[Haskell-cafe] Problems with strictness analysis?

[Don Stewart]

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