[Haskell-cafe] Problems with strictness analysis?

[frantisek kocun]

yet I need to add a $! to the recursive call of isum to get a truly
iterative ???

Wait a minute Patai. How would you do that? I'm only beginner I thought I
can only add strict "!" to data parameters. But to make isum function strict
would be helpful.

Thanks

On Mon, Nov 3, 2008 at 7:36 PM, Don Stewart <dons at galois.com> wrote:

> patai_gergely:
> > Hi everyone,
> >
> > I was experimenting with simple accumulator functions, and found that an
> > apparently tail recursive function can easily fill the stack. Playing
> > around with ghc and jhc gave consistently unpleasant results. Look at
> > this program:
> >
> > %%%%%%%%%%%
> >
> > -- ghc: no, ghc -O3: yes, jhc: no
> > isum 0 s = s
> > isum n s = isum (n-1) (s+n)
> >
> > -- ghc: no, ghc -O3: no, jhc: yes (because gcc -O3 is clever)
> > rsum 0 = 0
> > rsum n = n + rsum (n-1)
> >
> > main = case isum 10000000 0 {- rsum 10000000 -} of
> >          0 -> print 0
> >          x -> print x
> >
> > %%%%%%%%%%%
> >
> > I would expect the analysis to find out that the result of the function
> > is needed in every case (since we are matching a pattern to it), yet I
> > need to add a $! to the recursive call of isum to get a truly iterative
> > function. It's interesting how ghc and jhc seem to diverge, one
> > favouring the "iterative" version, the other the "recursive" one
> > (although it only works because gcc figures out that the function can be
> > expressed in an iterative way).
> >
> > Of course this is a real problem when I'm trying to write an actual
> > program, since it means I can be bitten by the laziness bug any time...
> > Is there any solution besides adding strictness annotations? Can I
> > expect the compiler to handle this situation better in the foreseeable
> > future?
>
> I think its sensible not to rely on an analysis to infer the correct
> reduction strategy for your code. Make the strictness explict, and your
> code will be more portable and more robust.
>
> Also, write in some type annotations. Particularly for atomic types like
> Int, these give the strictness analyser yet more information to work
> with.
>
> -- Don
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