Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Higher-order functions

Passing subroutines

To accept a subroutine as a parameter, declare it with the & sigil. Inside the body you can then call it by name:

sub apply(&f, $x) {
    f($x);
}

say apply(* + 3, 10); # 13

The parameter &f receives a piece of code, and f($x) calls it. Here we passed * + 3, a Whatever expression that means “add three to whatever you are given”, so apply(* + 3, 10) computes 13.

You can pass a named subroutine just as easily, by referring to it with the & sigil so that it is passed rather than called:

sub double($n) { $n * 2 }

sub apply(&f, $x) {
    f($x);
}

say apply(&double, 5); # 10

This is exactly how map and grep work: they are subroutines that take another subroutine — your block — and apply it to each element. Writing your own subroutines that take code makes them just as flexible.

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