[Haskell-cafe] a parallel mapM?

Clark Gaebel cgaebel at uwaterloo.ca
Wed Oct 3 17:06:45 CEST 2012

I'm not sure that exposing a liftIO for Monad.Par is the best idea. Since
all these parallel computations use runPar :: Par a -> a, it advertises
that the result is deterministic. I'm not really comfortable with a hidden
unsafePerformIO hiding in the background.

That said, I don't see a reason for not including a separate version of
runParIO :: ParIO a -> IO a for non-deterministic computations. It seems
really useful!

  - Clark

On Wed, Oct 3, 2012 at 10:24 AM, Ryan Newton <rrnewton at gmail.com> wrote:

> Several of the monad-par schedulers COULD provide a MonadIO instance and
> thus "liftIO", which would make them easy to use for this kind of parallel
> IO business:
> http://hackage.haskell.org/packages/archive/monad-par/0.3/doc/html/Control-Monad-Par-Scheds-Direct.html
> And that would be a little more scalable because you wouldn't get a
> separate IO thread for each parallel computation.  But, to be safe-haskell
> compliant, we don't currently expose IO capabilities. I can add another
> module that exposes this capability if you are interested...
>   -Ryan
> On Fri, Sep 28, 2012 at 4:48 PM, Alexander Solla <alex.solla at gmail.com>wrote:
>> On Fri, Sep 28, 2012 at 11:01 AM, Greg Fitzgerald <garious at gmail.com>wrote:
>>> I also tried Control.Parallel.Strategies [2].  While that route works,
>>> I had to use unsafePerformIO.  Considering that IO is for sequencing
>>> effects and my IO operation doesn't cause any side-effects (besides
>>> hogging a file handle), is this a proper use of unsafePerformIO?
>> That's actually a perfectly fine use for unsafePerformIO, since the IO
>> action you are performing is pure and therefore safe (modulo your file
>> handle stuff).
>> unsafePerformIO is a problem when the IO action being run has side
>> effects and their order of evaluation matters (since unsafePerformIO will
>> cause them to be run in an "unpredictable" order)
>> One common use for unsafePerformIO is to run a query against an external
>> library.  It has to be done in the IO monad, but it is a "pure" computation
>> insofar as it has no side-effects that matter.  Doing this lets us promote
>> values defined in external libraries to bona fide pure Haskell values.
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