecate the old form after a while, or we may choose

+to keep it, because it is more concise. We may even change the old form to

+allow omitting the name and not the type when only one identifier is

+specified, if this is not too much of a breaking change. As an intermediate

+step we could change the old form to always require both the type and the

+name, such that no type expressions will silently change meaning.</em></p>

+<p>There is a difference between declaring a generic function type and

+declaring a typedef which takes a type argument. The former is a

+declaration of a single type which describes a certain class of runtime

+entities: Functions that are capable of accepting some type arguments as

+well as some value arguments, both at runtime. The latter is a type-level

+function: It accepts a type argument at compile time and returns a type,

+which may be used, say, as a type annotation. Dart has had support for

+parameterized typedefs for a while, and the new syntax supports

+parameterized typedefs as well. Here is an example of a parameterized

+typedef, and a usage thereof:</p>

+<div class="highlight highlight-source-dart"><pre><span class="pl-k">typedef</span> I<T> = List<T> Function(T); <span class="pl-c">// New form.</span>

+<span class="pl-k">typedef</span> List<T> J<T>(T t); <span class="pl-c">// Old form.</span>

+I<span class="pl-k"><</span><span class="pl-k">int</span><span class="pl-k">></span> myFunction(J<span class="pl-k"><</span><span class="pl-k">int</span><span class="pl-k">></span> f) <span class="pl-k">=</span><span class="pl-k">></span> f;</pre></div>

+<p>Here, we have declared two equivalent parameterized typedefs <code>I</code> and <code>J</code>,

+and we have used an instantiation of each of them in the type annotations

+on <code>myFunction</code>. Note that the type of <code>myFunction</code> does not include <em>any</em>

+generic types, it is just a function that accepts an argument and returns a

+result, both of which have a non-generic function type that we have

+obtained by instantiating a parameterized typedef. The argument type might

+as well have been declared using the traditional function signature syntax,

+and the return type (and the argument type, by the way) might as well have

+been declared using a regular, non-parameterized typedef:</p>

+<div class="highlight highlight-source-dart"><pre><span class="pl-k">typedef</span> List<int> K(<span class="pl-k">int</span> i); <span class="pl-c">// Old form, non-generic.</span>

+K myFunction2(<span class="pl-k">List</span><span class="pl-k"><</span><span class="pl-k">int</span><span class="pl-k">></span> f(<span class="pl-k">int</span> i)) <span class="pl-k">=</span><span class="pl-k">></span> f; <span class="pl-c">// Same as myFunction.</span></pre></div>

+<p>The new syntax allows for using the two kinds of type parameters together:</p>

+<div class="highlight highlight-source-dart"><pre><span class="pl-k">typedef</span> L<T> = List<T> Function<S>(S, {T Function(<span class="pl-k">int</span>, S) factory});</pre></div>

+<p>This declares <code>L</code> to be a parameterized typedef; when instantiating <code>L</code>

+with an actual type argument as in <code>L<String></code>, it becomes the type of a

+generic function that accepts a type argument <code>S</code> and two value arguments:

+one required positional argument of type <code>S</code>, and one named optional

+argument with name <code>factory</code> and type <code>String Function(int, S)</code>; finally,

+it returns a value of type <code>List<String></code>.</p>

+<h2><a id="user-content-syntax" class="anchor" href="#syntax" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Syntax</h2>

+<p>The new form of <code>typedef</code> declaration uses the following syntax (there are

+no deletions from the grammar; addition of a new rule or a new alternative

+in a rule is marked with NEW and modified rules are marked CHANGED):</p>

+<pre><code>typeAlias:

+ metadata 'typedef' typeAliasBody |

+ metadata 'typedef' identifier typeParameters? '=' functionType ';' // NEW

+functionType: // NEW

+ returnType? 'Function' typeParameters? parameterTypeList

+parameterTypeList: // NEW

+ '(' ')' |

+ '(' normalParameterTypes ','? ')' |

+ '(' normalParameterTypes ',' optionalParameterTypes ')' |

+ '(' optionalParameterTypes ')'

+normalParameterTypes: // NEW

+ normalParameterType (',' normalParameterType)*

+normalParameterType: // NEW

+ type | typedIdentifier

+optionalParameterTypes: // NEW

+ optionalPositionalParameterTypes | namedParameterTypes

+optionalPositionalParameterTypes: // NEW

+ '[' normalParameterTypes ','? ']'

+namedParameterTypes: // NEW

+ '{' typedIdentifier (',' typedIdentifier)* ','? '}'

+typedIdentifier: // NEW

+ type identifier

+type: // CHANGED

+ typeWithoutFunction |

+ functionType

+typeWithoutFunction: // NEW

+ typeName typeArguments?

+typeWithoutFunctionList: // NEW

+ typeWithoutFunction (',' typeWithoutFunction)*

+mixins: // CHANGED

+ 'with' typeWithoutFunctionList

+interfaces: // CHANGED

+ 'implements' typeWithoutFunctionList

+superclass: // CHANGED

+ 'extends' typeWithoutFunction

+mixinApplication: // CHANGED

+ typeWithoutFunction mixins interfaces?

+newExpression: // CHANGED

+ 'new' typeWithoutFunction ('.' identifier)? arguments

+constObjectExpression: // CHANGED

+ 'const' typeWithoutFunction ('.' identifier)? arguments

+redirectingFactoryConstructorSignature: // CHANGED

+ 'const'? 'factory' identifier ('.' identifier)?

+ formalParameterList '=' typeWithoutFunction ('.' identifier)?

+</code></pre>

+<p>The syntax relies on treating <code>Function</code> as a fixed element in a function

+type, similar to a keyword or a symbol (many languages use symbols like

+<code>-></code> to mark function types).</p>

+<p><em>The rationale for using this form is that it makes a function type very

+similar to the header in a declaration of a function with that type: Just

+replace <code>Function</code> by the name of the function, and add missing parameter

+names and default values.</em></p>

+<p><em>The syntax differs from the existing function type syntax

+(<code>functionSignature</code>) in that the existing syntax allows the type of a

+parameter to be omitted, but the new syntax allows parameter names to be

+omitted. The rationale for this change is that a function type where a

+parameter has a specified name and no type is very likely to be a

+mistake. For instance, <code>int Function(int)</code> should not be the type of a

+function that accepts an argument named "int" of type <code>dynamic</code>, it should

+specify <code>int</code> as the parameter type and allow the name to be

+unspecified. It is still possible to opt in and specify the parameter name,

+which may be useful as documentation, e.g., if several arguments have the

+same type.</em></p>

+<p>The modification of the rule for the nonterminal <code>type</code> may cause parsing

+ambiguities. We intend to handle them by the following disambiguation rule

+in the parser: If the parser is at a location L where the tokens starting

+at L may be a <code>type</code> or some other construct (e.g., in the body of a

+method, when parsing something that may be a statement and may also be a

+declaration), the parser can commit to parsing a type by detecting that it

+is looking at the identifier <code>Function</code> followed by <code><</code> or <code>(</code>, or that it

+is looking at a type followed by the identifier <code>Function</code> followed by <code><</code>

+or <code>(</code>.</p>

+<p><em>Note that this disambiguation rule does require parsers to have unlimited

+lookahead. However, if a "diet parsing" strategy is used where the token

+stream already contains references from each opening bracket (such as <code><</code>

+or <code>(</code>) to the corresponding closing bracket then the decision can be

+taken in a fixed number of steps: If the current token is <code>Function</code> then

+check the immediate successor (<code><</code> or <code>(</code> means yes, we are looking at

+a <code>type</code>, everything else means no) and we're done; if the first token is

+an <code>identifier</code> other than <code>Function</code> then we can check whether it is a

+<code>qualified</code> by looking at no more than the two next tokens, and we may then

+check whether the next token again is <code><</code>; if it is not then we look for

+<code>Function</code> and the token after that, and if it is <code><</code> then look for the

+corresponding <code>></code> (we have now skipped a generic class type), and then

+the successor to that token again must be <code>Function</code>, and we finally check

+its successor (looking for <code><</code> or <code>(</code> again). This skips over the

+presumed type arguments to a generic class type without checking that they

+are actually type arguments, but we conjecture that there are no

+syntactically correct alternatives (for example, we conjecture that there

+is no syntactically correct statement, not a declaration, starting with

+<code>SomeIdentifier<...> Function(...</code> where the angle brackets are balanced).</em></p>

+<p><em>Note that this disambiguation rule will prevent parsing some otherwise

+correct programs. For instance, the declaration of an asynchronous function

+named <code>Function</code> with an omitted return type (meaning <code>dynamic</code>) and an

+argument named <code>int</code> of type <code>dynamic</code> using <code>Function(int) async {}</code> will

+be a parse error, because the parser will commit to parsing a type after

+having seen "<code>Function(</code>" as a lookahead. However, we do not expect that it

+will be a serious problem for developers to be unable to write such

+programs.</em></p>

+<h2><a id="user-content-scoping" class="anchor" href="#scoping" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Scoping</h2>

+<p>Consider a typedef declaration as introduced by this feature, i.e., a

+construct on the form</p>

+<pre><code>metadata 'typedef' identifier typeParameters? '=' functionType ';'

+</code></pre>

+<p>This declaration introduces <code>identifier</code> into the enclosing library scope.</p>

+<p>Consider a parameterized typedef, i.e., a construct on the form</p>

+<pre><code>metadata 'typedef' identifier typeParameters '=' functionType ';'

+</code></pre>

+<p>Note that in this case the <code>typeParameters</code> are present, not optional. This

+construct introduces a scope known as the <em>typedef scope</em>. Each typedef

+scope is nested inside the library scope of the enclosing library. Every

+formal type parameter declared by the <code>typeParameters</code> in this construct

+introduces a type variable into its enclosing typedef scope. The typedef

+scope is the current scope for the <code>typeParameters</code> themselves, and for the

+<code>functionType</code>.</p>

+<p>Consider a <code>functionType</code> specifying a generic function type, i.e., a

+construct on the form</p>

+<pre><code>returnType? 'Function' typeParameters parameterTypeList

+</code></pre>

+<p>Note again that <code>typeParameters</code> are present, not optional. This construct

+introduces a scope known as a <em>function type scope</em>. The function type

+scope is nested inside the current scope for the associated <code>functionType</code>.

+Every formal type parameter declared by the <code>typeParameters</code> introduces a

+type variable into its enclosing function type scope. The function type

+scope is the current scope for the entire <code>functionType</code>.</p>

+<p><em>This implies that parameterized typedefs and function types are capable of

+specifying F-bounded type parameters, because the type parameters are in

+scope in the type parameter list itself.</em></p>

+<h2><a id="user-content-static-analysis" class="anchor" href="#static-analysis" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Static Analysis</h2>

+<p>Consider a typedef declaration as introduced by this feature, i.e., a

+construct on the form</p>

+<pre><code>metadata 'typedef' identifier typeParameters? '=' functionType ';'

+</code></pre>

+<p>It is a compile-time error if a name <em>N</em> introduced into a library scope by

+a typedef has an associated <code>functionType</code> which depends directly or

+indirectly on <em>N</em>. It is a compile-time error if a bound on a formal type

+parameter in <code>typeParameters</code> is not a type. It is a compile-time error if

+a typedef has an associated <code>functionType</code> which is not a type when

+analyzed under the assumption that every identifier resolving to a formal

+type parameter in <code>typeParameters</code> is a type. It is a compile-time error if

+an instantiation <em>F<T1..Tk></em> of a parameterized typedef is mal-bounded.</p>

+<p><em>This implies that a typedef cannot be recursive. It can only introduce a

+name as an alias for a type which is already expressible as a

+<code>functionType</code>, or a name for a type-level function F where every

+well-bounded invocation <code>F<T1..Tk></code> denotes a type which could be expressed

+as a <code>functionType</code>. Following

+<a href="https://en.wikipedia.org/wiki/Kind_(type_theory)">common terminology</a>, we

+could say that a typedef can define entities of kind <code>*</code> and of kind

+<code>* -> *</code>, and, when it is assumed that every formal type parameter of the

+typedef (if any) has kind <code>*</code>, it is an error if the right hand side of the

+declaration denotes an entity of any other kind than <code>*</code>; in particular,

+declarations of entities of kind <code>* -> *</code> cannot be curried.</em></p>

+<p>It is a compile-time error if a name declared in a typedef, with or without

+actual type arguments, is used as a superclass, superinterface, or mixin.</p>

+<h2><a id="user-content-dynamic-semantics" class="anchor" href="#dynamic-semantics" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Dynamic Semantics</h2>

+<p>The addition of this feature does not change the dynamic semantics of

+Dart.</p>

+<h2><a id="user-content-changes" class="anchor" href="#changes" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Changes</h2>

+<p>2017-Jan-04: Adjusted the grammar to require named parameter types to have

+a type (previously, the type was optional).</p>

+<p>2016-Dec-21: Changed the grammar to prevent the new function type syntax

+in several locations (for instance, as a super class or as a mixin). The

+main change in the grammar is the introduction of <code>typeWithoutFunction</code>.</p>

+<p>2016-Dec-15: Changed the grammar to prevent the old style function types

+(derived from <code>functionSignature</code> in the grammar) from occurring inside

+the new style (<code>functionType</code>).</p>

+</article>

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