test.hs

import Control.Monad.Writer

type Tower = ([Int], Char)
data Hanoi = Hanoi Tower Tower Tower deriving Show

move :: Hanoi -> Hanoi
move (Hanoi a b c) = Hanoi (init (fst a), snd a) b ((fst c) ++ [last (fst a)], snd c)

solve :: Hanoi -> Writer [String] Hanoi
solve (Hanoi a b c) = solve' (length (fst a)) (Hanoi a b c)

solve' :: Int -> Hanoi -> Writer [String] Hanoi
solve' 1 (Hanoi a b c) = do
    tell ["Moved " ++ [snd a] ++" to " ++ [snd c]]
    return (move (Hanoi a b c))

solve' n (Hanoi a b c) = do
    (Hanoi x y z) <- solve' (n - 1) (Hanoi a c b)
    (Hanoi p q r) <- solve' 1 (Hanoi x z y)
    (Hanoi aa bb cc) <- solve' (n - 1) (Hanoi q p r)
    return (Hanoi bb aa cc)