Course of Raku / Advanced / Subroutines

Multiple dispatch

In the Essentials part you met multi-functions: several subroutines that share a name, where Raku picks the right one by looking at the types of the arguments. This mechanism is called multiple dispatch.

So far, the choice was made by the type of the arguments. A where clause lets you go further and dispatch on the value as well. It is written after a parameter and holds a condition that the argument must satisfy for that candidate to be chosen:

multi sub info(Int $n where $n < 0)  { say "$n is negative" }
multi sub info(Int $n where $n == 0) { say "$n is zero" }
multi sub info(Int $n where $n > 0)  { say "$n is positive" }

info(-5);
info(0);
info(7);

Each call goes to the candidate whose condition is true for the given value:

-5 is negative
0 is zero
7 is positive

All three candidates have the same signature as far as types go — a single Int — so without the where clauses they would clash. The conditions make them distinct.

A base case for recursion

A common use of where is to provide the stopping point of a recursion as a separate candidate. Here is the factorial again, split into two multi-subs:

multi sub fact(Int $n where $n <= 1) { 1 }
multi sub fact(Int $n)               { $n * fact($n - 1) }

say fact(5); # 120

The first candidate handles the base case ($n of 1 or less) and simply returns 1. Every other value goes to the second candidate, which calls fact again with a smaller number. There is no if inside the function — the choice of base case versus recursive step is made by the dispatcher.

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