Enum on Float/Double

[Hal Daume III]

> > My preference would be for succ (+-0) to return the smallest positive
> > real, since then you could define succ x to be the unique y with
> > x < y and forall z . z < y => not (x < z), where such a y exists, and
> > I'm not sure if the Haskell standard knows about signed zeros.
> 
> Is this really useful?  Why would you need this number?  Peano
> artithmetic on reals? :-)

Is there any way to do this (yet)?  I found a case where I really need:
  f :: Float -> Float
where
  f x is the least y such that x < y

even if i have to FFI to C, I'd really like a solution.

any help would be appreciated.

 - Hal