[Haskell] How to make sure that a function is evaluated only once?

Lemmih lemmih at gmail.com
Fri Dec 22 05:44:24 EST 2006

On 12/22/06, Axel Jantsch <axel at kth.se> wrote:
> Thanks to all who replied. I still haven't figured out a solution. I
> thought about memoizing the result, as several of you have suggested,
> but I am scared by the huge argument space.
> Lemmih <lemmih at gmail.com> wrote:
> > Do you have a minimal test case showing the undesirable behaviour?
> gf (actCnt,sndCnt, sizex, sizey, locx, locy) iPack
>   = (newSndCnt sndCnt outPack, outPack)
>     where
>     outPack = fWrapper sizex sizey locx locy actCnt iPack
>     newSndCnt n (Abst,_) = n
>     newSndCnt n (Prst _,_) = n + 1
> This is a simplified context for the call to the C function, which is
> wrapped by fWrapper.
> I figured out that one reason for double evaluation is that outPack is
> used twice in the return expression of gf.
> So my question now is, can I force the evaluation machine to evaluate
> fWrapper once and use the same result value in both places where outPack
> is used?
> I tried the NOINLINE pragma both on outPack and on fWrapper, but to no
> avail.
> Again, I am grateful for any hint to a work-around.

Try using a let-expression and/or make the tuple arguments strict.


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