Re: [Haskell-cafe] a parallel mapM?

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-M... 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 wrote:

On Fri, Sep 28, 2012 at 11:01 AM, Greg Fitzgerald 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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