Perl6::Gather - Implements the Perl 6 'gather/take' control structure in Perl 5

    use Perl6::Gather;

	@list = gather {
				# Try to extract odd numbers and odd number names...
				for (@data) {
					if (/(one|three|five|nine)$/) { take qq{'$_'}; }
					elsif (/^\d+$/ && $_ %2)      { take; }
				}
				# But use the default set if there aren't any of either...
				take @defaults unless gathered;
		    }

Perl 6 provides a new control structure -- gather -- that allows lists to be constructed procedurally, without the need for a temporary variable. Within the block/closure controlled by a gather any call to take pushes that call's argument list to an implicitly created array. take returns the number of elements it took.

At the end of the block's execution, the gather returns the list of values stored in the array (in a list context) or a reference to the array (in a scalar context).

For example, instead of writing:

    # Perl 6 code...
    print do {
                my @wanted;
                for <> -> $line {
                    push @wanted, $line  if $line ~~ /\D/;
                    push @wanted, -$line if some_other_condition($line);
                }
                push @wanted, 'EOF';
                @wanted;
              };

in Perl 6 we can write:

    # Perl 6 code...
    print gather {
                    for <> -> $line {
                        take $line  if $line ~~ /\D/;
                        take -$line if some_other_condition($line);
                    }
                    take 'EOF';
                 }

and instead of:

	$text = do {
				my $string;
                for <> {
                    next if /^#|^\s*$/;
					last if /^__[DATA|END]__\n$/;
                    $string .= $_;
                }
                $string;
              };

we could write:

	$text = ~gather {
                for <> {
                    next if /^#|^\s*$/;
					last if /^__[DATA|END]__\n$/;
                    take;
                }
              }

As the above example implies, if take is called without any arguments, it takes the current topic.

There is also a third function -- gathered -- which returns a reference to the implicit array being gathered. This is useful for handling defaults:

    @odds = gather {
                for @data {
                    take if $_ % 2;
                    take to_num($_) if /[one|three|five|nine]$/;
                }
                take 1,3,5,7,9 unless gathered;
            }

It's also handy for creating the implicit array by some process more complex than by simple sequential pushing. For example, if we needed to prepend a count of non-numeric items:

    @odds = gather {
                for @data {
                    take if $_ %2;
                    take to_num($_) if /[one|three|five|nine]$/;
                }
                unshift gathered,  +grep(/[a-z]/i, @data);
            }

Conceptually gather/take is the generalized form from which both map and grep derive. That is, we could implement those two functions as:

    sub map ($transform is Code, *@list) {
        return gather {  for @list { take $transform($_) }  };
    }

    sub grep ($selected is Code|Rule, *@list) {
        return gather {  for @list { take when $selected }  }
    }

A gather is also a very handy way of short-circuiting the construction of a list. For example, suppose we wanted to generate a single sorted list of lines from two sorted files, but only up to the first line they have in common. We could gather the lines like this:

    my @merged_diff = gather {
        my $a = <$fh_a>;
        my $b = <$fh_b>;
        loop {
            if defined all $a,$b {
                if    $a eq $b { last }     # Duplicate means end of list
                elsif $a lt $b { take $a; $a = <$fh_a>; }
                else           { take $b; $b = <$fh_b>; }
            }
            elsif defined $a   { take $a; $a = <$fh_a>; }
            elsif defined $b   { take $b; $b = <$fh_b>; }
            else               { last }
        }
    }

The Perl6::Gather module provides the same functionality in Perl 5. So we could code some of the previous examples like so:

    # Perl 5 code...
    use Perl6::Gather;

    print gather {
                    for my $line (<>) {
                        take $line  if $line =~ /\D/;
                        take -$line if some_other_condition($line);
                    }
                    take 'EOF';
                 };

and:

    # Perl 5 code...
    use Perl6::Gather;

	$" = "";
	$text = ~gather {
                for (<>) {
                    next if /^#|^\s*$/;
					last if /^__(?:DATA|END)__$/;
                    take;
                }
              };

and:

    # Perl 5 code...
    use Perl6::Gather;

    @odds = gather {
                for (@data) {
                    take if $_ % 2;
                    take to_num($_) if /(?:one|three|five|nine)\z/;
                }
                take 1,3,5,7,9 unless gathered;
            };

Note that -- as the second example above implies -- the gathered function returns a special Perl 5 array reference that acts like a Perl 6 array reference in boolean, numeric, and string contexts. Note too that that array reference has the ~ operator overloaded to provide string coercion (as in Perl 6).

The syntax and semantics of Perl 6 gather's is still being finalized and consequently is at any time subject to change. The the syntax and semantics of this module will track those changes when and if they occur.

Damian Conway (damian@conway.org)

Perl6::Export

It would be nice to be able to code the default case as:

    @odds = gather {
                for (@data) {
                    take if $_ % 2;
                    take to_num($_) if /(?:one|three|five|nine)\z/;
                }
            } or (1,3,5,7,9);

but Perl 5's or imposes a scalar context on its left argument. This is arguably a bug and definitely an irritation.

Comments, suggestions, and patches welcome.

 Copyright (c) 2003, Damian Conway. All Rights Reserved.
 This module is free software. It may be used, redistributed
 and/or modified under the same terms as Perl itself.