Course of Raku / Advanced / Subroutines

Signatures

In the Essentials part you defined subroutines with positional and named parameters and gave them default values. The list of parameters is called the signature. This section adds two more things a signature can do: mark a parameter as optional, and collect any number of extra arguments.

Optional parameters

A parameter followed by a question mark is optional: the caller may leave it out. When it is left out, the parameter is undefined, so you can supply a fallback with the defined-or operator //:

sub greet($name, $greeting?) {
    my $g = $greeting // 'Hello';
    say "$g, $name!";
}

greet('Anna');           # Hello, Anna!
greet('Anna', 'Hi');     # Hi, Anna!

When greet is called with one argument, $greeting is undefined, so // falls back to 'Hello'.

Slurpy parameters

A parameter marked with a * is slurpy: it gathers all the remaining arguments. A slurpy array, written *@, collects any number of positional arguments into an array:

sub count-them(*@items) {
    say @items.elems;
}

count-them(1, 2, 3, 4); # 4
count-them('a', 'b');   # 2

You can combine ordinary parameters with a slurpy one. The fixed parameters are filled first, and whatever is left goes into the slurpy array:

sub titles($name, *@titles) {
    say "$name has {@titles.elems} title(s)";
}

titles('Anna', 'Dr', 'Prof'); # Anna has 2 title(s)

In the same way, a slurpy hash, written *%, collects any extra named arguments into a hash:

sub register($name, *%options) {
    say "$name: {%options.elems} option(s)";
    say "role is %options<role>";
}

register('Anna', role => 'admin', active => True);

The two named arguments end up as keys of %options, so the program prints:

Anna: 2 option(s)
role is admin

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