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

[oleg at pobox.com]

Jean-Marie Gaillourdet wrote:
>> class T root pos sel | pos -> root, root -> sel where
>>    f :: pos -> sel -> Bool
>> instance T root (Any root) sel
> 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".

The class T has two functional dependencies: pos -> root and
root->sel. I believe you are talking about the former whereas my
previous message was talking about the latter.