continuing the list of odd cases in type class handling, here is a
small example where overlap resolution is not taken into account
when looking at FDs.
context: both hugs and ghc resolve instance overlaps in favour
of the most specific declaration.
so the following works in both ghc (darcs, 25022006) and
hugs (minhugs 20051031):
{- both ghc and hugs accept without 3rd par and FD
neither accepts with 3rd par and FD -}
data T = T deriving Show
data F = F deriving Show
class TEQ a b {- tBool | a b -> tBool -} where teq :: a -> b -> Bool
instance TEQ a a {- T -} where teq _ _ = True
instance TEQ a b {- F -} where teq _ _ = False
test = print (teq True 'c', teq True False)
and both print "(False,True)", so best-fit overlap resolution entirely
determines which instance to choose!
now, if we uncomment the third class parameter, they both complain
(as expected, though about different things).
however, if we also uncomment the functional dependency, to fix
the ambiguity wrt the 3rd parameter, both complain that this FD
is in conflict/inconsistent with the instances!
as far as i understand it, the potential inconsistency should have
been eliminated by the best-fit overlap resolution, so I submit this
is a bug (and unlike the earlier example I submitted to hugs-bugs,
this fails with both hugs and ghc; and it is less open to alternative
interpretations, I hope).
cheers,
claus
