[Haskell-cafe] Re[2]: Why Haskell?

Sebastian Sylvan sylvan at student.chalmers.se
Wed Jul 26 11:43:36 EDT 2006

On 7/26/06, SevenThunders <mattcbro at earthlink.net> wrote:
> Bulat Ziganshin-2 wrote:
> >
> > Hello Matthew,
> >
> > Sunday, July 23, 2006, 10:35:41 AM, you wrote:
> >
> >  >>     sequence $ [ reffill b s | s <- [0..(fi temits)-1], b <- [0..(fi
> >> nc)-1]]
> >
> >> Now thats interesting.  I can see that this function is more appropriate
> >> since I do not need to retrieve data from the IO monad,
> >> but what I don't understand is why it's actually faster.  I will give it
> >> a try and test it on a large set to see if things change.
> >
> > let's see at their (possible) definitions:
> >
> > sequence [] = return []
> > sequence (action:actions) = do x <- action
> >                                xs <- sequence actions
> >                                return (x:xs)
> >
> > sequence_ [] = return ()
> > sequence_ (action:actions) = do action
> >                                 sequence_ actions
> >
> > sequence_ is TAIL-RECURSIVE function. moreover, when it's inlined, the
> > result is what just all the action in list are sequentially performed
> > without creating any intermediate data structures. so, using sequence_
> > in above example is equivalent to implementing this cycle by hand.
> >
> > but when you use sequence, result of each action is saved and list
> > of all results is built. this list requires 12 bytes per element, so
> > you got 600 mb of garbage and much slower execution
> >
> >
> >
> > --
> > Best regards,
> >  Bulat                            mailto:Bulat.Ziganshin at gmail.com
> >
> > _______________________________________________
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> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
> Well lo and behold I also need decent performance when doing a lot of IO
> actions that do not discard their arguments.  For example suppose I want to
> generate a huge array of random numbers.

Well, why would you want a huge array of random numbers?
In Haskell there are two big ways to gain efficiency, strictness and
laziness. In this case I think laziness is a big candidate (the "huge
array" part gives it away).
Also there is no reason generating random numbers should be in the IO
monad - in fact the built-ins for random numbers are mainly regular
pure functions. You only need IO in the very beginning to get a seed.

So basically, try to use the extra modularity options that Haskell
gives you to separate things that *need* to be IO and things that can
just be regular lazy structures (like an infinite list of random
numbers occupying memory the size of a pointer). In your case you'd
use newStdGen in the IO monad to get a generator, and then produce a
"huge" (e.g. infinite) list of random numbers using that (in a pure
non-side-effect way), where only the numbers actually needed will ever
be computed.

If however you do need to do actual IO actions and you find it takes
too much space, you can add some extra laziness using
unsafeIntereaveIO. It basically just postpones an action until its
result is needed. This is what readFile uses, btw, to get lazy IO.
I think that's the way to go (rather than unsafePerformIO) because
it's still IO so you don't end up in the situation where you have a
pure function returning different results everytime you call it, and
you still get laziness. Be careful, though, that your particular IO
action won't behave strangely if it gets called at some random time or
not at all.

Sebastian Sylvan
UIN: 44640862

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