Sample Paper Haskell Q4.

xando.hs

import Data.List

data Piece = X | O deriving (Ord, Eq, Show)
data Board =  Board [(Int, Int, Piece)] deriving(Ord, Eq,Show)

winning_combinations = [[(a,0), (a,1), (a,2)] | a <- [0..2] ] ++ [[(0,a), (1,a), (2,a)] | a <- [0..2] ] ++ [[(0,0), (1,1), (2,2)], [(0,2), (1,1), (2,0)]]

over2 :: Board -> Bool
over2 (Board []) = False
over2 (Board x) = is_winner newx winning_combinations 
			where newx = sortBy compare x

is_winner x [] = False
is_winner x (w:wc)
		| (length newx == 3)  = all_X || all_Y || is_winner x wc
		| otherwise = is_winner x wc
		where newx = filter (\(x,y, _) -> (x, y) `elem` w ) x
		      all_X = foldr (&&) True $ map (\(_, _, p) -> p == X) newx
		      all_Y = foldr (&&) True $ map (\(_, _, p) -> p == O) newx

play :: Board -> Board
play (Board []) = (Board [(0, 0, X)])
play (Board x) = (Board (new_piece:x))
		 where next_piece = get_next x
		       coords = [ (x, y) | (x, y, _) <- x]
		       all = [(x, y) | x <- [0 .. 2], y <- [0..2]]
		       next_coord = filter (\x -> not (elem x coords)) all
		       new_piece = (\(x, y) p -> (x, y, p)) (next_coord !! 0) next_piece
		       
get_next :: [(Int, Int, Piece)] -> Piece
get_next x = get_next' x 0 0
get_next' :: [(Int, Int, Piece)] -> Int -> Int -> Piece
get_next' [] x o
	| x > o = O
	| otherwise = X

get_next' ((_, _, X):xs) x o = get_next' xs (x+1) o 
get_next' ((_, _, O):xs) x o = get_next' xs (x+1) o