Haskell 3x3 Tic-Tac-Toe

tictactoe.hs

data Cell = X | O | N deriving (Show, Eq)

board = [X,X,O, 
         O,X,X, 
         X,O,O]

winner [a,b,c,
        d,e,f, 
        g,h,i] | isWinner X = X
               | isWinner O = O
               | otherwise  = N
  where isWinner player = any (belongsTo player) $ concat [diag, rows, cols]
        belongsTo player = all (== player)
        diag = [[a,e,i], [c,e,g]]
        rows = [[a,b,c], [d,e,f], [g,h,i]]
        cols = [[a,d,g], [b,e,h], [c,f,i]]

main = do
  putStrLn $ show $ winner board