Torre de Hanoi em Haskell

hanoi.hs

hanoi :: Int -> a -> a -> a -> [(a,a)] 
hanoi 1 x y z = [(x,y)]
hanoi n x y z = hanoi (n-1) x y z ++ hanoi 1 x z y ++ hanoi (n-1) y z x

tamhanoi :: Int -> Int
tamhanoi n = 2^n - 1