On Fri, Sep 28, 2012 at 11:01 AM, Greg Fitzgerald <gari...@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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