rfc:list_default_value

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rfc:list_default_value [2015/11/09 12:18] – reezerfc:list_default_value [2017/09/22 13:28] (current) – external edit 127.0.0.1
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-====== PHP RFC:  Default Value in List Syntax ======+====== PHP RFC:  Default Value in List Assignment Syntax ======
   * Version: 0.1   * Version: 0.1
   * Date: 2015-11-08   * Date: 2015-11-08
   * Author: Reeze Xia, reeze@php.net   * Author: Reeze Xia, reeze@php.net
-  * Status: Draft+  * Status: Under Discussion
   * First Published at: http://wiki.php.net/rfc/list_default_value   * First Published at: http://wiki.php.net/rfc/list_default_value
  
 ===== Introduction ===== ===== Introduction =====
-We could destruct variable from an array, it may be nested or a simple array. But there is no guarantee that the array can fulfill all variables, it will be simple assigned with null with notice error (not good). We will need several redundant code to handle the possible cases if we want to check it and assign it with default values.+We could destruct variables from an array with [[http://php.net/list| list constuct]], it may be nested or a simple array. But there is no guarantee that the array can fulfill all variables, it will be simple assigned with null with notice error (not good). We will need several redundant code to handle the possible cases if we want to check it and assign it with default values. 
  
-For the similar purpose we introduced  [[http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison|Null coalesce "??"]] and [[http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary|Ternary Operator "?:"]] to help write clean code.+For the similar reason we introduced  [[http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison|Null coalesce "??"]] and [[http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary|Ternary Operator "?:"]] to help write clean code.
  
-One could check the array himself+Some clever users figured out  some workaround like this  [[http://php.net/manual/en/function.list.php#113189|User contributed notes]]. But that is not good enough and ugly.
  
 ===== Proposal ===== ===== Proposal =====
-All the features and examples of the proposal. 
  
-To [[http://news.php.net/php.internals/66051|paraphrase Zeev Suraski]], explain hows the proposal brings substantial value to be considered +Support new syntax to set default values for list elements, when the requested index it is not found in array:
-for inclusion in one of the world's most popular programming languages.+
  
-Remember that the RFC contents should be easily reusable in the PHP Documentation.+<code php> 
 +// basic syntax 
 +list($a, $b='default') = [1];      // a = 1, b = 'default' 
 +list($a, $b='default') = [1, 2];  // a = 1, b = 2
  
-===== Backward Incompatible Changes ===== +//  comparation  
-What breaks, and what is the justification for it?+list($a, list($b=1, $c=2)) = $arr; 
 +// or we need to check it ourself 
 +if (!isset($arr[1][0])) { 
 +    $arr[1][0] = 1; 
 +} 
 +if (!isset($arr[1][1])) { 
 +    $arr[1][0] = 2; 
 +}
  
-===== Proposed PHP Version(s) ===== +list($a, list($b, $c)) = $arr;
-List the proposed PHP versions that the feature will be included in.  Use relative versions such as "next PHP 5.x" or "next PHP 5.x.y".+
  
-===== RFC Impact ===== 
-==== To SAPIs ==== 
-Describe the impact to CLI, Development web server, embedded PHP etc. 
  
-==== To Existing Extensions ==== +// other examples 
-Will existing extensions be affected?+function say_hello() 
 +{ 
 +    return "Hello"; 
 +} 
 +$name = 'PHP'; 
 +list($a=say_hello(), $b=$name."7.0") = []; // a = 'Hello', b = 'PHP7.0'
  
-==== To Opcache ==== +list($a, list($b=1, $c=2)) = [1]; // a = 1, b = 1, c = 2
-It is necessary to develop RFC's with opcache in mind, since opcache is a core extension distributed with PHP.+
  
-Please explain how you have verified your RFC's compatibility with opcache.+</code>
  
-==== New Constants ==== +The assignment could be considered as a shortcut for:
-Describe any new constants so they can be accurately and comprehensively explained in the PHP documentation.+
  
-==== php.ini Defaults ==== +<code php>
-If there are any php.ini settings then list: +
-  * hardcoded default values +
-  * php.ini-development values +
-  * php.ini-production values+
  
-===== Open Issues ===== +list($a, $b='default') = $arr;
-Make sure there are no open issues when the vote starts!+
  
-===== Unaffected PHP Functionality ===== +// equals
-List existing areas/features of PHP that will not be changed by the RFC.+
  
-This helps avoid any ambiguity, shows that you have thought deeply about the RFC's impact, and helps reduces mail list noise.+$a = $arr[0]; 
 +$b = isset($arr[1]) ? $arr[1] : 'default'; 
 +</code> 
 + 
 +This seems not a lot of work than the new syntax. But sometimes we need to destruct nested array and there are morn than two variables, anyway list() itself is designed to avoid assign by hand for short and clean code. 
 + 
 + 
 +The syntax is: 
 + 
 +<code flex> 
 +assignment_list_element: 
 + variable  
 ++ | variable '=' expr # the new syntax. value could be any expression 
 + | T_LIST '(' assignment_list ')' 
 + | /* empty */  
 +</code> 
 + 
 +The syntax is same as function's default value for parameters. 
 + 
 + 
 +We could also set default value in foreach list context: 
 + 
 +<code php> 
 +foreach($arr as list($k, list($v1='deafult', $v2='default2'))) { 
 +    // do something 
 +} 
 +</code> 
 + 
 +More case could be found from PR's tests:  [[https://github.com/php/php-src/pull/1623/files]] 
 + 
 +This feature could also be found in [[https://clojurebridge.github.io/community-docs/docs/clojure/destructuring/|Clojure]] and [[https://hacks.mozilla.org/2015/05/es6-in-depth-destructuring/|Javascript]] for destructing.  
 + 
 +===== Backward Incompatible Changes ===== 
 + 
 +No BC break. 
 + 
 +===== Proposed PHP Version(s) ===== 
 +PHP 7.1 
 + 
 +===== RFC Impact ===== 
 + 
 +==== To Opcache ==== 
 + 
 +I am working on opcache compatibility. 
 + 
 +===== Open Issues ===== 
 +None for now
  
 ===== Future Scope ===== ===== Future Scope =====
-This sections details areas where the feature might be improved in future, but that are not currently proposed in this RFC. 
  
 ===== Proposed Voting Choices ===== ===== Proposed Voting Choices =====
-Include these so readers know where you are heading and can discuss the proposed voting options. 
  
-State whether this project requires a 2/3 or 50%+1 majority (see [[voting]])+* Whether accept the RFC for PHP 7.1 
 + 
 +This project requires a 2/3 majority (see [[voting]])
  
 ===== Patches and Tests ===== ===== Patches and Tests =====
-Links to any external patches and tests go here. 
  
-If there is no patch, make it clear who will create a patch, or whether a volunteer to help with implementation is needed.+* Patch: [[https://github.com/php/php-src/pull/1623]]
  
-Make it clear if the patch is intended to be the final patch, or is just a prototype.+I am working on opcache compatibility
  
 ===== Implementation ===== ===== Implementation =====
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 ===== References ===== ===== References =====
-Links to external references, discussions or RFCs+ 
 +  * Javascript destructing:  [[https://hacks.mozilla.org/2015/05/es6-in-depth-destructuring/]] 
 +  * Clojure destructing [[https://clojurebridge.github.io/community-docs/docs/clojure/destructuring/]]
  
 ===== Rejected Features ===== ===== Rejected Features =====
-Keep this updated with features that were discussed on the mail lists.+ 
 +===== Changelog ===== 
 + 
 +  * v0.1 - Initial version
rfc/list_default_value.1447071502.txt.gz · Last modified: 2017/09/22 13:28 (external edit)

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