signum (recip (-1 :% 1)) should not be 1 % 1

[Ross Paterson]

On Thu, Dec 05, 2002 at 07:04:34PM +0100, Ralf Welter wrote:
> I think I have found a bug in the hugs prelude:
> Bug: definition of recip in the instance declaration of breaks silent 
> assumptions about the internal data in Ratio types.
> 
> Description:
> data Integral a => Ratio a = !a :% !a deriving (Eq)
> by deriving Eq a normal form is assumed. Further places do at least 
> assume a positive denominator:
> 
> >instance Integral a => Num (Ratio a) where
> >    ...
> >    abs (x :% y)      = abs x :% y
> >    signum (x :% y)   = signum x :% 1
> silently assume the denominator is positive
> 
> unforturtunately the definition of recip
> >instance Integral a => Fractional (Ratio a) where
> >    ...
> >    recip (x:%y)        = y :% x
> will break these properties.
> fix: remove the colon on the right hand side of the recip definition.

Thanks for the report and fix -- you're right, this is what the Report
specifies (and what GHC does).  It's now fixed in CVS.