====== PHP RFC: Power Operator ======
  * Version: 0.3
  * Date: 2013-11-23
  * Author: Tjerk Meesters, datibbaw@php.net
  * Status: Implemented in PHP 5.6
  * First Published at: http://wiki.php.net/rfc/pow-operator
  * Revision (0.1 → 0.2): 2013-12-19
  * Revision (0.2 → 0.3): 2013-12-21

===== Introduction =====

This proposal is two-fold:

  - Introduce an exponential (right associative) operator ''<nowiki>**</nowiki>''.
    * Avoids a function call.
    * Support for [[rfc:operator_overloading_gmp|GMP overloading]].
    * Easier to read and shorter to write.
    * Can be found in other languages.
  - Introduce an exponential assignment operator ''<nowiki>**=</nowiki>''

===== Proposal =====

A short working example:

<code php>
echo 2 ** 3; // 8
</code>

It supports [[operator_overloading_gmp|GMP overloading]]:

<code php>
$base = gmp_init(2);
$exponent = 3;
var_dump($base ** $exponent);

// output
object(GMP)#3 (1) {
  ["num"]=>
  string(1) "8"
}
</code>

Example of exponent assignment:

<code php>
$x = 2;
$x **= 3;
echo $x; // 8
</code>

**Important**

The proposed associativity is right, just like how power towers work.

The operator precedence is:
  * higher than the bitwise not (~) and unary minus,
  * lower than array dereferencing.

Examples:

<code php>
echo 2 ** 3 ** 2; // 512 (not 64)
echo -3 ** 2; // -9 (not 9)
echo 1 - 3 ** 2; // -8
echo ~3 ** 2; // -10 (not 16)
</code>

===== Changelog =====

21-Dec-2013: 
  * Removed turning pow() into a language construct from the proposal, due to BC breaks.
  * Closed vote and moved RFC back to Discussion status.
  * Reverted commit [[https://github.com/datibbaw/php-src/commit/f60b98cf7a8371233d800a6faa286ddba4432d02|f60b98c]].

22-Dec-2013:
  * Moved RFC back to Voting status

===== Discussion =====

> Should ''-3 <nowiki>**</nowiki> 2'' evaluate to ''9'' instead of ''-9''?

According to the following resources, the scale tips more towards having the exponent precede the unary minus:

  * http://mathforum.org/library/drmath/view/53194.html
  * http://math.stackexchange.com/questions/491933/exponent-rules-with-negative-numbers
  * http://math.stackexchange.com/questions/68833/what-does-22-evaluate-to/68834#68834

**Similar languages**

  * Ada
  * D
  * F#
  * Fortran
  * Freemat
  * Haskell
  * Mathematica / Matlab / Scilab
  * Octave
  * Perl
  * Python
  * R
  * Ruby
  * Sage
  * VB / Basic

**Dissimilar languages**

  * Bash
  * Cobol
  * ColdFusion
  * Excel
  * Tcl

----

> Should ''2 <nowiki>**</nowiki> 3 <nowiki>**</nowiki> 2'' yield ''64'' (left associative), ''512'' (right associative) or throw an error (non associative)?

The exponent operator evaluation order should be based on [[http://en.wikipedia.org/wiki/Tetration|Tetration]] and therefore be right associative. 

**Languages with left associative exponential operator**

  * VB (not by choice imho)
  * Basic
  * Octave
  * Matlab
  * ColdFusion

** Languages with right associative exponential operator**

  * D
  * Haskell
  * R
  * F#
  * Ruby
  * Perl
  * Python
  * Mathematica
  * Freemat
  * Scilab
  * Tcl (changed from left associative!)
  * Cobol
  * Fortran
  * Sage
  * Bash

**Languages with non associative exponential operator**

  * Ada

**Sources**
  * https://plus.google.com/u/0/104277466162910953762/posts/e3jCt51VfmD
  * http://perldoc.perl.org/perlop.html
  * http://www.tcl.tk/cgi-bin/tct/tip/274.html
  * http://book.realworldhaskell.org/read/getting-started.html
  * http://www.nku.edu/~foxr/CSC407/NOTES/ch7.ppt‎ (second slide)

===== Proposed PHP Version(s) =====

PHP 5.6

===== Impact to Existing Extensions =====

The opcode ''ZEND_POW <165>'' and ''ZEND_ASSIGN_POW <166>'' is added.

External extensions such as [[http://pecl.php.net/package/vld|vld]] or [[https://github.com/krakjoe/phpdbg|phpdbg]] would have to be updated, but I'm not aware of any core extensions that would otherwise be affected.

===== Vote =====

Voting will be based on the following:

  * Add the power operators ''<nowiki>**</nowiki>'' and ''<nowiki>**</nowiki>='',
  * Add ''ZEND_POW'' and ''ZEND_ASSIGN_POW'' opcodes.

Changes from 0.1:

  * A option is added to vote for a non-associative ''<nowiki>**</nowiki>'' operator. See also: Discussion. This counts as an inclusion vote.

A two third majority is required for acceptance.

----

<doodle title="Should PHP get a power operator in 5.6?" auth="datibbaw" voteType="single" closed="true">
   * Yes, right associative
   * Yes, non associative
   * No
</doodle>

----

Voting ends on 5th of January 2014.



===== Patches and Tests =====

Power operator PR: https://github.com/php/php-src/pull/543

PHP 5.6 Commits: [[http://git.php.net/?p=php-src.git;a=commit;h=aff56f3c4539869910cf2778cf0ece2d8c2dd671|1]] [[http://git.php.net/?p=php-src.git;a=commit;h=363ff60475d93716722034b8f7a2486229bf4cfb|2]]