[Haskell-cafe] Re: Parallel foldl doesn't work correctly

Simon Marlow marlowsd at gmail.com
Mon Dec 14 08:19:58 EST 2009

On 13/12/2009 18:09, Philip Beadling wrote:

> I've come to the conclusion that, yep, you can't (directly) parallelise
> of fold operation, as fold guarantees order of processing.

True, but you can evaluate the elements in the input list to foldl' in 
parallel, as you were doing.  Presumably this doesn't give you enough 
parallelism in your case, though.

May I recommend that you pick up the latest parallel package:


and also the GHC 6.12.1 release (due any time now).  The new parallel 
package in particular cures a nasty space leak when using Strategies, 
and the new GHC includes a host of improvements to parallel performance.

If you still have trouble, then try using ThreadScope


with GHC 6.12.1.  You can use ThreadScope directly from the darcs 
repository on code.haskell.org, and we hope to do a proper release soon.


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