Course of Raku / Essentials / More about functions

Multi-functions

Raku implements multiple dispatch based on function signature. It allows creating functions that share the name but have different types of input data. Use the multi declarator for each of the variants of the function.

multi sub add(Int $x, Int $y) { $x + $y }
multi sub add(Str $x, Str $y) { $x ~ $y }

(It is fine to omit sub after multi.)

If you have the two variants of the same function, the compiler performs multiple dispatch depending on which arguments it sees in the function call. Compare the following two calls:

say add(10, 20); # 30
say add('10', '20'); # 1020

The first call triggers the function with integer parameters, while the second call runs the second variant of the function that expects two strings.

Literal parameters

An interesting case for multi-functions is to have variants that have literal values as parameters. Consider the following two variants:

multi sub f(42) {say 'This is the answer'}
multi sub f($x) {say "$x is not the answer"}

The first variant is only run when you call the function with the exact value of 42. In other cases, the second variant is used. The order in which these functions are defined does not matter.

f(10); # 10 is not the answer
f(42); # This is the answer

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