Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Lazy and infinite sequences

The sequence operator

The sequence operator ... builds a list from a starting pattern up to an end value. Give it the first few elements and the endpoint, and it works out how the series continues:

say (2, 4 ... 10); # (2 4 6 8 10)

From 2, 4 it infers “add two each time”, and continues until it reaches 10. The step can be downward too:

say (10, 8 ... 2); # (10 8 6 4 2)

If the elements grow by multiplication rather than addition, the operator notices that as well:

say (1, 2, 4 ... 64); # (1 2 4 8 16 32 64)

Here each term is double the previous one, so the sequence is geometric.

When the rule is more complex than the operator can guess, you can supply it explicitly as a closure of the previous elements. For instance, an explicit “add the step” rule:

say (1, * + 2 ... 9); # (1 3 5 7 9)

The block * + 2 says how to get the next element from the current one. With this, ... can express any sequence you can describe by a rule.

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