Haskell parTraversable不产生任何火花

当我使用

-s

运行此程序时：

module Main where

import Control.Parallel.Strategies

main :: IO ()
main = do let xs = take 1000 $ product . take 1000 . repeat <$> [1..]
              x  = product (xs `using` parList rseq)
          putStrLn (show x)

不会产生火花：

火花：0（0转换，0溢出，0无效，0 GC'd，0失效）

如果我将

rseq

更改为

rdeepseq

：

main = do let xs = (take 1000 $ product . take 1000 . repeat <$> [1..]) :: [Integer]
              x  = product (xs `using` parList rdeepseq)
          putStrLn (show x)

---

我遗漏了什么？

我可以重现你的行为（ghc-8.2.1，parallel-3.2.1.1）

稍后往下看

Strategies.hs

是一个

RULES

pragma，表示特殊情况

parList-rseq

。我猜这是一个bug，它与parTraversable有不同的行为（我对内部结构了解不够，无法确定bug的位置）。我建议在

parallel

issue tracker提交一张罚单：

以下是有问题的代码，从文件的第505行开始：

-- Non-compositional version of 'parList', evaluating list elements
-- to weak head normal form.
-- Not to be exported; used for optimisation.

-- | DEPRECATED: use @'parList' 'rseq'@ instead
parListWHNF :: Strategy [a]
parListWHNF xs = go xs `pseq` return xs
  where -- go :: [a] -> [a]
           go []     = []
           go (y:ys) = y `par` go ys

-- The non-compositional 'parListWHNF' might be more efficient than its
-- more compositional counterpart; use RULES to do the specialisation.

{-# NOINLINE [1] parList #-}
{-# NOINLINE [1] rseq #-}
{-# RULES
 "parList/rseq" parList rseq = parListWHNF
 #-}

parList :: Strategy a -> Strategy [a]
parList = parTraversable

-- Non-compositional version of 'parList', evaluating list elements
-- to weak head normal form.
-- Not to be exported; used for optimisation.

-- | DEPRECATED: use @'parList' 'rseq'@ instead
parListWHNF :: Strategy [a]
parListWHNF xs = go xs `pseq` return xs
  where -- go :: [a] -> [a]
           go []     = []
           go (y:ys) = y `par` go ys

-- The non-compositional 'parListWHNF' might be more efficient than its
-- more compositional counterpart; use RULES to do the specialisation.

{-# NOINLINE [1] parList #-}
{-# NOINLINE [1] rseq #-}
{-# RULES
 "parList/rseq" parList rseq = parListWHNF
 #-}