loop + default control structure

• Version: 1.0
• Date: 2014-09-18
• Author: Leigh, leight -> gmail
• Status: Draft
• First Published at: http://wiki.php.net/rfc/loop_default

It is often desirable to perform a set of actions if the conditions to enter a loop are not met. This can require a significant amount of boilerplate code if the conditions are complex.

This RFC proposes an optional default control block for loops, that is executed in the event that the loop is not entered.

Pre-condition loops (for, foreach and while) will be allowed to have an optional default block (using the existing default keyword) after their loop body, which is executed when the body of the loop is not entered (condition failed, empty array, etc.).

Usage:

while ($cond) {
	// loop body
}
default {
	// did not enter while loop
}
 
for ($i = 0, $j = $k; $i < 4 && $j < 65536; $i++, $j <<= 1) {
	// loop body
}
default {
	// did not enter for loop
}
 
foreach (generator() as $k => $v) {
	// loop body
}
default {
	// did not enter foreach loop
}

In the case of for and foreach loops it's immediately obvious that separate tracking variables would be required to monitor whether the loop had been entered. This proposal does away with that necessity and increases performance as a side-effect.

do {} while(); loops have been deliberately excluded from this behaviour as they always enter the loop at least once. The only available behaviour for a default block on a do while loop is to have it execute if the loop only iterates once, which feels inconsistent.

Alternate syntax loops also gain this functionality with a default: clause that does not break switch statements.

This type of behaviour has been suggested before usually using the else keyword, however using else has several drawbacks that make default more attractive:

* Not backwards compatible by default due to dangling else

if ($something)
	while ($cond)
		// loop body
else
	// this now belongs to the while loop

* Making it backwards compatible leads to inconsistent behaviour

if ($something)
	while ($cond)
		// loop body
	else
		// this still belongs to the if statement

* It breaks familiarity with similar behaviour in other languages

while ($cond) {
	// loop body
}
else {
	// In Python this will always execute unless break; is used in the loop body
}

This could be solved by introducing a new keyword, however to maintain backward compatibility as far as possible it is more sane to borrow an existing keyword with a similar semantic meaning, in this case from switch(); if the condition is not met then the default is used.

The intention is to implement this by duplicating loop prologues to avoid the requirement for tracking variables, and keep performance on-par with pre-patch looping.

As an example here is the opcode dump of a pre-patch basic while loop.

$i = 3;
while ($i--) {
    print 'loop';
}

line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
   3     0  >   ASSIGN                                                   !0, 3
   4     1  >   POST_DEC                                         ~1      !0
         2    > JMPZ                                                     ~1, ->6
   5     3  >   PRINT                                            ~2      'loop'
         4      FREE                                                     ~2
   6     5    > JMP                                                      ->1
         6  > > RETURN                                                   1

And a post-patch basic while loop with default block (labels added to help visualise flow)

$i = 0;
while ($i--) {
    print 'loop';
}
default {
    print 'default';
}

         # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
         0  >   ASSIGN                                                   !0, 0
cond_1:  1  >   POST_DEC                                         ~1      !0
         2    > JMPZNZ                                   loop            ~1, ->default
cond_2:  3  >   POST_DEC                                         ~1      !0
         4    > JMPZ                                                     ~1, ->nxt_op
loop:    5  >   PRINT                                            ~2      'loop'
         6      FREE                                                     ~2
         7    > JMP                                                      ->cond_2
default: 8  >   PRINT                                            ~3      'default'
         9      FREE                                                     ~3
nxt_op: 10  > > RETURN                                                   1

The key here is that cond_1 uses JMPZNZ to either jump over cond_2 straight to loop or to the default block on the first iteration, but after the loop is entered cond_2 is used for all subsequent iterations and jumps to nxt_op on failure, skipping the default block. for and foreach loops are handled in a similar manner.

Nothing forseen.

PHP.next (whether next is 7.0 or 5.7)

All SAPIs gain the same functionality

No standard extensions should be affected, only the parser and compiler are modified.

TODO - There may be an impact here, but any patch will review Opcache. This section needs commentary from internals.

None

break and continue both continue to function sanely.

Both require the loop to be entered to have an effect, which means the default block cannot be executed at the point these constructs are used.

Loops without bodies also get to have default blocks

There may be an opporunity to support the python style loop+else, but this will require a new keyword to preserve dangling else backward compatibility

This is a language change and requires a 2/3 majority in favour of the feature.

A proof of concept is being worked on by Leigh - while and for are working - foreach currently segfaulting

TODO

Original RFC:

loop_else (by Dmitri Ravazin)

Original discussions in bug tracker:

while {} else {} (by php at bellytime dot com)

Build in foreach else support (by kjarli at gmail dot com)

For... else construct (by jeroenvandenenden at gmail dot com)

2014-09-18 - v1.0 - RFC created