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

Returning subroutines

A subroutine can also return a subroutine. This lets you build new functions on the fly, tailored to the arguments you give:

sub adder($n) {
    sub ($x) { $x + $n };
}

my &add5 = adder(5);
say add5(3); # 8

adder(5) builds and returns an anonymous subroutine that adds 5 to its argument. We store it in &add5 and call it like any other subroutine. Calling adder(10) would give a different adder that adds ten.

Notice that the returned subroutine remembers the value of $n from the call that created it — add5 keeps its 5 even after adder has finished. A subroutine that captures values from the scope where it was created is called a closure, the subject of the next section.

Returning functions is a compact way to produce families of related operations without repeating yourself: one adder definition yields an unlimited number of specific adders.

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