[Haskell-cafe] a parallel mapM?
rrnewton at gmail.com
Wed Oct 3 16:24:44 CEST 2012
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
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...
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 . 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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