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POShared variable in Haskell parMap
 

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    <p>I have a computation which does essentially the following:</p> <pre><code>f :: [a] -&gt; ([b],Bool) </code></pre> <p>This function can actually be written</p> <pre><code>f = foldr h ([],False) . map g where h (b,bool) (bs,boolSoFar) = (b:bs,bool || boolSoFar) </code></pre> <p>where <code>g :: a -&gt; (b,Bool)</code> is some function that takes a lot of time. Also f is usually called on small lists, so it seemed like it might be fun to try computing the map in parallel. This can be accomplished with Control.Parallel.Strategies parMap. So now we use</p> <pre><code>f = foldr h ([],False) . parMap rseq g where h (b,bool) (bs,boolSoFar) = (b:bs, bool || boolSoFar) </code></pre> <p>This all works great. Now, you'll note that there is a sequential optimization that can be performed in the first definition of <code>f</code>. Namely, I can use map-fold fusion to write it as a single fold, so one loop through the list. However, then I lose the benefits of doing parallel.</p> <p>Now, one might say that in the second definition of <code>f</code>, looping through list again is not so bad, so why not just do it. I guess what I am thinking is that if Haskell had mutable variables, then one could just, in the body of map, update this boolean variable (I guess you would have to lock and unlock it). Are there any suggestions for doing things like this?</p>
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    1. COI highly doubt that using a mutable variable to contain the boolean value is going to make it any faster. Have you run a profiler on your code to see where the slowdowns occur? You may be too focused on optimizing something simple when your efficiency problem lies elsewhere. Are you sure that `g` is getting evaluated in parallel, or is it creating thunks in parallel and letting the single-core `foldr` do the evaluation?
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    2. COHi thanks for the comment. Actually, I want to know more if this is possible. I have been playing with profiling, and what not. Certainly the majority of time is spent in the map. I know this, and it is stated in the question. But I have seen this pattern come up in more than one place in my code, where I do something in parallel, and then something else in sequence -- and the sequential part is something tacked on to the end of a computation and could be done with a shared variable in the parallel part. So I'm asking how to do this in Haskell.
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      1. USJonathan Gallagher
    3. COIt's certainly possible to use `MVar`s to do this, but I think you might find that the overhead of using a mutable variable would, in most cases, vastly outweigh doing a quick second loop. It would depend on your specific problem, though. I can definitely imagine where there could be cases that you would want to do something like that.
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