TITLE

Allow multiple loop variables in foreach statements

VERSION

  Maintainer: Peter Scott <peter@psdt.com>
  Date: 29 Aug 2000
  Last Modified: 6 Sep 2000
  Mailing List: perl6-language@perl.org
  Number: 173
  Version: 2
  Status: Frozen

ABSTRACT

Allow for a list of loop variables in for(each) statements, i.e. & e.g.,

      foreach my ($x, $y, $z) (@list) { ... }

DESCRIPTION

This has been raised by several people in the past, most recently on the perl6-* lists by Graham Barr, who does not have time to RFC it currently. The semantics are (hopefully) obvious: where

      foreach $var (@list)

iterates through @list, assigning $var to each element in turn,

      foreach ($var_1, ... $var_n) (@list)

iterates through @list n elements at a time, assigning $var_1 through $var_n to them respectively. If the number of elements in @list is not evenly divisible by n, the remaining loop variables on the last iteration become undef.

The most obvious usefulness is traversing a hash which has been passed in an argument list:

      sub foo {
          foreach my($key, $value) (@_) { ... }
      }
      foo(%hash);

instead of having to make a temporary copy:

      sub foo {
          my %h = @_;
          while (my ($key, $value) = each %h) { ... }
      }

This extension does not apply to the foreach statement modifier, since that doesn't accept a loop variable anyway.

Alternatives

While splice is frequently adduced as an alternative, this requires an array instead of just a list, and is destructive to boot.

IMPLEMENTATION

The potential parsing difficulty I came up with was how to tell that

      foreach ($a,$b,$c) (@list) ...

was not

      foreach $_ ($a, $b, $c) (@list) ...

Graham tells me that the required parentheses and block braces make it possible for the parser to be able to tell the difference. John Porter suggests an alternative syntax

     foreach [$a, $b, $c] (@list)

which could be employed instead if there is a parsing problem.

REFERENCES

"Foreach Loops" in perlsyn