Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Iterators

Lazy gather

A gather block is lazy: it does not run all at once. It produces values only as they are pulled out, which means a gather can describe an endless series and still be useful — you simply stop taking values when you have enough.

The catch is that you must keep the result lazy. Storing it in a plain @array would try to collect every value at once, and for an endless series that never finishes. Instead, keep the sequence in a scalar (where it stays a lazy Seq) and pull a slice from it:

my $naturals = gather {
    my $n = 1;
    loop {
        take $n++;
    }
}

say $naturals[^5]; # (1 2 3 4 5)

The loop never ends, but only five values are ever produced, because that is all we asked for with [^5]. You can also pull from the front with .head:

say (gather { my $n = 1; loop { take $n++ } }).head(3); # (1 2 3)

Laziness is what lets gather model infinite streams safely: nothing is computed until it is needed, and you take exactly as much as you want.

← Quiz — gather and take | Gather the cubes →

💪 Or jump directly to the exercises in this section.

Lazy gather

Course navigation