Author image Moritz Lenz
and 1 contributors


Perl6::GatherTake - Perl 6 like gather { take() } for Perl 5


    use Perl6::GatherTake; 

    my $powers_of_two = gather {
        my $i = 1;
        for (;;) {
            take $i;
            $i *= 2;

    print $powers_of_two->[3], "\n";
    # output: 8


Perl6::GatherTake implements an API for producing partial computation results on the fly, storing them in a lazy list.

A word of warning: This module tries to explore some language concepts. It is not suitable for any productive work.

A gather { ... }; block returns a reference to a (tied) array. Each call to take inside the block pushes its arguments to that array. The block is only run as needed to produce results (but see "BUGS AND LIMITATIONS" below), which means that you can put infinite loops inside the gather block as long as it calls take on a regular basis.

Instead of this common construct:

    my @results;
    for (@data){
        # computations here
        if ($result =~ m/super regex/){
            push @results, $result;

You can now write

    my $results = gather {
        for (@data){
            # computations here
            if ($result =~ m/super regex/){
                take $result;

It has the nice side effect that the computations are only executed as the array elements are accessed, so if the end of the array is never used you can save much time here.

Nested gather { take }; blocks are supported, a take always supplies data to the innermost gather block.

Note that if a gather block is an infinite loop, you're responsible for not accessing all elements. If you do something stupid like iterating over all items, joining them or copying the array (my @other = @$array_ref) you have an infinite loop (until you run out of memory).

Assigning to an array element triggers evaluation until the index of the changed item is reached.


This is a prototype module and is neither stable nor well-tested at the moment.

  • Due to the Coro based implementation (and the author's missing understanding of Coro's concepts) the lazyness is limited: gather-blocks might be run up to the first occurance of take before a element is fetched from the associated array.

  • scalar @$array_ref doesn't return "the right" value for an array reference that is returend by a gather-take block. More precisely it returns the number of already computed values plus one (unless the gather block is exhausted). This means that iterating over for (@$list) will result in an undefined element at the end if the block returns only a finite number of elements.

  • This module consumes much more resources than desirable: for each gather-take-block it (currently) maintains a tied array (which is implemented as a blessed hash) which holds all the computed values so far, a Coro and a Coro::Channel object.

  • take doesn't default to $_.

  • More advanced array operations (like slices, splice etc.) aren't tested yet.


This package is free software, you can use it under the same terms as Perl itself.

All example and test code in this distribution is "Public Domain" (*), i.e. you may use it in any way you want.

(*) German copyright laws always grant the original author some rights, so I can't really place things in the "Public Domain". But don't let that bother you.


Moritz Lenz,, E-Mail <>.


You can obtain the latest development version via subversion:

    svn co

Patches and comments are welcome.