Conway's Game Of Life.

GameOfLife.hs

import qualified Data.Set as S
import qualified Data.Map as M
import Control.Monad
import Data.Bool
 
step :: S.Set (Int, Int) -> S.Set (Int, Int)
step grid = let
    neighs (i, j) = [((i + di, j + dj), 1) |
        (di, dj) <- tail $ liftM2 (,) [0, -1, 1] [0, -1, 1]]
    freqs = M.fromListWith (+) $ neighs =<< S.toList grid
    in S.fromList [i |
        (i, n) <- M.toList freqs,
        (n == 3) || (n == 2 && S.member i grid)]
 
showGrid :: [Int] -> [Int] -> S.Set (Int, Int) -> String
showGrid w h grid = 
    unlines [[bool ' ' 'x' (S.member (i, j) grid) | j <- w] | i <- h]
 
parseGrid :: [String] -> S.Set (Int, Int)
parseGrid ls = S.fromList [(i, j) |
    (i,  l ) <- zip [0..] ls,
    (j, 'x') <- zip [0..] l]
 
glider = parseGrid [
    " x ",
    "  x",
    "xxx"]
 
main = forM_ (take 20 $ iterate step glider) $ \grid -> do
    putStrLn $ showGrid [-10..10] [-10..10] grid
    putStrLn $ replicate 20 '-'