Entropy

sub entropy(@a) {
    [+] map -> \p { p * -log p }, bag(@a).values »/» +@a;
}

say log(2) R/ entropy '1223334444'.comb;

Output:

1.84643934467102

In case we would like to add this function to Raku's core, here is one way it could be done:

use MONKEY-TYPING;
augment class Bag {
    method entropy {
	[+] map -> \p { - p * log p },
	self.values »/» +self;
    }
}

say '1223334444'.comb.Bag.entropy / log 2;