Force single evaluation?
sven.panne at aedion.de
Thu Dec 21 06:27:06 EST 2006
Am Mittwoch, 20. Dezember 2006 16:10 schrieb Axel Jantsch:
> [...] My simplified codes is as follows:
> f :: InparT -> IO CInt
> f inPar = do
> inP <- malloc
> poke inP inPar
> r <- cfun inP
> free inP
> return r
This can be simplified to 'flip with cFun' (malloc/free pair => alloca,
alloca/poke => with).
> [...] So even if I make cfun pure (which it is), the enclosing function f is
This depends on the viewpoint: Unless malloc fails, f will return the same
value for the same argument, so it can be considered pure.
> Shall I pretend f is pure and wrap it into unsafePerformIO?
You can do this, but you still depend on the compiler/interpreter not doing
any funny transformations which could duplicate an application of f. I don't
think that this will be a problem in practice, but to be on the safe side you
could memoize f or cFun and perhaps even a 1-element cache might be enough to
avoid all this trickery.
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