Hi, oleg@pobox.com wrote:
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'
As I understand functional dependencies, and I am definitely _not_ an expert on them, they state, that given the lhs of a dependency is instantiated, the type variables of the rhs are determined, too. If that is correct, I don't understand why this instance should be to general, as every instantiation of "root" exactly determines the corresponding instantiation of "Any root".
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?
GHC 6.6 does not accept such instances. Add the following code to the module TestCase
instance T Int Int Int instance T Int Int Bool
and you get the following error: TestCase.hs:10:0: Functional dependencies conflict between instance declarations: instance T Int Int Int -- Defined at TestCase.hs:10:0 instance T Int Int Bool -- Defined at TestCase.hs:11:0 -- Jean-Marie