Hi!
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.
Descriptiion:
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.
Hope this helps - I havn't had the time to check if ghc and other
implementations suffer from the same issue.
regards,
ralf welter
