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).