Factorial

via User-defined Postfix Operator

[*] is a reduction operator that multiplies all the following values together. Note that we don't need to start at 1, since the degenerate case of [*]() correctly returns 1, and multiplying by 1 to start off with is silly in any case.

sub postfix:<!> (Int $n) { [*] 2..$n }
say 5!;

Output:

120

via Memoized Constant Sequence

This approach is much more efficient for repeated use, since it automatically caches. [\*] is the so-called triangular version of [*]. It returns the intermediate results as a list. Note that Raku allows you to define constants lazily, which is rather helpful when your constant is of infinite size...

constant fact = 1, |[\*] 1..*;
say fact[5]

Output:

120