Multiple dispatch

Declare several multi subs with one name; Raku picks the right one by the arguments.

Declare several subs with the same name using multi, and Raku chooses among them by the arguments' types:

multi describe(Int $n) { "an integer: $n" }
multi describe(Str $s) { "a string: $s" }
say describe(42);
say describe('camelia');

an integer: 42
a string: camelia

Dispatch on values #

A parameter can be a literal value, which turns recursion base cases into their own candidates — no if in sight:

multi fact(0) { 1 }
multi fact(Int $n where * > 0) { $n * fact($n - 1) }
say fact(5);
say fact(20);

120
2432902008176640000

The where * > 0 is a constraint: that candidate only matches positive integers. (The * builds a small anonymous function — "the thing being tested, greater than zero". More of it in the next lesson.)

Constraints do the branching #

multi bottles(0) { 'no more bottles' }
multi bottles(1) { '1 bottle' }
multi bottles($n) { "$n bottles" }
say bottles($_) for 2, 1, 0;

2 bottles
1 bottle
no more bottles

Try it #

Write the Fibonacci function with three multis: fib(0) is 0, fib(1) is 1, and everything else is the sum of the two before it.

multi fib(0) { 0 }

say fib(10);

multi fib(0) { 0 }
multi fib(1) { 1 }
multi fib(Int $n) { fib($n - 1) + fib($n - 2) }
say fib(10);

55