eight_queens.hs

-- Eight queens solution in 5 lines of haskell

solution :: [Int]
solution = head (allSolutions !! 8)

allSolutions :: [[[Int]]]
allSolutions = iterate (concatMap extendSolution) [[]]

extendSolution :: [Int] -> [[Int]]
extendSolution qs = map (:qs) $ filter (`okToAdd` qs) [0..7]

okToAdd :: Int -> [Int] -> Bool
okToAdd q qs = all (okToAddGivenDir q qs) [ succ, pred, id ]

okToAddGivenDir :: Int -> [Int] -> (Int -> Int) -> Bool
okToAddGivenDir q0 qs f = and $ zipWith (/=) (tail $ iterate f q0) qs

main = do
  putStrLn (show solution)