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README

NAME
    Syntax::Keyword::Gather - Implements the Perl 6 'gather/take' control
    structure in Perl 5

VERSION
    version 1.003001

SYNOPSIS
     use Syntax::Keyword::Gather;

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

    or to use the stuff that Sub::Exporter gives us, try

     # this is a silly idea
     use syntax gather => {
       gather => { -as => 'bake' },
       take   => { -as => 'cake' },
     };

     my @vals = bake { cake (1...10) };

DESCRIPTION
    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. This module implements
    that control structure.

    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:

     print do {
        my @wanted;
        while (my $line = <>) {
           push @wanted, $line  if $line =~ /\D/;
           push @wanted, -$line if some_other_condition($line);
        }
        push @wanted, 'EOF';
        join q{, }, @wanted;
     };

    instead we can write:

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

    and instead of:

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

    we could write:

     my $text = join q{}, gather {
        while (<>) {
           next if /^#|^\s*$/;
           last if /^__[DATA|END]__\n$/;
           take $_;
        }
     };

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

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

    Note that -- as the 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.

    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:

     my @odds = gather {
        for @data {
           take $_ if $_ %2;
           take to_num($_) if /[one|three|five|seven|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 (&@) {
       my $coderef = shift;
       my @list = @{shift @_};

       return gather {
          take $coderef->($_) for (@list)
       };
     }

     sub grep (&@) {
       my $coderef = shift;
       my @list = @{shift @_};

       return gather {
          take $_ if $coderef->($_) for @list
       };
     }

    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>;
        while (1) {
           if ( defined $a && defined $b ) {
              if    ($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 }
        }
     }

    If you like it really short, you can also "gather"/"take" $_ magically:

    my @numbers_with_two = gather { for (1..20) { take if /2/ } }; #
    @numbers_with_two contains 2, 12, 20

    Be aware that $_ in Perl5 is a global variable rather than the current
    topic like in Perl6.

HISTORY
    This module was forked from Damian Conway's Perl6::Gather for a few
    reasons.

    to avoid the slightly incendiary name
    to avoid the use of the Perl6::Exporter
    ~ doesn't overload to mean string context

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

     my @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.

AUTHORS
    *   Arthur Axel "fREW" Schmidt <frioux+cpan@gmail.com>

    *   Damian Conway

COPYRIGHT AND LICENSE
    This software is copyright (c) 2014 by Arthur Axel "fREW" Schmidt.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.

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