[Haskell-cafe] parMap doesn't work fine

Brandon Moore brandon_m_moore at yahoo.com
Thu May 12 16:36:55 CEST 2011

> Hello!
> I've just started using parallel computations in Haskell. parMap  works fine, 
>it is so easy to use. However, parMap fails with functions returning  lazy 
>structures, e.g. tuples.
> This code works as expected:
> (parMap  rpar) bm tvalues

Using rpar is redundant. That strategy schedules a value to maybe be computed 
later in parallel, but parMap already distributed work between
cores, so you might as well use rseq or something.

> bm :: Double -> Double is some heavy function. But if  I want to return list of 
>pairs (t, bm t) it doesn't use cpu cores fine (only one  is in use):
> (parMap rpar) (\t -> (t, bm t)) tvalues

That's only evaluating the pair in parallel. bm t will only be computed when you 
open up the pairs afterwards.
Try rdeepseq instead.


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