[Haskell-cafe] Simplest way to move a ... `using` ... line into the IO monad?

Viktor Dukhovni ietf-dane at dukhovni.org
Fri Oct 27 18:51:52 UTC 2023

On Fri, Oct 27, 2023 at 07:43:07PM +0100, Tom Ellis wrote:
> On Fri, Oct 27, 2023 at 11:36:53AM -0700, David Banas wrote:
> > What is the simplest way to translate a non-monadic line of Haskell code, such as:
> > 
> > res = map foo xs `using` parList rseq
> > 
> > For use in the IO monad?
> > 
> > This doesn’t work:
> > 
> > res <- mapM fooM xs `using` parTraversable rseq
> The docs for Control.Parallel.Strategies say "The parallel tasks
> evaluated by a Strategy may have no side effects. For
> non-deterministic parallel programming, see Control.Concurrent."[1].
> So I guess you should look into Control.Concurrent[2].

For a more friendly interface:


If the number of concurrent tasks is reasonably modest, a good fit could
be `mapConcurrently`.

For much larger numbers of tasks, it is best to run the jobs at a
bounded concurrency, which may bring a BoundedChan to which the items to
process are written, another to which results are written and a set of
threads that do the processing, repeatedly reading more data until the
channel is drained. 

*The* book on subject is: https://simonmar.github.io/pages/pcph.html


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