[Haskell-cafe] forLoop + strict State monad is much faster than foldl'

[Patrick Wheeler]

@Niklas

You can use the `foldM` form the FoldL package to achieve equal results as
the fastest loops in your current benchmark. foldM will allow you to deal
with your loops at a high level of abstraction though. See the following
post, by Gabriel Gonzalez, for an example:
http://www.haskellforall.com/2013/08/foldl-100-composable-streaming-and.html

I have added the bench mark to my fork of your repo, and made a pull
request:
https://github.com/Davorak/loop

It looks like the only reason that `foldM` does not preform well with
`Word32` is because of the naive implementation of `enumFromTo` for Word32
as explained in my other email in more detail.

Here is the Criterion report:
http://htmlpreview.github.io/?https://github.com/Davorak/loop/blob/master/results/bench.html



On Tue, Apr 29, 2014 at 11:31 AM, Niklas Hambüchen <mail at nh2.me> wrote:

> This is just a short notice that using
>
>    foldl' (+) 0 [0..100000::Int]
>
> is over 10 times slower than using
>
>   flip execState 0 $
>     forLoop (0 :: Int) (< n) (+1) $ \i -> do
>       x <- get
>       put $! x + i
>
> with `loopFor` as on https://github.com/nh2/loop.
>
> Even using an IORef is twice as fast as the pure foldl' (but still 5
> times slower than strict State).
>
> The benchmark is at
>
> http://htmlpreview.github.io/?https://github.com/nh2/loop/blob/master/results/bench-foldl-and-iorefs-are-slow.html
> .
>
> (All benchmarks are linked from https://github.com/nh2/loop.)
>
> You can see there that the problem is gone when using Vector.foldl', but
> only for Int - for Word32 it persists.
>
> It seems that manual looping is beneficial even when dealing with prime
> examples of pure code that GHC ought to optimize well.
>
> Niklas
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-- 
Patrick Wheeler
Patrick.John.Wheeler at gmail.com
Patrick.J.Wheeler at rice.edu
Patrick.Wheeler at colorado.edu
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