Course of Raku / Advanced / Subroutines

Nested subroutines

A subroutine may be defined inside another subroutine. A nested subroutine is a normal subroutine, but it is visible only within the subroutine that contains it.

sub outer {
    sub inner {
        say 'I am the inner subroutine';
    }

    inner;
}

outer;

The program prints:

I am the inner subroutine

Here inner is declared inside outer, and outer calls it. This works because inner is in scope throughout the body of outer.

Outside outer, however, the name inner does not exist. The following program fails to compile:

sub outer {
    sub inner { 42 }
}

inner; # error: Undeclared routine: inner

Nesting is useful when a helper routine is needed by one subroutine only. Keeping it inside makes clear that it is a private detail and prevents it from being called from anywhere else by mistake.

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