rfc:closures

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rfc:closures [2008/07/02 17:36]
chris_se typo
rfc:closures [2009/12/15 22:41]
rquadling Fix list numbering
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   * Date: 2008-07-01   * Date: 2008-07-01
   * Author: Christian Seiler <chris_se@gmx.net>, Dmitry Stogov <dmitry@zend.com>   * Author: Christian Seiler <chris_se@gmx.net>, Dmitry Stogov <dmitry@zend.com>
-  * Status: Under Discussion+  * Status: Implemented
  
 This RFC discusses the introduction of compile-time lambda functions and closures in PHP. This RFC discusses the introduction of compile-time lambda functions and closures in PHP.
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 Closures and lambda functions can make programming much easier in several ways: Closures and lambda functions can make programming much easier in several ways:
- 
 ==== Lambda Functions ==== ==== Lambda Functions ====
  
-Lambda functions allow the quick definition of throw-away functions that are not used elsewhere. Imaging for example a piece of code that needs to call preg_replace_callback(). Currently, there are three possibilities to achieve this: +Lambda functions allow the quick definition of throw-away functions that are not used elsewhere. Imagine for example a piece of code that needs to call preg_replace_callback(). Currently, there are three possibilities to achieve this: 
- +  - Define the callback function elsewhere. This distributes code that belongs together throughout the file and decreases readability. 
-   - Define the callback function elsewhere. This distributes code that belongs together throughout the file and decreases readability. +  - Define the callback function in-place (but with a name). In that case one has to use function_exists() to make sure the function is only defined once. Here, the additional if() around the function definition makes the source code difficult to read. Example code:<code php>
- +
-   - Define the callback function in-place (but with a name). In that case one has to use function_exists() to make sure the function is only defined once. Here, the additional if() around the function definition makes the source code difficult to read. Example code: +
- +
-<code php>+
    function replace_spaces ($text) {    function replace_spaces ($text) {
      if (!function_exists ('replace_spaces_helper')) {      if (!function_exists ('replace_spaces_helper')) {
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    }    }
 </code> </code>
- +  - Use the present create_function() in order to create a function at runtime. This approach has several disadvantages: First of all, syntax highlighting does not work because a string is passed to the function. It also compiles the function at run time and not at compile time so opcode caches can't cache the function.
-   - Use the present create_function() in order to create a function at runtime. This approach has several disadvantages: First of all, syntax highlighting does not work because a string is passed to the function. It also compiles the function at run time and not at compile time so opcode caches can't cache the function. +
 ==== Closures ==== ==== Closures ====
  
-Closures provide a very useful tool in order to make lambda functions even more useful. Just imagine you want to replace 'hello' through 'goodbye' in all elements of an array. PHP provides the array_map() function which accepts a callback. If you don'wan'to hard-code 'hello' and 'goodbye' into your sourcecode, you have only four choices: +Closures provide a very useful tool in order to make lambda functions even more useful. Just imagine you want to replace 'hello' through 'goodbye' in all elements of an array. PHP provides the array_map() function which accepts a callback. If you don'want to hard-code 'hello' and 'goodbye' into your sourcecode, you have only four choices: 
- +  - Use create_function(). But then you may only pass literal values (strings, integers, floats) into the function, objects at best as clones (if var_export() allows for it) and resources not at all. And you have to worry about escaping everything correctly. Especially when handling user input this can lead to all sorts of security issues. 
-   - Use create_function(). But then you may only pass literal values (strings, integers, floats) into the function, objects at best as clones (if var_export() allows for it) and resources not at all. And you have to worry about escaping everything correctly. Especially when handling user input this can lead to all sorts of security issues. +  - Write a function that uses global variables. This is ugly, non-reentrant and bad style. 
- +  - Create an entire class, instantiate it and pass the member function as a callback. This is perhaps the cleanest solution for this problem with current PHP but just think about it: Creating an entire class for this extremely simple purpose and nothing else seems overkill. 
-   - Write a function that uses global variables. This is ugly, non-reentrant and bad style. +  - Don't use array_map() but simply do it manually (foreach). In this simple case it may not be that much of an issue (because one simply wants to iterate over an array) but there are cases where doing something manually that a function with a callback as parameter does for you is quite tedious.
- +
-   - Create an entire class, instantiate it and pass the member function as a callback. This is perhaps the cleanest solution for this problem with current PHP but just think about it: Creating an entire class for this extremely simple purpose and nothing else seems overkill. +
- +
-   - Don't use array_map() but simply do it manually (foreach). In this simple case it may not be that much of an issue (because one simply wants to iterate over an array) but there are cases where doing something manually that a function with a callback as parameter does for you is quite tedious.+
  
 Note: str_replace also accepts arrays as a third parameter so this example may be a bit useless. But imagine you want to do a more complex operation than simple search and replace. Note: str_replace also accepts arrays as a third parameter so this example may be a bit useless. But imagine you want to do a more complex operation than simple search and replace.
- 
 ===== Common misconceptions ===== ===== Common misconceptions =====
  
    - Lambda functions / closures are **not** a way of dynamically extending classes by additional methods at runtime. There are several other possibilities to do this, including the already present _ _call semantic.    - Lambda functions / closures are **not** a way of dynamically extending classes by additional methods at runtime. There are several other possibilities to do this, including the already present _ _call semantic.
- +   PHP'notion of scope is quite different than the notion of scope other languages define. Combine this with variable variables ($$var) and it becomes clear that automatically detecting which variables from the outer scope are referenced inside are closure is impossible. Also, since for example global variables are not visible inside functions either by default, automatically making the parent scope available would break with the current language concept PHP follows.
-   PHPs notion of scope is quite differnet than the notion of scope other languages define. Combine this with variable variables ($$var) and it becomes clear that automatically detecting which variables from the outer scope are referenced inside are closure is impossible. Also, since for example global variables are not visible inside functions either by default, automatically making the parent scope available would break with the current language concept PHP follows.+
  
 ===== Proposal and Patch ===== ===== Proposal and Patch =====
  
 The following proposal and patch implement compile-time lambda functions and closures for PHP while keeping the patch as simple as possible. The following proposal and patch implement compile-time lambda functions and closures for PHP while keeping the patch as simple as possible.
- 
 ==== Userland perspective ==== ==== Userland perspective ====
  
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 </code> </code>
  
-The & is optional and indicates that the function should return a refernce. The use followed by the paranthesis is optional and indicates that several variables from the current scope should be imported into the closure.+The & is optional and indicates that the function should return a reference. The use followed by the parentheses is optional and indicates that several variables from the current scope should be imported into the closure.
  
 Example usage: Example usage:
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      return preg_replace_callback ('/( +) /', $replacement, $text);      return preg_replace_callback ('/( +) /', $replacement, $text);
    }    }
 +</code>
 +
 +You can even put the lambda function inline, for example:
 +
 +<code php>
 +  function replace_spaces ($text) {
 +    return preg_replace_callback ('/( +) /',
 +      function ($matches) {
 +        return str_replace ($matches[1], ' ', '&nbsp;').' ';
 +      }, $text);
 +  }
 </code> </code>
  
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        }        }
      };      };
-     return array_map ($map, array);+     return array_map ($map, $array);
    }    }
 </code> </code>
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 </code> </code>
  
-=== Refernces vs. Copies ===+=== References vs. Copies ===
  
-By default, all imported variables are copied as values into the closure. This makes it impossible for a closure to modify the variable in the parent scope. By prepending an & in front of the variable name in the use declaration, the variable is imported as a refernce instead. In that case, changes to the variable inside the closure will affect the outside scope.+By default, all imported variables are copied as values into the closure. This makes it impossible for a closure to modify the variable in the parent scope. By prepending an & in front of the variable name in the use declaration, the variable is imported as a reference instead. In that case, changes to the variable inside the closure will affect the outside scope.
  
 Example: Example:
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 === Interaction with OOP === === Interaction with OOP ===
  
-If a closure is defined inside an object, the closure has full access to the current object through $this (without the need to import it explicitely) and all private and protected methods of that class. This also applies to nested closures. Example:+$this support has been removed, see [[rfc/closures/removal-of-this|removal of this]] 
 + 
 +If a closure is defined inside an object, the closure has full access to the current object through $this (without the need to import it explicitly) and all private and protected methods of that class. This also applies to nested closures. Example:
  
 <code php> <code php>
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          $x = 4;          $x = 4;
          $closure = static function ($y) use ($x) {          $closure = static function ($y) use ($x) {
-           return $x + $y;+           return $x + $y;
          };          };
          return $closure (6);          return $closure (6);
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 In this case, $this is not available inside the closure. This may save a lot of memory if saves many closures that originated in longer needed objects. In this case, $this is not available inside the closure. This may save a lot of memory if saves many closures that originated in longer needed objects.
 +
  
 ==== Additional goody: _ _invoke ==== ==== Additional goody: _ _invoke ====
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 </code> </code>
  
-==== Interaction with reflection ====+==== Interaction with reflection (1) ====
  
 Since closures are anonymous, they do **not** appear in reflection. Since closures are anonymous, they do **not** appear in reflection.
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 $object->x = 5; $object->x = 5;
 $closure (); $closure ();
 +</code>
 +
 +==== Interaction with reflection (2) ====
 +
 +In addition to the previous patch, reflection support was augmented to support reflecting closure objects and returning the correct function pointer.
 +
 +<code php>
 +$closure = function ($a, &$b, $c = null) { };
 +$m = new ReflectionMethod ($closure, '__invoke');
 +Reflection::export ($m);
 +</code>
 +
 +This will yield:
 +
 +<code>
 +Method [ <internal> public method __invoke ] {
 +
 +  - Parameters [3] {
 +    Parameter #0 [ <required> $a ]
 +    Parameter #1 [ <required> &$b ]
 +    Parameter #2 [ <optional> $c ]
 +  }
 +}
 +</code>
 +
 +The following will also work (invoke is implied if no method name is specified):
 +
 +<code php>
 +$m = new ReflectionMethod ($closure);
 +$p = new ReflectionParameter ($closure, 0);
 +$p = new ReflectionParameter ($closure, 'a');
 +$p = new ReflectionParameter (array ($closure, '__invoke'), 0);
 </code> </code>
  
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 ==== The patch ==== ==== The patch ====
 +
 +**Note:** The patches were already applied to PHP_5_3 and HEAD (with some minor modifications and fixes).
  
 Current patches: Current patches:
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 ==== BC breaks ==== ==== BC breaks ====
  
-   None by designA new previously invalid syntax is added. No additional keywords are defined.+   Creates an additional class named "Closure" that may break existing code. Apparently classes by this name [[http://google.com/codesearch?hl=en&lr=&q=%22class+Closure%22+lang%3Aphp&sbtn=Search|are used to emulate closures]] in current PHP versions. 
 +   * None otherwise (no new keywords)
  
 ==== Caveats / possible WTFs ==== ==== Caveats / possible WTFs ====
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 === Trailing '';'' === === Trailing '';'' ===
  
-On writing ''$func = function () { };'' there is a semicolon necessary. If left out it will produce a compile error. Since any attempt to remove that necessity would unecessarily bloat the grammar, I suggest we simply keep it the way it is. Also, Lukas Kahwe Smith pointed out that a single trailing semicolon after a closing brace already exists: ''do { } while ();''+On writing ''$func = function () { };'' there is a semicolon necessary. If left out it will produce a compile error. Since any attempt to remove that necessity would unnecessarily bloat the grammar, I suggest we simply keep it the way it is. Also, Lukas Kahwe Smith pointed out that a single trailing semicolon after a closing brace already exists: ''do { } while ();''
  
 === Misinterpretations of the goal of closures === === Misinterpretations of the goal of closures ===
  
 As the discussion on the mailing list showed, there were quite a few misconceptions on what closures may or may not achieve. One often used suggestion was to use closures in order to extend classes by additional methods at run time. This is **not** the goal of closures **and** it can already be achieved without closures just by using _ _call, see for example [[http://phpfi.com/328105]]. As the discussion on the mailing list showed, there were quite a few misconceptions on what closures may or may not achieve. One often used suggestion was to use closures in order to extend classes by additional methods at run time. This is **not** the goal of closures **and** it can already be achieved without closures just by using _ _call, see for example [[http://phpfi.com/328105]].
 +
 +===== Example code =====
 +
 +The example code in this document is available [[http://www.christian-seiler.de/temp/php-5.3-test-closures.txt|here]].
  
 ===== Changelog ==== ===== Changelog ====
  
 +   * 2008-08-11 Christian Seiler: Documented additional reflection improvements (see php-internals)
 +   * 2008-07-15 Christian Seiler: Updated status of this RFC
    * 2008-07-01 Christian Seiler: Updated patch yet again    * 2008-07-01 Christian Seiler: Updated patch yet again
    * 2008-06-26 Christian Seiler: Revised patch, using objects instead of resources, added tests    * 2008-06-26 Christian Seiler: Revised patch, using objects instead of resources, added tests
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    * 2008-06-16 Christian Seiler: Small changes    * 2008-06-16 Christian Seiler: Small changes
    * 2008-06-16 Christian Seiler: Initial creation    * 2008-06-16 Christian Seiler: Initial creation
- 
rfc/closures.txt · Last modified: 2017/09/22 13:28 (external edit)