nqueens.hs

import qualified Data.Set as Set

(x1, y1) |+ (x2, y2) = (x1+x2, y1+y2)

size = 8

out_of_greed (x, y) =
  let check v = v < 0 || v == size
  in check x || check y

add_dangerous pos set dir =
  let iter set pos
        | out_of_greed pos = set
        | otherwise = iter (Set.insert pos set) (pos |+ dir)
  in
   iter set pos

make_dangerous pos set =
  foldl (add_dangerous pos) set [(0, 1), (-1, 1), (1, 1)]

place pos@(x, y) set | y >= size = Just []
                     | otherwise =
  fmap (pos :) (try 0 (y+1))
  where
    dang = make_dangerous pos set
    unsafe pos = Set.member pos dang
    try x y | x == size = Nothing
            | unsafe (x, y) = try (x+1) y
            | otherwise =
              case place (x, y) dang of
                Nothing -> try (x+1) y
                sol -> sol

draw_list xs =
  let get_cell pos
        | pos `elem` xs = '#'
        | otherwise = '0'
      draw_line line = do
        mapM_ (putChar . get_cell) line
        putChar '\n'
  in
   mapM_ draw_line [[(x, y) | x <- [0..size]] | y <- [0..size]]

main = do
  case place (0, 0) Set.empty of
    Just sol -> draw_list sol
    Nothing -> putStrLn "failed :("