我正在完成 Andre Loh 在 haskell 练习中的确定性并行编程练习。我试图通过使用策略将 N-Queens 顺序代码转换为并行代码，但我注意到并行代码的运行速度比顺序代码慢得多，并且还会因堆栈空间不足而出错。
这是并行 N-Queens 的代码，
import Control.Monad
import System.Environment
import GHC.Conc
import Control.Parallel.Strategies
import Data.List
import Data.Function
type PartialSolution = [Int] -- per column, list the row the queen is in
type Solution = PartialSolution
type BoardSize = Int
chunk :: Int -> [a] -> [[a]]
chunk n [] = []
chunk n xs = case splitAt n xs of
(ys, zs) -> ys : chunk n zs
-- Generate all solutions for a given board size.
queens :: BoardSize -> [Solution]
--queens n = iterate (concatMap (addQueen n)) [[]] !! n
queens n = iterate (\l -> concat (map (addQueen n) l `using` parListChunk (n `div` numCapabilities) rdeepseq)) [[]] !! n
-- Given the size of the problem and a partial solution for the
-- first few columns, find all possible assignments for the next
-- column and extend the partial solution.
addQueen :: BoardSize -> PartialSolution -> [PartialSolution]
addQueen n s = [ x : s | x <- [1..n], safe x s 1 ]
-- Given a row number, a partial solution and an offset, check
-- that a queen placed at that row threatens no queen in the
-- partial solution.
safe :: Int -> PartialSolution -> Int -> Bool
safe x [] n = True
safe x (c:y) n = x /= c && x /= c + n && x /= c - n && safe x y (n + 1)
main = do
[n] <- getArgs
print $ length $ queens (read n)
该行(\l -> concat (map (addQueen n) l using parListChunk (n div numCapabilities) rdeepseq))是我从原始代码中更改的内容。我已经看过 Simon Marlow 的解决方案，但我想知道我的代码速度变慢和错误的原因。
提前致谢。