[Haskell-cafe] Re: A question about functional dependencies and existential quantification

[Felipe Almeida Lessa]

On 3/27/07, Jean-Marie Gaillourdet <jmg at informatik.uni-kl.de> wrote:
> > 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


I'm not an expert as well, but I think oleg was actually talking about

> instance T Int (Any Int) Int
> instance T Int (Any Int) Bool

which also fails (on GHC 6.6) with:

    Functional dependencies conflict between instance declarations:
      instance T Int (Any Int) Int -- Defined at ...
      instance T Int (Any Int) Bool -- Defined at ...

although just

> instance T root (Any root) sel

is accepted.

-- 
Felipe.