Course of Raku / Essentials / Creating and calling functions

Default values

Both positional and named parameters can have default values. If the function has a default value for the given parameter, that parameter may be skipped when calling the function.

Positional parameters

An example of a default value for a function with positional parameters:

sub greet($name = 'World') {
    say "Hello, $name!";
}

greet('Merinda');
greet();

The first call uses the name passed as the only argument and thus prints Hello, Merinda!. The second call uses the default value and prints Hello, World!.

If a function has more than one parameters, then the default values can only be used at the end of the list of them:

sub f($a, $b, $c = 42, $d = 50) { . . . } # correct
# sub f($a = 10, $b = 20, $c, $d) { . . . } # WRONG

Named parameters

The same syntax is used for setting default values for named parameters:

sub greet(:$name = 'World') {
    say "Hello, $name!";
}

greet(name => 'Merinda'); # Hello, Merinda!
greet(); # Hello, World!

The order of the named parameters does not matter, so any of them can have default values, even the first in the list:

sub greet(:$greeting = 'Hello', :$name) {
    say "$greeting, $name!";
}

greet(name => 'Alla'); # Hello, Alla!

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