[ghc-steering-committee] Proposal: Binding existential type variables (#96)

[Roman Leshchinskiy]

On Tue, Mar 27, 2018 at 6:24 AM, Joachim Breitner <mail at joachim-breitner.de>
wrote:

>
> 2. The distinction between existentials and universals is oddly abrupt.
>

This is a good point. In addition to what Joachim said, universals can also
depend on existentials which I find weird.

As an aside, here is one corner case which I don't think is handled in the
proposal:

data T a where
  MkT :: forall a. T a

With -XPolyKinds, the type inferred for T is forall k (a :: k). T a. Now, k
is an existential but can it be bound in a pattern? The proposal only
mentions 2 cases: no forall and a forall that mentions all type variables
and neither applies here.

Roman
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