> > > The compiler cannot guess that some primitive Float->Double
> > > function can
> > > be used instead of going through Rational.
> >
> > If enough inlining is done, then it should be able to deforest the
> > intermediate Rational and generate the same code. But I
> agree, using RULES
> > here is quicker and doesn't rely on some hefty unfoldings.
> Unless ghc has gotten incredibly clever I don't believe that. Look at
> the code involved in such a conversion. It involves, among
> many other things,
> encodeFloat and decodeFloat. So unless you have taught ghc about how
> floating point numbers are represented on all your target
> machines (I mean,
> it doesn't have to be IEEE) you can't really inline these at
> compile time.
> No, this is a perfect example of where you need a RULE.
>
> (BTW, hbc has optimized these conversion since about 6 years ago.)
Ah yes, I forgot how complex fromRat was. Good point.
Simon
