Towers of Hanoi, with command-line interface

hanoi.hs

-- Towers of Hanoi for three pegs
--   outputs a list of moves to transport a tower of specified size
--   from peg A to peg B, with peg C as reserve space.
--
-- $ runghc hanoi.hs 3
-- [("A","B"),("A","C"),("B","C"),("A","B"),("C","A"),("C","B"),("A","B")]

import System.Environment (getArgs)

main :: IO ()
main = getArgs >>= print . hanoi "A" "B" "C" . read . head

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

hanoi :: Peg -> Peg -> Peg -> Integer -> [Move]
hanoi _ _ _ n | n < 1 = []
hanoi p q _ 1         = [(p, q)]
hanoi p q r n         = hanoi p r q (n - 1)
                     ++ hanoi p q r      1
                     ++ hanoi r q p (n - 1)