rfc:auto-capture-closure
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rfc:auto-capture-closure [2022/05/26 20:17] crell Wordsmithing, editing, and an extra example. |
rfc:auto-capture-closure [2022/07/02 13:12] (current) imsop update "Vote" heading |
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- | ====== PHP RFC: Auto-capturing multi-statement closures | + | ====== PHP RFC: Short Closures 2.0 ====== |
* Version: 2.0 | * Version: 2.0 | ||
* Date: 2022-05-25 | * Date: 2022-05-25 | ||
Line 5: | Line 5: | ||
* Author: Larry Garfield (larry@garfieldtech.com) | * Author: Larry Garfield (larry@garfieldtech.com) | ||
* Author: Arnaud Le Blanc (arnaud.lb@gmail.com) | * Author: Arnaud Le Blanc (arnaud.lb@gmail.com) | ||
- | * Status: In Discussion | + | * Status: In Voting |
* First Published at: http:// | * First Published at: http:// | ||
===== Introduction ===== | ===== Introduction ===== | ||
- | Closures (also known as lambdas or anonymous | + | Anonymous |
- | <code php> | + | [[rfc:arrow_functions_v2|Arrow Functions]] were introduced in PHP 7.4 as an alternative. However, the single-expression limitation can lead to complex one-liners, or makes Arrow Functions unfit in many use-cases that would benefit from a more concise syntax. |
- | // As of 8.1: | + | |
- | $y = 1; | + | This RFC proposes an extension of the Arrow Function syntax supporting multiple statements: |
- | $fn1 = fn($x) => $x + $y; // auto-capture + single expression | + | <code php> |
+ | $guests | ||
+ | $guest | ||
+ | return | ||
+ | }); | ||
+ | </code> | ||
- | $fn2 = function ($x) use ($y): int { // manual-capture + statement list | + | ===== Proposal ===== |
- | // ... | + | |
- | return $x + $y; | + | Short Closures extend Arrow Functions by allowing multiple statements enclosed in '' |
- | }; | + | |
+ | <code php> | ||
+ | fn (parameter_list) { | ||
+ | statement_list; | ||
+ | } | ||
</ | </ | ||
- | The proposed | + | The '' |
+ | |||
+ | The syntax | ||
+ | |||
+ | ==== Auto capture by-value ==== | ||
+ | |||
+ | Like Arrow Functions, Short Closures use auto capture | ||
<code php> | <code php> | ||
- | $fn3 = fn ($x): int { // auto-capture | + | $y = 1; |
- | // ... | + | |
+ | $fn1 = fn ($x) => $x + $y; | ||
+ | $fn2 = fn ($x) { | ||
+ | return $x + $y; | ||
+ | }; | ||
+ | |||
+ | $fn3 = function ($x) use ($y) { | ||
return $x + $y; | return $x + $y; | ||
}; | }; | ||
</ | </ | ||
- | ===== Proposal ===== | + | ==== No explicit capture |
- | ==== Background ==== | + | Explicit capture is not included in the new syntax. |
- | As of PHP 8.1, the following syntaxes around functions have the following meaning: | + | ==== Syntax ==== |
+ | |||
+ | The signature accepts the same syntax as that of Arrow Functions: | ||
<code php> | <code php> | ||
+ | fn () { } | ||
+ | fn ($a, $b) { } | ||
+ | fn ($a, ...$args) { } // Variadic parameter | ||
+ | fn (int $a): string { } // Type hints | ||
+ | fn ($a = 42) { } // Parameter default value | ||
+ | fn &($a) { } // Return by-reference | ||
+ | fn (&$a) { } // Pass by-reference | ||
+ | </ | ||
- | // A named, globally available function. | + | The signature must be followed by '' |
- | // No variables are auto-captured from the environment. | + | |
- | // The body is a statement list, with possibly a return statement. | + | |
- | function foo($a, $b, $c): int { | + | |
- | return $a * $b * $c; | + | |
- | } | + | |
- | // An anonymous, locally available function. | + | <code php> |
- | // Variables are explicitly captured lexically. | + | fn () { return |
- | // The body is a statement list, with possibly a return | + | fn () { print 1; } |
- | $foo = function | + | fn () { |
- | return $a * $b * $c; | + | |
- | }; | + | return |
- | + | } | |
- | // An anonymous, locally available function. | + | |
- | // Variables are auto-captured lexically. | + | |
- | // The body is a single-expression, | + | |
- | $foo = fn($a, $b): int => $a * $b * $c; | + | |
</ | </ | ||
- | That is, a function may be named or local/ | + | Note that Short Closures with a multi-statement |
- | The declined [[rfc:short-functions|Short Functions]] RFC sought to add one additional combination: | + | The syntax choice here is consistent with other language constructs: |
- | This RFC seeks to add a different combination: | + | * '' |
+ | * Conversely, the '' | ||
+ | * The '' | ||
+ | * The '' | ||
- | The remaining potential combinations would be: | + | These rules are easily recognizable and learnable by developers. |
- | * named function, auto-capture, | + | ===== Why extend Arrow Functions? ===== |
- | * named function, auto-capture, | + | |
- | * anonymous function, manual-capture, | + | |
- | None of these additional variants are included in this RFC. | + | Arrow Functions were added as an alternative to Anonymous Functions. The latter can be quite verbose, even when they only perform a simple operation. This is due to a large amount |
- | ==== Auto-capture multi-statement closures ==== | + | While Arrow Functions solve this problem to some extent, the one-expression limit can lead to one-liners with non ideal readability, |
- | Specifically, this RFC adds the following | + | As an example, writing |
<code php> | <code php> | ||
- | // An anonymous, locally available function. | + | $guests |
- | // Variables are auto-captured lexically. | + | $guest = $repository-> |
- | // The body is a statement list, with possibly a return statement; | + | return $guest !== null && in_array($guest-> |
- | $c = 1; | + | }); |
- | $foo = fn($a, $b):int { | + | |
- | $val = $a * $b; | + | |
- | return $val * $c; | + | |
- | }; | + | |
</ | </ | ||
- | The syntax choice here leads to the following consistent syntactic meanings: | + | ===== Discussion on auto-capture ===== |
- | * The '' | + | Auto capture |
- | * '' | + | |
- | * The '' | + | |
- | * The '' | + | |
- | * A function with a name is declared globally at compile time. A function without a name is declared locally as a closure at runtime. | + | |
- | These rules are easily recognizable | + | In the past, there had been reticence about auto-capture that has kept it out of evolutions in closures. |
- | The '' | + | Implementation difficulties arise from by-reference or by-variable semantics, especially when supporting dynamic means of accessing |
- | <code php> | + | As noted in the benchmarks section, the implementation offered here has effectively no performance impact either way. |
- | $c = 1; | + | |
- | $foo = fn($a, $b) use (&$c):int { | + | In the majority of cases where closures are used in practice, the code involved is short enough that debugging is not hampered by automatic capture. |
- | $val = $a * $b; | + | |
- | | + | Potential confusing behavior is further mitigated by PHP' |
- | }; | + | |
- | </ | + | Furthermore, |
+ | |||
+ | For those few cases in which, for whatever reason, the developer is concerned about auto-capture reducing debugability or about accidental capture, the existing explicit-only syntax remains valid and unchanged. | ||
- | In practice, we anticipate | + | ==== Using variables from the parent block ==== |
- | ==== Explicit capture ==== | + | Using variables from the parent block is not unusual in PHP. We do it all the time in loops. |
- | The proposed syntax supports explicit capture with the '' | + | In the following example, the loop uses three variables from the parent block. We have learned to recognize that what follows a '' |
<code php> | <code php> | ||
- | $c = 1; | + | $guests |
- | fn () use ($a, &$b) { | + | foreach |
- | | + | $guest = $repository-> |
- | // $b is explicitly captured by reference | + | if ($guest !== null && in_array($guest->id, $guestsIds)) { |
- | // | + | $guests[] = $guest; |
+ | } | ||
} | } | ||
</ | </ | ||
- | This allows auto-capturing multi-statement closures to match long closures in functionality. Without this, it could be necessary to switch back and forth between | + | In the following example, the function uses two variables from the parent block, which should not be more surprising than with a loop once we have learned that what follows a '' |
- | We expect that explicitly capturing by value will be rare in practice. | + | <code php> |
+ | $guests = array_filter($users, | ||
+ | $guest = $repository-> | ||
+ | return $guest !== null && in_array($guest-> | ||
+ | }); | ||
+ | </ | ||
- | ==== Auto-capture semantics ==== | + | However the comparison stops here. These two examples do not behave equally with regard to side effects: Variable assignments to the '' |
- | The auto-capture semantics presented here are designed to be intuitive and have negligible performance impact. | + | ==== Capture is by-value, no unintended side-effects ==== |
- | Auto-capturing multi-statement closures can access all variables | + | It is important to note that the default capture mode in Anonymous Functions, Arrow Functions, and Short Closures is by-value. This purposefully differs from the semantics commonly found in other programming languages. |
+ | |||
+ | A by-value capture means that it is not possible to modify any variables | ||
<code php> | <code php> | ||
$a = 1; | $a = 1; | ||
- | $b = 2; | ||
$f = fn () { | $f = fn () { | ||
- | | + | $a++; // Has no effect outside of the function |
+ | $tmp = $a + 1; // Has no effect outside of the function | ||
+ | return | ||
}; | }; | ||
- | $f(); // prints "3" | + | print $a; // prints " |
+ | $f(); | ||
+ | print $a; // prints "1" | ||
</ | </ | ||
- | Accessed variables are bound //by value// at the time of the function | + | Conversely, |
<code php> | <code php> | ||
Line 160: | Line 186: | ||
$f(); // prints " | $f(); // prints " | ||
</ | </ | ||
+ | |||
+ | Because variables are bound by-value, the confusing behaviors often associated with closures do not exist. As an example, the following code snippet demonstrates such a behavior in JavaScript: | ||
+ | |||
+ | <code javascript> | ||
+ | // JavaScript | ||
+ | var fns = []; | ||
+ | for (var i = 0; i < 3; i++) { | ||
+ | fns.push(function() { | ||
+ | console.log(i); | ||
+ | }); | ||
+ | } | ||
+ | for (var k in fns) { | ||
+ | var fn = fns[k]; | ||
+ | fn(); // Prints " | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | In PHP the behavior is intuitive and less confusing: | ||
<code php> | <code php> | ||
- | $a = 1; | + | // PHP |
- | $f = fn () { | + | $fns = []; |
- | $a++; | + | for ($i = 0; $i < 3; $i++) { |
+ | $fns[] = fn () { | ||
+ | print $i; | ||
+ | }; | ||
+ | } | ||
+ | foreach ($fns as $fn) { | ||
+ | $fn(); // Prints " | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | In JavaScript the same output can be obtained by declaring '' | ||
+ | |||
+ | In PHP, the variable is captured by-value, thus entirely avoiding the confusion. | ||
+ | |||
+ | Of course, functions can have side-effects when accessing mutable values such as objects or resources. The following example demonstrates this: | ||
+ | <code php> | ||
+ | $d = new DateTime(); | ||
+ | |||
+ | $fn1 = fn () { | ||
+ | $d-> | ||
}; | }; | ||
- | print $a; // prints " | + | $fn2 = function () use ($d) { |
- | $f(); | + | $d-> |
- | print $a; // prints " | + | }; |
+ | |||
+ | $fn3 = function | ||
+ | $d-> | ||
+ | }; | ||
</ | </ | ||
- | Because variables are bound by value, the potential for " | + | ===== Auto-capture semantics ===== |
- | This is the behavior of long closures with explicit | + | The RFC inherits |
- | For performance reasons, only the variables that are directly accessed with the variable | + | > Short Closures can access a snapshot of the variable |
- | Additionally, | + | This can also be stated as follows: |
- | We can express | + | > Short Closures |
- | The "at least" part has only marginal effect aside from performance, | + | This is implemented by binding the value of the declaring scope variables to local variables in the function. This is referred to as //capture// in this RFC. |
- | ==== Implementation details | + | This RFC leaves unspecified which variables are captured, as long as these semantics are maintained. |
+ | |||
+ | ==== Optimization | ||
- | Auto-capturing | + | A naive approach would capture //all// the variables |
<code php> | <code php> | ||
$tmp = 5; | $tmp = 5; | ||
- | fn() { | + | fn () { |
$tmp = foo(); | $tmp = foo(); | ||
bar($tmp); | bar($tmp); | ||
Line 197: | Line 266: | ||
</ | </ | ||
- | A naive capture mechanism | + | This approach |
- | Capture analysis, | + | The implementation proposed in this RFC prevents this by attempting to capture |
- | In practice, auto-capturing multi-statement closures end up capturing the same set of variables | + | These implementation details are irrelevant for most purposes, as they do not have an effect |
- | This retains | + | * If there is a possibility that a variable may be read by the function before binding it, it is captured |
+ | * When inspecting the code, the following operations are assumed to always bind a variable without reading it: | ||
+ | * Variable assignments | ||
+ | * Variable assignments by reference | ||
+ | * '' | ||
+ | * '' | ||
+ | * '' | ||
+ | * This excludes assignments to object properties (they never bind the variable), assignments to array dimensions (they read the variable) | ||
+ | * In all other situations in which a variable is used, it is assumed that it is read | ||
- | ==== Benchmarks ==== | + | This optimization is not applied to Arrow Functions because variable bindings are unusual in these functions. |
- | In benchmarks, the implementation in the 1.0 version | + | ==== Observable effects |
- | The 2.0 version, proposed here, has only marginal | + | As long as the semantics are maintained, whether a variable is captured or not is largely irrelevant for most purposes, and can be observed |
- | The capture | + | * When debugging: Whether a variable is captured or not may be visible in the list of variables in scope in debuggers. Captured variables are local variables in the Closure, initialized to the captured value.\\ \\ |
+ | * Via reflection: Captured variables will be visible in ReflectionFunction.\\ \\ | ||
+ | * Via dynamic variable access: Means to access variables dynamically, | ||
+ | * Via destructors: | ||
+ | * Via resource usage: Capturing too much could increase memory or CPU usage. | ||
- | For more benchmark | + | ==== Implementation |
- | ==== Changes to arrow functions ==== | + | The capture analysis used in this RFC will only capture the variables that may be read before being assigned by the function. This uses the Optimizer' |
- | This RFC proposes to change | + | This maintains |
- | There is a very rare case of breaking change, described in the breaking changes section. | + | ===== Benchmarks ===== |
- | ==== Why add another function mechanism? ==== | + | In benchmarks, the implementation in the 1.0 version of this RFC showed a notable CPU and memory increase when using auto-capturing multi-statement closure in some cases. |
- | Long Closures in PHP can be quite verbose, even when they only perform a simple operation. This is due to a large amount | + | The 2.0 version, proposed here, has only marginal impact compared |
- | While one-line arrow functions solve this problem to some extent, there are ample cases that require a 2-3 statement body. That is still short enough that the chances of a developer confusing in-function and out-of-function variables is very remote, but the burden of manually closing over 3-4 variables is relatively high. | + | The capture analysis approach described above makes Short Closures as efficient as Anonymous Functions. |
- | One example is when you are within a class method with multiple arguments and you want to simply return a closure that uses all the arguments, using the “use” keyword to list all the arguments is entirely redundant and pointless. | + | For more benchmark details, see: https:// |
- | + | ||
- | Then there are often use-cases with '' | + | |
- | + | ||
- | The trend in PHP in recent years has been toward more compact but still readable syntax that eliminates redundancy. | + | |
- | + | ||
- | ==== Methods ==== | + | |
- | + | ||
- | As methods cannot be anonymous, there are no impacts on methods from this RFC. | + | |
- | ==== What about long-closures? ==== | + | ===== What about Anonymous Functions? ===== |
- | The existing | + | The existing |
- | ==== Multi-line expressions ==== | + | ===== Multi-line expressions |
There has been related discussion of multi-line expressions, | There has been related discussion of multi-line expressions, | ||
Line 251: | Line 324: | ||
$c = ...; | $c = ...; | ||
$ret = match ($a) { | $ret = match ($a) { | ||
- | 1, 3, 5 => (fn() { | + | 1, 3, 5 => (fn () { |
$val = $a * $b; | $val = $a * $b; | ||
return $val * $c; | return $val * $c; | ||
})(), | })(), | ||
- | 2, 4, 6 => (fn() { | + | 2, 4, 6 => (fn () { |
$val = $a + $b; | $val = $a + $b; | ||
return $val + $c; | return $val + $c; | ||
Line 264: | Line 337: | ||
While sub-optimal, | While sub-optimal, | ||
- | ==== Examples | + | ===== Comparison to other languages ===== |
- | Closures | + | As far as we are aware, only two languages in widespread use require variables |
- | <code php> | + | Languages commonly capture by-variable |
- | $x = function | + | |
- | // ... | + | |
- | }; | + | |
- | </ | + | |
- | From Mark: "That was just to get those variables inside a callback that could be | + | ===== History ===== |
- | invoked inside a throw-aware buffering helper." | + | |
- | Another similar example is for wrapping behavior in a transaction. Often, that is done by passing a callable to an '' | + | The first discussion [[https:// |
- | <code php> | + | In the same and subsequent discussions [[https:// |
- | public function savePost($user, $date, $title, $body, $tags) { | + | |
- | return $this->db-> | + | |
- | $this-> | + | |
- | $this-> | + | |
- | return $this-> | + | |
- | }); | + | |
- | } | + | |
- | </ | + | |
- | In this case, the '' | + | It is unclear whether |
- | As noted, inline callbacks may also need to capture | + | The [[rfc: |
- | <code php> | + | The [[rfc: |
- | /** @var Product[] */ | + | |
- | $arr = [ ... ]; | + | |
- | $wantApproved | + | ===== Alternative implementations ===== |
- | $size = ' | + | |
- | $filtered = array_filter($arr, function ($item) use ($wantApproved, $size): bool { | + | A few people suggested implementing the same functionality via a different syntax, that is, basing it on the long-closure syntax with a '' |
- | if ($wantApproved) { | + | |
- | return $item-> | + | |
- | } else if ($size) { | + | |
- | return $item->size() == $size; | + | |
- | } else { | + | |
- | return false; | + | |
- | } | + | |
- | }); | + | |
- | </ | + | |
- | In this case, again, | + | The resulting behavior in either |
- | ==== Comparison | + | - The longer form introduces more visual noise to achieve the same result. |
+ | - PHP developers have been using the '' | ||
+ | - With the improved capture logic, many of the arguments for the explicit capture syntax go away. | ||
+ | - Using the longer '' | ||
+ | - If converting from a single line short-lambda to a 2 line closure, switching to the long-form syntax is more work than just switching '' | ||
- | As far as we are aware, only two languages in widespread use require variables to be explicitly closed over: PHP and C++. All other major languages capture implicitly, as is proposed | + | For those reasons, the authors went with the '' |
- | + | ||
- | Many languages tend to capture by variable. In practice this can lead to surprising effects, especially in loops. | + | |
===== Backward Incompatible Changes ===== | ===== Backward Incompatible Changes ===== | ||
- | Changing capture analysis in arrow functions in an incompatible change in rare occurrences. Impacted functions are those that access a variable indirectly before assigning the same variable, like this: | + | None. |
- | + | ||
- | <code php> | + | |
- | $var = ' | + | |
- | fn () => $$var && $a = 1; | + | |
- | </ | + | |
- | + | ||
- | Occurrences of this should be rare because assignments in arrow functions have no effect on the outer scope. | + | |
===== Proposed PHP Version(s) ===== | ===== Proposed PHP Version(s) ===== | ||
Line 344: | Line 387: | ||
===== Future Scope ===== | ===== Future Scope ===== | ||
- | The proposal section detailed three additional possible combinations of function functionality that are not included here. While it is not likely that they have much use, the pattern here clearly lays out what they would be were a future RFC to try and implement | + | These are some possible future extensions, but the authors don't necessarily endorse |
- | Specifically, | + | ==== Explicit use list on Short Closures ==== |
+ | |||
+ | It would be possible to extend the Short Closure syntax to allow an explicit use list: | ||
<code php> | <code php> | ||
- | // Global scope | + | $fn = fn () use ($a, &$b) { |
- | $c = 1; | + | }; |
+ | </ | ||
- | fn foo($a, $b): int { | + | One anticipated use-case is to selectively capture some variables by-reference. |
- | $val = $a * $b; | + | |
- | return $val * $c; | + | |
- | } | + | |
- | fn foo($a, $b): int => $a * $b * $c; | + | There are at least two possible variations of this extension. In one of them, the use list is merged with auto-capture, |
- | $foo = function($a, | + | This RFC initially proposed the first possibility. This is not included in the current version because this appeared to create confusion. |
- | </ | + | |
- | Those versions are //not// included in this RFC. | + | ==== Optimize Arrow Functions ==== |
- | ===== Proposed Voting Choices ===== | + | This RFC proposes an optimized auto-capture. It would be possible to apply this optimization to Arrow Functions as well, but this would be a breaking change in some rare cases. |
- | This is a simple Yes/No vote, requiring 2/3 to pass. | + | This is not included in this RFC because most Arrow Functions would not benefit from this. |
+ | |||
+ | ===== Vote ===== | ||
+ | |||
+ | This is a simple Yes/No vote, requiring 2/3 to pass. Vote ends on 15 July 2022. | ||
+ | |||
+ | |||
+ | <doodle title=" | ||
+ | * Yes | ||
+ | * No | ||
+ | </ | ||
===== Patches and Tests ===== | ===== Patches and Tests ===== | ||
Line 380: | Line 432: | ||
===== References ===== | ===== References ===== | ||
- | [[rfc: | + | * [[rfc: |
+ | * [[rfc: | ||
+ | * [[rfc: | ||
===== Changelog ===== | ===== Changelog ===== | ||
- | 2.0: Updated for new patch; reduced discussion of short-function RFC and related topics; expanded discussion of the capture rules and noted benchmarks showing minimal performance impact | + | 2.0: Updated for new patch; reduced discussion of short-function RFC and related topics; expanded discussion of the capture rules and noted benchmarks showing minimal performance impact; renamed to "Short Closures 2.0" |
rfc/auto-capture-closure.1653596268.txt.gz · Last modified: 2022/05/26 20:17 by crell