[Haskell-cafe] Data structure containing elements which are instances of the same type class

[David Feuer]

On Aug 15, 2012 3:21 AM, "wren ng thornton" <wren at freegeek.org> wrote:
> It's even easier than that.
>
>     (forall a. P(a)) -> Q  <=>  exists a. (P(a) -> Q)
>
> Where P and Q are metatheoretic/schematic variables. This is just the
usual thing about antecedents being in a "negative" position, and thus
flipping as you move into/out of that position.

Most of this conversation is going over my head. I can certainly see how
exists a. (P(a)->Q) implies that (forall a. P(a))->Q. The opposite
certainly doesn't hold in classical logic. What sort of logic are you folks
working in?
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