[Haskell-cafe] Strange behavior with listArray

Alex Stangl alex at stangl.us
Mon Nov 12 14:43:32 CET 2012

On Mon, Nov 12, 2012 at 08:36:49AM +0100, Bas van Dijk wrote:
> On 12 November 2012 04:50, Alex Stangl <alex at stangl.us> wrote:
> > I'm stymied trying to figure out why the program below blows up with
> > <<<loop>>> when I use "f 0"
> If you replace the a!0 in f by its value 0, f is equivalent to:
>             f k = if k > 0
>                     then f 0
>                     else 0 : f 1
> Do you see the loop now?

I realize it loops/recurses, just like h does, or any instance
of building lazy infinite data structures. It works fine when you
only extract a finite number of elements from the infinite structure.
It's not clear why that is not happening here, as if there is a failure
of laziness.  f 0 should effectively yield [0, 0, ...], correct?

> Maybe you meant f to be:
>             f k = if k > 0
>                     then f (a!k)
>                     else 0 : f 1

Actually it was that way in the original program. I switched it to 0
the process of trying to "distill" it down to a simplest test. Either
way yield the same result, <<<loop>>>. If you take the array reference
out, this code works fine, as it obviously should. But with the array
reference intact, it appears listArray isn't accessing the list lazily



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