[Haskell-cafe] GHC optimisations

[Rodrigo Queiro]

> Using the fromInteger (and fromRational) axioms should only *increase*
> precission, I don't see how that is such a bad thing.

I think it's bad if the behaviour of your program depends on the
optimisation level.

On 22/08/07, Twan van Laarhoven <twanvl at gmail.com> wrote:
> Stefan O'Rear wrote:
> > On Wed, Aug 22, 2007 at 06:36:15PM +0100, Neil Mitchell wrote:
> >
> >>Hi
> >>
> >>
> >>>If Num obeys ring axioms, fromInteger is a perfectly fine
> >>>ring-homomorphism. (It's also the first or second homomorphism taught.)
> >>
> >>Does Int obey these axioms? I'm thinking that assuming properties
> >>about things such as numbers is very likely to go wrong very quickly.
> >>Monads you might be able to get away with, Numbers you probably can't.
> >
> >
> > Int does obey the axioms, it's the classical ring ℤ[4294967296].
> > Double, however, does not:
>
> But Double is already quite badly behaved:
>  > let x = 1e20
>  > Prelude> 1 + (x - x)
>  > 1.0
>  > Prelude> (1 + x) - x
>  > 0.0
>
> Using the fromInteger (and fromRational) axioms should only *increase*
> precission, I don't see how that is such a bad thing.
>
> Also, as far as I can see GHC already does this optimizations if the
> type is specialized to Double. Except for the fact that the PrelRules
> rules don't seem to fire, because the constants get floated out.
>
> Twan
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