On Friday 21 May 2010 22:06:43, Henning Thielemann wrote:
> On Fri, 21 May 2010, Daniel van den Eijkel wrote:
> > Dear Haskellers,
> >
> > I just want to share an observation. I had to convert a Double to a
> > Float value in an inner loop of an application, and I used somethin
> > like this:
> >
> > xf = (fromRational $ toRational xd) :: Float
>
> I think realToFrac is the function to use here, and this might be
> replaced by double2Float by an optimizer rule. I think double2Float is
> from a GHC package and thus one should avoid to call double2Float
> directly.
In GHC.Real:
-- | general coercion to fractional types
realToFrac :: (Real a, Fractional b) => a -> b
realToFrac = fromRational . toRational
{-# RULES
"realToFrac/Int->Int" realToFrac = id :: Int -> Int
#-}
There are more rules elsewhere. If you compile with optimisations, GHC
turns your realToFrac into double2Float# nicely, so it's okay to use
realToFrac.
However, without optimisations, no rules fire, so you'll get
(fromRational . toRational).
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