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

Jean-Marie Gaillourdet wrote:

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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.