gather / take

Build a lazy list by taking values from anywhere inside a block.

gather runs a block and collects every value handed to take inside it — however deeply nested — into a single lazy list. It decouples producing values from the loop or logic that decides which to emit.

gather over a loop #

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

[2 4 6 8 10]

Only the numbers that reach a take end up in the list — the if filters inline.

take from nested code #

take can appear anywhere the gather block reaches, including inside called blocks, so production logic can be spread out.

my @out = gather {
    take "start";
    take $_ for <a b>;
    take "end";
}
say @out;

[start a b end]

Notes #

• The result is lazy: gather doesn't run the block until the list is consumed, so gather { … take … } can even be infinite and taken from with [^n].
• take returns its argument, so my $v = take $x both emits and keeps the value.
• Assigning the result to an @array reifies it; a bare gather is the lazy Seq, which you take from with [^n] or reify with .list/.Array.