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, 7 months ago

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	Lasse Reichstein Nielsen 2017/04/24 13:02:05 The document has extension .md, but contains HTML. The document has extension .md, but contains HTML. That is probably a mistake :) eernst 2017/04/25 11:57:15 Fixed. Show quoted text On 2017/04/24 13:02:05, Lasse Reichstein Nielsen wrote: > The document has extension .md, but contains HTML. > That is probably a mistake :) Fixed.
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	380 INFORMAL SPECIFICATION: Generic Function Type Alias

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

	405 <article class="markdown-body entry-content" itemprop="text"><h1><a id="user -content-feature-generic-function-type-alias" class="anchor" href="#feature-gene ric-function-type-alias" aria-hidden="true"><svg aria-hidden="true" class="octic on 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 1 3h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Feature: Generic Function Type Alias</h1>

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

	407 <p><strong>This document</strong> is an informal specification of a feature supp orting the

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

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

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

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

	412 <code>typedef</code>. This feature is being introduced into Dart starting Q4, 2 016.</p>

	413 <p><strong>This feature</strong> introduces a new syntactic form of typedef decl aration

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

	415 sign, <code>=</code>. The effect of such a declaration is that the name is decla red to

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

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

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

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

	420 <p>The <strong>motivation</strong> for adding this feature is that it allows dev elopers to

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

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

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

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

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

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

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

	428 <h2><a id="user-content-examples" class="anchor" href="#examples" aria-hidden="t rue"><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>

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

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

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

	432 parameter <code>T</code> and one value parameter of type <code>T</code> whose na me is

	433 unspecified, and returns a result of type <code>List<T></code>. It is poss ible to use

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

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

	436 the same type:</p>

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

	438 <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>

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

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

	441 the name may be omitted.</p>

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

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

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

	445 readability.</p>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	464 I<span class="pl-k"><</span><span class="pl-k">int</span><span class="pl-k">& gt;</span> myFunction(J<span class="pl-k"><</span><span class="pl-k">int</spa n><span class="pl-k">></span> f) <span class="pl-k">=</span><span class="pl-k ">></span> f;</pre></div>

	465 <p>Here, we have declared two equivalent parameterized typedefs <code>I</code> a nd <code>J</code>,

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

	467 on <code>myFunction</code>. Note that the type of <code>myFunction</code> does n ot include <em>any</em>

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

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

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

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

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

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

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

	475 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 cla ss="pl-c">// Same as myFunction.</span></pre></div>

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

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

	478 <p>This declares <code>L</code> to be a parameterized typedef; when instantiatin g <code>L</code>

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

	480 generic function that accepts a type argument <code>S</code> and two value argum ents:

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

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

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

	484 <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"></pat h></svg></a>Syntax</h2>

	485 <p>The new form of <code>typedef</code> declaration uses the following syntax (t here are

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

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

	488 <pre><code>typeAlias:

	489 metadata 'typedef' typeAliasBody \|

	490 metadata 'typedef' identifier typeParameters? '=' functionType ';' // NE W

	491 functionType: // NEW

	492 returnType? 'Function' typeParameters? parameterTypeList

	493 parameterTypeList: // NEW

	494 '(' ')' \|

	495 '(' normalParameterTypes ','? ')' \|

	496 '(' normalParameterTypes ',' optionalParameterTypes ')' \|

	497 '(' optionalParameterTypes ')'

	498 normalParameterTypes: // NEW

	499 normalParameterType (',' normalParameterType)*

	500 normalParameterType: // NEW

	501 type \| typedIdentifier

	502 optionalParameterTypes: // NEW

	503 optionalPositionalParameterTypes \| namedParameterTypes

	504 optionalPositionalParameterTypes: // NEW

	505 '[' normalParameterTypes ','? ']'

	506 namedParameterTypes: // NEW

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

	508 typedIdentifier: // NEW

	509 type identifier

	510 type: // CHANGED

	511 typeWithoutFunction \|

	512 functionType

	513 typeWithoutFunction: // NEW

	514 typeName typeArguments?

	515 typeWithoutFunctionList: // NEW

	516 typeWithoutFunction (',' typeWithoutFunction)*

	517 mixins: // CHANGED

	518 'with' typeWithoutFunctionList

	519 interfaces: // CHANGED

	520 'implements' typeWithoutFunctionList

	521 superclass: // CHANGED

	522 'extends' typeWithoutFunction

	523 mixinApplication: // CHANGED

	524 typeWithoutFunction mixins interfaces?

	525 newExpression: // CHANGED

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

	527 constObjectExpression: // CHANGED

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

	529 redirectingFactoryConstructorSignature: // CHANGED

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

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

	532 </code></pre>

	533 <p>The syntax relies on treating <code>Function</code> as a fixed element in a f unction

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

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

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

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

	538 replace <code>Function</code> by the name of the function, and add missing param eter

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

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

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

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

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

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

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

	546 function that accepts an argument named "int" of type <code>dynamic</code>, it s hould

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

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

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

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

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

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

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

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

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

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

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

	558 is looking at a type followed by the identifier <code>Function</code> followed b y <code><</code>

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

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

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

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

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

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

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

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

	567 an <code>identifier</code> other than <code>Function</code> then we can check wh ether it is a

	568 <code>qualified</code> by looking at no more than the two next tokens, and we ma y then

	569 check whether the next token again is <code><</code>; if it is not then we lo ok for

	570 <code>Function</code> and the token after that, and if it is <code><</code> t hen look for the

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

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

	573 its successor (looking for <code><</code> or <code>(</code> again). This skip s over the

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

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

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

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

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

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

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

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

	582 argument named <code>int</code> of type <code>dynamic</code> using <code>Functio n(int) async {}</code> will

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

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

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

	586 programs.</em></p>

	587 <h2><a id="user-content-scoping" class="anchor" href="#scoping" aria-hidden="tru e"><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-.4 5 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"></p ath></svg></a>Scoping</h2>

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

	589 construct on the form</p>

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

	591 </code></pre>

	592 <p>This declaration introduces <code>identifier</code> into the enclosing librar y scope.</p>

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

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

	595 </code></pre>

	596 <p>Note that in this case the <code>typeParameters</code> are present, not optio nal. This

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

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

	599 formal type parameter declared by the <code>typeParameters</code> in this constr uct

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

	601 scope is the current scope for the <code>typeParameters</code> themselves, and f or the

	602 <code>functionType</code>.</p>

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

	604 construct on the form</p>

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

	606 </code></pre>

	607 <p>Note again that <code>typeParameters</code> are present, not optional. This c onstruct

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

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

	610 Every formal type parameter declared by the <code>typeParameters</code> introduc es a

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

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

	613 <p><em>This implies that parameterized typedefs and function types are capable o f

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

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

	616 <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 9 zm9-3h-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 3-3.5S1 4.5 6 13 6z"></path></svg></a>Static Analysis</h2>

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

	618 construct on the form</p>

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

	620 </code></pre>

	621 <p>It is a compile-time error if a name <em>N</em> introduced into a library sco pe by

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

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

	624 parameter in <code>typeParameters</code> is not a type. It is a compile-time err or if

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

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

	627 type parameter in <code>typeParameters</code> is a type. It is a compile-time er ror if

	628 an instantiation <em>F<T1..Tk></em> of a parameterized typedef is mal-boun ded.</p>

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

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

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

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

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

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

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

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

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

	638 declaration denotes an entity of any other kind than <code>*</code>; in particul ar,

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

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

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

	642 <h2><a id="user-content-dynamic-semantics" class="anchor" href="#dynamic-semanti cs" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" heig ht="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 2.7 2-2 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>

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

	644 Dart.</p>

	645 <h2><a id="user-content-changes" class="anchor" href="#changes" aria-hidden="tru e"><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-.4 5 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"></p ath></svg></a>Changes</h2>

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

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

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

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

	650 main change in the grammar is the introduction of <code>typeWithoutFunction</cod e>.</p>

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

	652 (derived from <code>functionSignature</code> in the grammar) from occurring insi de

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

	654 </article>

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