rfc:allow_casting_closures_into_single-method_interface_implementations

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rfc:allow_casting_closures_into_single-method_interface_implementations [2023/04/25 09:48] – nicolasgrekasrfc:allow_casting_closures_into_single-method_interface_implementations [2023/04/25 19:34] – nicolasgrekas
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-===== Title: Allow casting closures into single-method interface implementations =====+====== PHP RFC: Allow casting closures into single-method interface implementations ======
  
   * Version: 1.0   * Version: 1.0
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   * Implementation: TBD   * Implementation: TBD
  
-==== Introduction ====+===== Introduction =====
  
 This RFC proposes a new method for the <php>Closure</php> class called <php>castTo()</php>. This method would allow developers to create an instance of a class that implements a specified interface and uses the closure as the implementation. This RFC proposes a new method for the <php>Closure</php> class called <php>castTo()</php>. This method would allow developers to create an instance of a class that implements a specified interface and uses the closure as the implementation.
  
-==== Proposal ====+===== Proposal =====
  
 A new method, <php>castTo()</php>, would be added to the <php>Closure</php> class with the following signature (generics added for extra clarity): A new method, <php>castTo()</php>, would be added to the <php>Closure</php> class with the following signature (generics added for extra clarity):
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   * **Providing a type-safe alternative to the <php>Closure</php> type**: instead of typing against the <php>Closure</php> type, developers could declare and use interfaces that have a single <php>__invoke()</php> method. While this is possible already, this is quite rare in practice because it comes at a high syntactic cost for end users, which are then required to define a full class. By allowing to quickly turn a closure into an implementation of such interfaces, the <php>castTo()</php> method would greatly reduce this boilerplate, leading to a wider adoption of type-safe code for closures.   * **Providing a type-safe alternative to the <php>Closure</php> type**: instead of typing against the <php>Closure</php> type, developers could declare and use interfaces that have a single <php>__invoke()</php> method. While this is possible already, this is quite rare in practice because it comes at a high syntactic cost for end users, which are then required to define a full class. By allowing to quickly turn a closure into an implementation of such interfaces, the <php>castTo()</php> method would greatly reduce this boilerplate, leading to a wider adoption of type-safe code for closures.
  
-=== Example 1: Using a closure as a default implementation for a TranslatorInterface ===+==== Example 1: Using a closure as a default implementation for a TranslatorInterface ====
  
 This example demonstrates how to use the <php>castTo()</php> method to create a default implementation for a ''TranslatorInterface'' using a closure. It combines this proposal with the first-class callable syntax. This example demonstrates how to use the <php>castTo()</php> method to create a default implementation for a ''TranslatorInterface'' using a closure. It combines this proposal with the first-class callable syntax.
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 If no custom implementation of ''TranslatorInterface'' is provided during the instantiation of ''SomeClass'', a default implementation is  derived using a closure created from the <php>strtr()</php> function. If no custom implementation of ''TranslatorInterface'' is provided during the instantiation of ''SomeClass'', a default implementation is  derived using a closure created from the <php>strtr()</php> function.
  
-=== Example 2: Type-safe alternative to using Closure ===+==== Example 2: Type-safe alternative to using Closure ====
  
-Imagine that you have the following function, that takes a closure and two number to do some operation on them:+Imagine that you have the following function, that takes a closure and two numbers to do some operation on them:
  
 <code php> <code php>
-function executeOperation(Closure $operation, int $a, int $b): int { +function executeOperation(Closure $operator, int $a, int $b): int { 
-    return $operation($a, $b);+    return $operator($a, $b);
 } }
 </code> </code>
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 } }
  
-function executeOperation(InvokableInterface $operation, int $a, int $b): int { +function executeOperation(OperatorInterface $operator, int $a, int $b): int { 
-    return $operation($a, $b);+    return $operator($a, $b);
 } }
  
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 In this example, an ''OperatorInterface'' is defined with a single <php>__invoke()</php> method, which can be used to replace the <php>Closure</php> type on the <php>executeOperation()</php> function. The <php>castTo()</php> method is then used to create instances of the interface, with different behaviors implemented by closures. This approach provides a more type-safe and expressive way to handle closures while adhering to the required interface. In this example, an ''OperatorInterface'' is defined with a single <php>__invoke()</php> method, which can be used to replace the <php>Closure</php> type on the <php>executeOperation()</php> function. The <php>castTo()</php> method is then used to create instances of the interface, with different behaviors implemented by closures. This approach provides a more type-safe and expressive way to handle closures while adhering to the required interface.
  
-=== Example 3: Decoupling URI template implementations using adapters ===+==== Example 3: Decoupling URI template implementations using adapters ====
  
 This example demonstrates how the <php>castTo()</php> method can be used to create adapters for This example demonstrates how the <php>castTo()</php> method can be used to create adapters for
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 </code> </code>
  
-In this example, an ''UriExpanderInterface''' is defined with an <php>expand()</php> method.+In this example, an ''UriExpanderInterface'' is defined with an <php>expand()</php> method. 
 3 different implementations of the interface are derived using the <php>castTo()</php> method. 3 different implementations of the interface are derived using the <php>castTo()</php> method.
  
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 illustrating the decoupling and flexibility provided by using adapters created with the <php>castTo()</php> method. illustrating the decoupling and flexibility provided by using adapters created with the <php>castTo()</php> method.
  
-==== Backward Incompatible Changes ====+==== Prototype implementation ==== 
 + 
 +Using <php>eval()</php> and anonymous classes, it's already possible to create a function that turns a closure into an implementation of a single-method interface. Conceptually, this RFC could be coded this way: 
 + 
 +<code php> 
 +class Closure 
 +{ 
 +    // [...] Closure is an internal class 
 +     
 +    public function castTo(self $closure, string $interface): object 
 +    { 
 +        static $cache = []; 
 + 
 +        $cacheKey = <compute-key-from-$closure-and-$interface> 
 + 
 +        if (isset($cache[$cacheKey])) { 
 +            return $cache[$cacheKey]($closure); 
 +        } 
 +         
 +        $method = <name-of-the-method-in-$interface>; 
 +        $signature = <signature-of-$closure>; 
 +        $return = str_ends_with($signature, '): never') || str_ends_with($signature, '): void') ? '' : 'return '; 
 + 
 +        $object = eval(<<<PHP 
 +            return new class (\$closure) implements {$interface} { 
 +                public function __construct(private readonly Closure \$closure) { 
 +                } 
 +                public function {$method}{$signature} { 
 +                    {$return}\$this->closure->__invoke(<list-of-arguments-in-$closure>); 
 +                } 
 +            }; 
 +            PHP); 
 + 
 +        $cache[$cacheKey] = $object::class; 
 + 
 +        return $object; 
 +    } 
 +} 
 +</code> 
 + 
 +A working prototype implementation is provided by this package: 
 +https://github.com/tchwork/closure-caster/blob/main/src/function.php 
 + 
 +===== Backward Incompatible Changes =====
  
 This proposal introduces no backward incompatible changes, as it only adds a new method to the <php>Closure</php> class. This proposal introduces no backward incompatible changes, as it only adds a new method to the <php>Closure</php> class.
  
-==== Open Issues ====+===== Open Issues =====
  
   * exact behavior when checks fail   * exact behavior when checks fail
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   * would the engine allow implementing this without adding any extra frame in the call stack?   * would the engine allow implementing this without adding any extra frame in the call stack?
  
-==== Unaffected PHP Functionality ====+===== Unaffected PHP Functionality =====
  
 No existing PHP functionality would be affected by this proposal. No existing PHP functionality would be affected by this proposal.
  
-==== Future Scope ====+===== Future Scope =====
  
   * Allow a closure to declare a single-method interface it implements, as in e.g. <php>function () implements FooInterface {...}</php>. See https://wiki.php.net/rfc/allow-closures-to-declare-interfaces-they-implement   * Allow a closure to declare a single-method interface it implements, as in e.g. <php>function () implements FooInterface {...}</php>. See https://wiki.php.net/rfc/allow-closures-to-declare-interfaces-they-implement
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 Both ideas are complementary to the one proposed in this RFC. Both ideas are complementary to the one proposed in this RFC.
  
-==== Proposed PHP Version ====+===== Proposed PHP Version =====
  
 This RFC targets PHP version 8.3. This RFC targets PHP version 8.3.
  
-==== Vote ====+===== Vote =====
  
 The vote will require a 2/3 majority to be accepted. Voting will start on [Vote start date] and end on [Vote end date]. The vote will require a 2/3 majority to be accepted. Voting will start on [Vote start date] and end on [Vote end date].
  
  
rfc/allow_casting_closures_into_single-method_interface_implementations.txt · Last modified: 2023/04/25 19:36 by nicolasgrekas

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