Course of Raku / Advanced / More about built-in types / Sequences

Lazy and infinite sequences

A sequence does not have to compute all of its values at once. Raku sequences are lazy: each value is produced only when it is actually needed. This makes it possible to describe a sequence that has no end at all.

To write an infinite sequence, use Inf as the endpoint. Wrap the sequence in parentheses, because ... binds more loosely than =:

my $naturals = (1 ... Inf);

Without the parentheses, my $naturals = 1 ... Inf would assign just 1 to the variable and then build the sequence on its own — and evaluating an endless sequence for no result would hang the program.

Nothing is computed yet. The sequence will hand out values only as you ask for them. The head method takes the first few:

say (1 ... Inf).head(5); # (1 2 3 4 5)

Even though the sequence is infinite, only the first five numbers are generated. Without laziness, a program that tried to build all of an endless sequence would never finish.

For a finite sequence, the tail method takes values from the end instead:

say (1 ... 100).tail(3); # (98 99 100)

Laziness is what lets you separate the description of a sequence from the decision of how much of it to use.

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