f 0 = l
f n = g (f (n-1))
But more functional would be:
f 0 l = l
f n l = g (f (n-1) l)
This could also be done with folds or morfisms, but this is easier to understand.
For example here's using a hylomorphism, but it doesn't make it clearer really:
f g l = hylo l (.) (\n -> (g, n-1)) (==0)
It says some thing like compose (.) g(l) until n==0
