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--- rfc:void_return_type [2015/02/14 03:17] – ajf
+++ rfc:void_return_type [2017/09/22 13:28] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== PHP RFC: Void Return Type ======
-  * Version: 0.1
+  * Version: 0.2.1
-  * Date: 2015-02-14
+  * Date: 2015-02-14 (v0.1, later withdrawn), 2015-10-14 (v0.2, revival)
   * Author: Andrea Faulds, ajf@ajf.me
-  * Status: Under Discussion
+  * Status: Implemented (PHP 7.1)
   * First Published at: http://wiki.php.net/rfc/void_return_type
 ===== Introduction =====
-The [[rfc:return_types|Return Types]] RFC has introduced return types to PHP. While there is already a means to specify that any value may be returned (by omitting the return type), there is no means to specify that no value can be returned, unlike many other languages with return types. This is unfortunate, as this can be useful for documentation and error-checking purposes. In particular, it makes it clear that a function performs an action, rather than producing a result. This RFC proposes the introduction of a ''void'' return type for this purpose.
+The [[rfc:return_types|Return Types]] RFC has introduced return types to PHP. While there is already a means to specify that any value may be returned (by omitting the return type), there is no way to specify that //no// value should be returned, unlike many other languages with return types. This is unfortunate, as this can be useful for documentation and error-checking purposes. In particular, it makes it clear that a function performs an action, rather than producing a result. This RFC proposes the introduction of a ''void'' return type for this purpose, similar to that in other programming languages.
 ===== Proposal =====
@@ Line 16: / Line 16: @@
 <code php>
 function should_return_nothing(): void {
-    return 1; // Catchable fatal error: should_return_nothing() must not return a value, integer returned
+    return 1; // Fatal error: A void function must not return a value
 }
-should_return_nothing();
 </code>
-It accepts either an implicit return, or an explicit value-less return:
+Unlike other return types which are enforced when a function is called, this type is checked at compile-time, which means that an error is produced without the function needing to be called.
+A function with a ''void'' return type, or //void function//, may either return implicitly, or have a return statement without a value:
 <code php>
@@ Line 27: / Line 28: @@
     // valid
 }
-lacks_return();
+</code>
+<code php>
 function returns_nothing(): void {
     return; // valid
 }
-returns_nothing();
 </code>
-However, it does not accept returning a ''NULL'' value:
+A void function may not return a value:
 <code php>
-function returns_null(): void {
+function returns_one(): void {
-    return NULL; // Catchable fatal eerror: returns_null() must not return a value, null returned
+    return 1; // Fatal error: A void function must not return a value
 }
-returns_null();
 </code>
-This might seem odd given that ''return;'' is technically equivalent. However, allowing ''NULL'' would also allow this:
 <code php>
-function returns_null2(): void {
+function returns_null(): void {
-    $x = some_computation(); // produces null most of the time
+    return null; // Fatal error: A void function must not return a value
-    return $x; // Catchable fatal error: returns_null() must not return a value, null returned
 }
-returns_null2();
 </code>
-If we were to allow returning a value which evaluates to null, we'd have to allow the above, and this doesn't make sense: a ''void'' function shouldn't be trying to return a value.
+Note that ''void'' is only valid as a return type, not as a parameter type:
-Furthermore, there is a code style point to be made here. ''return;'' signals different intent to ''return NULL;''. The former returns control flow and, while it does implicitly produce a null return value, signals that the return value is unimportant. The latter, on the other hand, explicitly produces a null return value, suggesting that the null value is significant.
-The ''void'' type cannot be used for parameters:
 <code php>
@@ Line 80: / Line 72: @@
 }
 </code>
+===== Rationale =====
+==== Why check at compile-time? ====
+Generally-speaking, you should warn of problems sooner rather than later. It just so happens that it's trivial (a three-line change) to make PHP to check this at compile-time rather than run-time. So, this proposal suggests just that.
+==== Why isn't ''return null;'' permitted? ====
+Some people have asked about this, since ''return;'' and ''return null;'' are technically equivalent in PHP; when a return value isn't specified, PHP will produce ''null'' for you. However, choosing one over the other suggests intent. If you specify a value, it suggests the value is significant. In a void function, the return value is insignificant: it's always the same and has no actual usefulness. Specifying it explicitly with ''return null;'' is pointless, because it doesn't really matter what value the function is going to return.
+Since ''void'' signifies an unimportant return value that won't be used, this proposal requires you to not specify one. This extends to ''null'' just as it does to any other value.
+==== Use of void functions in expressions ====
+In some other languages, such as C, a void function can't be used in an expression, only as a statement. Since this RFC adds a way to specify a void function to PHP's syntax, it might be expected the same restriction would now apply in PHP. However, this wouldn't match precedent. PHP has had 'void functions' of a kind since its inception, in the form of built-in functions, which are documented as "void" in the manual. Such functions //can// be used in expressions, unlike in C.
+We could change PHP's rules on void functions and disallow their use in expressions, but this would create a backwards-compatibility issue: it's not inconceivable that existing PHP code relies on being able to call built-in void functions in expressions, and plenty of code assumes that you can take the return value of an arbitrary PHP function (a callback, perhaps).
+Moreover, IDEs and other tools can warn the user when the return value of a void function is being used. It isn't strictly necessary for the language itself to cover this.
+==== Why call it void and not null? ====
+Some have suggested that the return type be named ''null'' instead, since (as previously mentioned) PHP implicitly produces null as the result value for functions which don't explicitly return something, so ''void'' would be almost the same as just enforcing that a function returns null. Plus, ''void'' might suggest a function that can't be used in an expression, whereas ''null'' wouldn't. Also, ''void'' would be a new "type" in a sense, whereas ''null'' is preëxisting.
+The main reason to choose ''void'' over ''null'' is that it is the customary name to use for such a return type. We already use ''void'' rather than ''null'' when documenting functions in PHP, both built-in and userland functions: PHP's function prototypes in source code ([[https://github.com/php/php-src/blob/b00a315806ca1ac24b3e1ff97764f9813ad664e6/ext/standard/var.c#L227|e.g.]]), the PHP manual ([[http://php.net/manual/en/function.var-dump.php|e.g.]]) and phpDocumentor's docblock format ([[http://phpdoc.org/docs/latest/guides/types.html#keywords|see definition]]) all use ''void''. In addition, Hack, a PHP-derived and -compatible language which adds its own typing system, [[http://docs.hhvm.com/manual/en/hack.annotations.types.php|also uses void]]. This is just looking at existing PHP practice; most contemporary programming languages use ''void'' here. Some of these ([[https://en.wikipedia.org/wiki/Void_type#In_C_and_C.2B.2B|C, Objective-C and C++]], [[http://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-Result|Java]], [[https://msdn.microsoft.com/en-us/library/yah0tteb.aspx|C#]], etc.) prohibit the use of a void function in an expression, but note that others ([[http://www.typescriptlang.org/Content/TypeScript%20Language%20Specification.pdf|TypeScript]], [[http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/specialTypes.html#void|ActionScript]], [[https://developer.apple.com/library/watchos/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html|Swift]]) //do// allow void functions in expressions, just as PHP does, by making them implicitly return some unit type. Since ''void'' seems to be the most popular choice for such a return type, both in PHP and elsewhere, why should we name it something different? There's no precedent for it and the name doesn't seem to have been an issue until now.
+The other reason is that ''void'' more clearly conveys that the function is supposed to not return a value, rather than return null specifically.
 ===== Backward Incompatible Changes =====
-Like the scalar types proposed by the [[rfc:scalar_type_hints|Scalar Type Hints]] RFC, the ''void'' return type does not become a reserved word, but is instead restricted from use in class and interface names. This avoids confusion while minimising backwards-compatibility breakage.
+Like the names reserved by the [[rfc:scalar_type_hints_v5|Scalar Type Declarations]] and [[rfc:reserve_more_types_in_php_7|Reserve More Types in PHP 7]] RFCs, the ''void'' return type does not become a reserved word proper, but is instead prohibited from use as the name of a class or interface. This avoids confusion while minimising backwards-compatibility issues. I do not expect that ''Void'' is a very common class name, and so the backwards-compatibility impact should be limited.
 ===== Proposed PHP Version(s) =====
-This is proposed for the next major version of PHP, currently PHP 7.
+This is proposed for the next minor version of PHP, currently PHP 7.1.
-===== Proposed Voting Choices =====
+===== Vote =====
-As this is a language change, a 2/3 majority is required. The vote is a straight Yes/No vote for accepting the RFC merging the patch.
+As this is a language change, a 2/3 majority is required. The vote is a straight Yes/No vote for accepting the RFC and merging the patch.
+Voting started 2015-10-29 and ended <del>10</del> 11 days later on 2015-11-09.
+<doodle title="Accept the Void Return Type RFC for PHP 7.1 and merge patch into master?" auth="ajf" voteType="single" closed="true">
+   * Yes
+   * No
+</doodle>
 ===== Patches and Tests =====
-There is a patch for php-src with tests here: https://github.com/php/php-src/pull/1084 It is based on patch used by the [[rfc:scalar_type_hints|Scalar Type Hints]] RFC, since it already contains the infrastructure for reserving class and interface names for parameter and return types. However, should that RFC fail to pass, it would be possible to base the patch directly on master.
+There is a patch for php-src's master branch with tests: https://github.com/php/php-src/pull/1576
-There is currently no patch for the language specification.
+There is also a patch for the language specification's master branch with tests: https://github.com/php/php-langspec/pull/150
 ===== Implementation =====
+The patch for the Zend Engine was merged into php-src master here: https://github.com/php/php-src/commit/366ba41334870f325f248d8e486e3ebf8bafb984
+The patch for the language specification was merged into master here: https://github.com/php/php-langspec/commit/ad1a8bdba48ed23f118fe743f6930d1d57cd8042
+The feature will go into PHP 7.1.
 After the project is implemented, this section should contain
-  - the version(s) it was merged to
-  - a link to the git commit(s)
   - a link to the PHP manual entry for the feature
 ===== Changelog =====
+  * v0.2.1 - Add subsection explaining name choice
+  * v0.2 - Revived, cleaned up proposal and rationale, added compile-time checking
+  * v0.1.1 - Detailed implicit null return value
   * v0.1 - Initial version

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