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

gather and take

A gather block collects values. Anywhere inside it — including in loops and subroutines it calls — a take adds a value to the list that gather produces:

my @squares = gather {
    take $_ ** 2 for 1..3;
}

say @squares; # [1 4 9]

The take runs once for each number 1, 2, 3, contributing 1, 4, and 9. The whole gather block evaluates to the list of everything taken.

take can appear under any control flow, which makes gather ideal for building a list with conditions:

my @evens = gather {
    for 1..10 {
        take $_ if $_ %% 2;
    }
}

say @evens; # [2 4 6 8 10]

Here a value is taken only when the if succeeds, so only the even numbers end up in the list. This is often clearer than building the list by hand with push, because the logic reads as a normal loop and take simply marks the values to keep.

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