Andre W B Furtado wrote:

[sorry for the silly question]

It's not *that* silly, most people have to browse through Haskell's strange numeric class hierarchy to answer it... :-[

Which is the correct way to convert a GLfloat to a GLdouble? (There is any Prelude function that converts a Float to a Double?)

Given the fact that one doesn't know what floating types GLfloat/GLdouble correspond to exactly, one has to go a rather generic way via Rational: In class Real there is toRational :: Real a => a -> Rational and class Fractional contains fromRational :: Fractional a => Rational -> a As a convenience function, the Prelude contains realToFrac :: (Real a, Fractional b) => a -> b realToFrac = fromRational . toRational which does exactly what you want, since Float and Double have both instances for Real and Fractional. Not very intuitive, but that's the way Haskell is (currently) defined... A performance note: GHC's Prelude has rules in its Prelude to avoid going via Rational for all 4 possible combinations of Float and Double for realToFrac, so it's relatively cheap. Cheers, S.