Course of Raku / Essentials / Functions 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 to 1. With multi-functions, you can achieve that by extracting the case with $n == 1 to a separate multi-function.

Code

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.

Output

$ raku exercises/more-on-functions/factorial-with-multi-functions.raku 5
120

Comment

Note that the input argument is explicitly converted to an integer: @*ARGS[0].Int. This is to prevent an 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 contrast, 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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