Perl 6 and Parrot Essentials, Second Edition

Curried Subroutines

Currying^[14] allows you to create a shortcut for calling a subroutine with some preset parameter values. The assuming method takes a list of named arguments and returns a subroutine reference, with each of the named arguments bound to the original subroutine’s parameter list. If you have a subroutine multiply that multiplies two numbers, you might create a subref $six_times that sets the value for the $multiplier parameter, so you can reuse it several times:

sub multiply ($multiplicand, $multiplier) {
    return $multiplicand * $multiplier;
}

$six_times = &multiply.assuming(multiplier => 6);

$six_times(9); # 54
$six_times(7); # 42
 . . .

You can also use binding assignment to alias a curried subroutine to an ordinary subroutine name instead of a scalar variable:

&six_times := &multiply.assuming(multiplier => 6);

six_times(7); # 42

^[14] The term “currying” is drawn from functional languages and is named in honor of logician Haskell Curry.