[Haskell-cafe] Re: Are there performant mutable Arrays in Haskell?

[Don Stewart]

barsoap:
> "Brettschneider, Matthias" <Brettschneider at hs-albsig.de> wrote:
> 
> > Thx for your hints, I played around with them and the performance
> > gets slightly better. But the major boost is still missing :) 
> > 
> > I noticed, that one real bottleneck seems to be the conversion of the
> > array back into a list. The interesting part is, if I use the elems
> > function (Data.Array.Base) the performance is about 4x better then
> > with my own function. So I thought, I write my own version of elems,
> > (that just converts a part of the array to a list) and I fall back
> > into the same performance as my first approach. 
> > 
> > To make a long story short, here is the library code: 
> > elems arr = case bounds arr of
> >       (_l, _u) -> [unsafeAt arr i | i <- [0 .. numElements arr - 1]
> > 
> > And my version:
> > boundedElems arr = case bounds arr of
> >       (_l, _u) -> [unsafeAt arr i | i <- [1737 .. 1752]]
> > 
> > Is there a reason, why the library version is 4 times faster, than
> > mine?
> >
> Uhhhmmm... I've got no idea. But as the list is constructed lazily, you
> shouldn't expect a real speedup by being lazy manually, anyway. You
> might want to try the stream-fusion list implementation of lists, or
> get rid of lists, alltogether.

Yes, avoid constructing the list, if you're not doing that in C.