Re: [Haskell-cafe] Problems with strictness analysis?

lrpalmer:

On Mon, Nov 3, 2008 at 2:35 PM, Don Stewart wrote:

...

Consider this program,

isum 0 s = s isum n s = isum (n-1) (s+n)

main = case isum 10000000 0 {- rsum 10000000 -} of 0 -> print 0 x -> print x

Now, isum is *not* strict in 's', [...]

Of course, we make this strict in a number of ways:

* Turning on optimisations:

I am confused about your usage of "strict". Optimizations are not supposed to change semantics, so I don't know how it is possible to make a function strict by turning on optimizations. This function was always strict in s, given a strict numeral type. By induction on n:

isum 0 _|_ = _|_ isum (n+1) _|_ = isum n (s+_|_) = isum n _|_ = _|_

For negative n, it either wraps around to positive in which case the above analysis applies, or it does not halt, which is strict (in the same way "const _|_" is strict).

"Optimisations" enable strictness analysis. -- Don