Named parameters & arguments

Pass arguments by name with :$x parameters and the :$var / key => value forms.

A parameter written :$name is named, not positional: the caller supplies it by name, in any order. Named arguments make call sites self-documenting, especially when there are several.

Declaring and passing named parameters #

sub area(:$w, :$h) {
    $w * $h;
}
say area(w => 3, h => 4);

12

Order is irrelevant — area(h => 4, w => 3) gives the same result.

The :$var shortcut #

When you already have variables of the same name, :$w at the call site is short for w => $w. This pairs neatly with :$w parameters.

sub area(:$w, :$h) {
    $w * $h;
}
my ($w, $h) = 3, 4;
say area(:$w, :$h);

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Notes #

• Named parameters are optional by default; add ! to require one: :$w!.
• A named parameter can have a default like any other: :$h = 1.
• :$w (colon-pair shorthand), w => $w, and :w($w) are three spellings of the same argument; :w alone is w => True.