CS194 Homework 2 - Towers of Hanoi

HW01.2.hs

module Main where

type Peg = String
type Move = (Peg, Peg)
{-
    | Towers of hanoi solution
    >>> hanoi 2 "a" "b" "c"
    [("a","c"),("a","b"),("c","b")]

    move n-1 disc from a to c using b as temporary storage
    move the top disc from a to b
    move n-1 discs from c to b using a as a temporary storage
-}
hanoi :: Integer -> Peg -> Peg -> Peg -> [Move]
hanoi 0 _ _ _ = []
hanoi n a b c = hanoi (n-1) a c b ++ [(a, b)] ++ hanoi (n-1) c b a

main :: IO()
main = do
    putStrLn "Enter number of disks:"
    n <- getLine
    print $ hanoi (read n) "a" "b" "c"