Course of Raku / Essentials / More about functions / Exercises / Factorial with multi-functions

Solution: Factorial with multi-functions

This exercise is about factorials again. Indeed, the task allows us to illustrate more features of Raku.

When computing a factorial by starting from the given number and going downwards, you need to stop when the number gets 1. With multi-functions, you can achieve that by extracting the case with $n == 1 to a separate multi-function.


Here is the solution:

multi sub factorial(1)  { 1 }
multi sub factorial($n) { $n * factorial($n - 1) }

say factorial(@*ARGS[0].Int);

🦋 Find the program in the file factorial-with-multi-functions.raku.


$ raku exercises/more-on-functions/factorial-with-multi-functions.raku 5


Note that the input argument is explicitly converted to an integer: @*ARGS[0].Int. This is to prevent infinite loop when the input number is 1. In this case, the type of parameter passed to the factorial function is IntStr, and the first multi-variant cannot catch the call. In contract, when factorial(2 - 1) is called recursively, the argument of the function is an integer, which makes it possible to call the first variant.

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