#5672: parBufferWHNF could be less subtle
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Reporter: duncan | Owner:
Type: feature request | Status: new
Priority: normal | Component: libraries (other)
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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I would suggest modifying `parBufferWHNF` as follows, with the `{- x
``seq`` -}` actually included.
{{{
parBufferWHNF :: Int -> Strategy [a]
parBufferWHNF n0 xs0 = return (ret xs0 (start n0 xs0))
where -- ret :: [a] -> [a] -> [a]
ret (x:xs) (y:ys) = y `par` (x : ({-x `seq`-} ret xs ys))
ret xs _ = xs
-- start :: Int -> [a] -> [a]
start 0 ys = ys
start !_n [] = []
start !n (y:ys) = y `par` start (n-1) ys
}}}
The point of the extra `x ``seq`` ` is so that consumers that do not
evaluate the list element before evaluating the next cons cell don't get
such surprising loss of parallelism. For example non-strict left folds
like `sum` will rush to the end of the list spine without evaluating the
list elements, and then evaluate the elements later. That does not work
well with the `parBufferWHNF` strategy. Adding the extra `seq` fixes it.
Note that in this case I think `seq` or `pseq` would work.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5672>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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