[Haskell-cafe] Problems with strictness analysis?

[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?

Gergely

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