ticTacToe.hs

import Data.Array
import Data.Maybe

data Piece = X | O | Empty deriving (Eq)

instance Show Piece where
  show X = "X"
  show O = "O"
  show Empty = " "

type Board = Array Integer Piece

emptyBoard = listArray (0,8) $ replicate 9 Empty

move i p board = board // [(i, p)]

checkWin board = filter isJust . map (checkRow board) $ 
    [ (0, 1, 2), (3, 4, 5), (6, 7, 8)
    , (0, 3, 6), (1, 4, 7), (2, 5, 8)
    , (0, 4, 8), (2, 4, 7)
    ]

checkRow board (a,b,c) = 
    case (board ! a, board ! b, board ! c) of
      (X,X,X) -> Just X
      (O,O,O) -> Just O
      _       -> Nothing

printBoard :: Board -> IO ()
printBoard b = let p = show . (b!) in 
    putStr . unlines . map unwords $ 
      [ [p 0, "|", p 1, "|", p 2]  
      , ["---------"]
      , [p 3, "|", p 4, "|", p 5]
      , ["---------"]
      , [p 6, "|", p 7, "|", p 8]
      ]