Jean-Marie Gaillourdet wrote
I am trying to do something like the following:
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module TestCase where
data Any root = forall pos sel . T root pos sel => ANY pos
class T root pos sel | pos -> root, root -> sel where f :: pos -> sel -> Bool
instance T root (Any root) sel where f (ANY p) s = f p s
The code as posted seems to have a problem. The problem is noted by GHC 6.4 and hugs but is accepted by GHC 6.6. Such a change in GHC seems strange. The problem is not related to existentials, so we just drop them
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module TestCase where
data Any root = ANY
class T root pos sel | pos -> root, root -> sel where f :: pos -> sel -> Bool instance T root (Any root) sel
test x = f ANY x
Hugs and GHC 6.4 both complain about the instance of T: Hush (September 2006 version) says ERROR "/tmp/j.hs":9 - Instance is more general than a dependency allows and GHC 6.4.2 says Illegal instance declaration for `T root (Any root) sel' (the instance types do not agree with the functional dependencies of the class) In the instance declaration for `T root (Any root) sel' I concur. The class declares T as being a ternary relation such that the following holds forall r p p' s s'. T(r,p,s) && T(r,p',s') -> s = s' Now, the instance `T root (Any root) sel' is satisfied when root=Int, sel = Bool and when root=Int, sel = Int. Does it not? That falsifies the above proposition. In other words, the instance T is not functional with respect to the first and the third arguments. That is not surprising. What is surprising is why GHC 6.6 accepts such an instance?