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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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