Exponential operator View as Markdown

Introduction

This proposal is two-fold:

1. Introduce an exponential (right associative) operator **.
   • Avoids a function call.
   • Support for GMP overloading.
   • Easier to read and shorter to write.
   • Can be found in other languages.
2. Introduce an exponential assignment operator **=

Proposal

A short working example:

echo 2 ** 3; // 8

It supports GMP overloading:

$base = gmp_init(2);
$exponent = 3;
var_dump($base ** $exponent);
 
// output
object(GMP)#3 (1) {
  ["num"]=>
  string(1) "8"
}

Example of exponent assignment:

$x = 2;
$x **= 3;
echo $x; // 8

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:

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

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 f60b98c.

22-Dec-2013:

• Moved RFC back to Voting status

Discussion

Should -3 ** 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 ** 3 ** 2 yield 64 (left associative), 512 (right associative) or throw an error (non associative)?

The exponent operator evaluation order should be based on 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 vld or 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 ** and **=,
• Add ZEND_POW and ZEND_ASSIGN_POW opcodes.

Changes from 0.1:

• A option is added to vote for a non-associative ** operator. See also: Discussion. This counts as an inclusion vote.

A two third majority is required for acceptance.

Vote: Should PHP get a power operator in 5.6? — Yes, right associative: 22 · Yes, non associative: 1 · No: 9 (closed)

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: 1 2