The eight queens puzzle is the problem of placing eight chess queens on an 8×8 chessboard so that no two queens threaten each other.

The task is to implement the extended version, queens :: Int -> [[Int]]; given an integer specifying the dimension of the chess board, it returns
all valid solutions.

Since it’s a well-established problem, many standard techniques exist, such as those listed at https://rosettacode.org/wiki/N-queens_problem#Haskell.
I have reprinted two versions I am quite fond of. Now that there are multiple versions available, I am curious about their performance, so I used the
following main function, compiled with -O, to do simple evaluation, and the result is attached along with the corresponding source code.

import Control.Monad import Data.List (delete) queens n = map fst $ foldM oneMorequeens ([],[1..n]) [1..n] where oneMorequeens (y,d) _ = [(x:y, delete x d) | x <- d, safe x] where safe x = and [x /= c + n && x /= c - n | (n,c) <- zip [1..] y] -- 73712 -- 3.256558s

queens n = foldM (\y _ -> [ x : y | x <- [1..n], safe x y 1]) [] [1..n] where safe x [] _ = True safe x (c:y) n = and [ x /= c , x /= c + n , x /= c - n , safe x y (n+1)]

-- 73712 -- 11.298376s

In the process of trying to understand such concise solutions, I have attempted to craft two versions without using foldM.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

typeBoard = [Int]

queens :: Int -> [Board] queens n = loop [[]] 0 where loop :: [Board] -> Int -> [Board] loop boards counter | counter == n = boards | otherwise = loop (concatMap expand boards) (counter+1)

expand :: Board -> [Board] expand board = [x : board | x <- [1..n], safe x board 1]

safe x [] _ = True safe x (c:y) n = and [ x /= c , x /= c + n , x /= c - n , safe x y (n+1)] -- 73712 -- 2.569084s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

import Data.List (delete) typeBoard = [Int]

queens :: Int -> [Board] queens n = map fst $ loop [([], [1..n])] 0 where loop :: [(Board, [Int])] -> Int -> [(Board, [Int])] loop boards counter | counter == n = boards | otherwise = loop (concatMap expand boards) (counter+1)

expand :: (Board, [Int]) -> [(Board, [Int])] expand (board, candidates) = [(x : board, delete x candidates) | x <- candidates, safe x board]

safe x board = and [ x /= c + n && x /= c - n | (n,c) <- zip [1..] board] 73712 1.378314s

I am quite happy to see that they both perform better than existing ones. (Actually, I like the one without using tuple best, even though it’s not
the fastest.)