Course of Raku / Advanced / Control flow / Statement prefixes 🆕

Laziness and eagerness

Some lists in Raku are computed only as their elements are needed — they are lazy. The lazy and eager prefixes let you control this explicitly.

The eager prefix forces a list to be produced all at once, immediately:

my @squares = eager (1..3).map(* ** 2);
say @squares; # [1 4 9]

Without eager the map would still produce the values, but eager guarantees they are all computed there and then rather than on demand.

The lazy prefix does the opposite: it marks a list as lazy, so its elements are produced only when pulled. This is what lets a list be conceptually infinite:

my $numbers = lazy (1 .. Inf);
say $numbers.is-lazy; # True

The .is-lazy method confirms the list will not try to compute all of its (endless) elements up front.

This particular example is exactly a case where Raku would assume laziness anyway: an unbounded range like 1 .. Inf is already lazy, so say (1 .. Inf).is-lazy prints True without the prefix. Writing lazy here changes nothing — it only makes the intent explicit. The prefix earns its keep when you want to make lazy a list that would otherwise be computed eagerly.

Most of the time Raku chooses sensibly on its own — ranges and sequences are lazy, ordinary array assignment is eager. Reach for these prefixes when you need to override that choice: eager to force an expensive computation to happen now, lazy to defer one that might be large or unbounded.

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