Haskell: Tower of Hanoi

hanoi.hs

type Peg = String
type Move = (Peg, Peg)

hanoi :: Integer -> Peg -> Peg -> Peg -> [Move]
hanoi 0 _ _ _ = []
hanoi n a b c = hanoi (n - 1) a c b ++ [(a, c)] ++ hanoi (n - 1) b a c

main = do
    print $ hanoi 1 "a" "b" "c"
    print $ hanoi 2 "a" "b" "c"
    print $ hanoi 3 "a" "b" "c"
    print $ hanoi 4 "a" "b" "c"
    print $ length (hanoi 20 "a" "b" "c")