[Haskell-cafe] Parallel lazy zip

[Daniel Peebles]

Would using

zipWith (\x y -> x `par` y `pseq` x + y) (expensiveList 1) (expensiveList 2)

do it? it seems to help a bit on my machine, but doesn't give me twice
the performance

On Wed, Sep 16, 2009 at 10:59 AM, Henning Thielemann
<lemming at henning-thielemann.de> wrote:
>
> When reading
>  http://www.macs.hw.ac.uk/~dsg/gph/papers/html/Strategies/strategies.html
>  I got the impression, that when I want to compute in parallel I have to
> suppress laziness at all costs, otherwise only a neglible portion of the
> code is run in parallel. How can I parallelize the computation of two lazily
> generated lists, where the list generation is expensive, but the combination
> of the lists is cheap?
>
> For example:
>
>
> module Main where
>
> expensiveList :: Int -> [Int]
> expensiveList n =
>   map (\m -> sum [n..m]) [10000000..]
>
> sequentialZip :: [Int]
> sequentialZip =
>   zipWith (+)
>      (expensiveList 1)
>      (expensiveList 2)
>
> main :: IO ()
> main =
>   mapM_ print $ take 10 sequentialZip
>
>
> It seems to me that this program must run almost twice as fast when using
> two cores, because the expensive lists can be computed perfectly in
> parallel. It requires however, that the zipWith can fetch data lazily across
> threads. However applying a Parallel strategy to the expensive list, will
> certainly try to evaluate it completely.
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>