Subroutines

Declare subs with typed, optional, and named parameters; the last expression is the return value.

sub declares a function. Parameters go in the signature; the last evaluated expression is returned automatically (an explicit return also works):

sub greet($name) {
    "Hello, $name!"
}
say greet('Ada');
say greet('Grace');

Hello, Ada!
Hello, Grace!

Defaults and named parameters #

A parameter can have a default. Prefix one with : and it becomes named — callers pass it by name, in any order:

sub power($base, :$exp = 2) {
    $base ** $exp
}
say power(5);
say power(2, exp => 10);

25
1024

Types #

Add a type to a parameter and Raku enforces it. This signature also shows a return type after the arrow:

sub double(Int $n --> Int) {
    $n * 2
}
say double(21);

42

Call it as double('fish') and you get a type error instead of a silent mistake — try it.

Try it #

Write max2($a, $b) returning the larger of the two (the ternary ?? !! from lesson 5 is perfect here).

sub max2($a, $b) {
}
say max2(3, 8);
say max2(9, 4);

sub max2($a, $b) {
    $a > $b ?? $a !! $b
}
say max2(3, 8);
say max2(9, 4);

8
9