[Haskell-cafe] Non-existant existential?

Jim Apple jbapple+haskell-cafe at gmail.com
Thu Sep 21 11:58:50 EDT 2006

Consider the following:

data SimpExist a = SimpExist (forall x . x -> a)
f :: SimpExist Bool
f = SimpExist (const True)
g = SimpExist id

What is the type of g? In a similar example, GHC tells me it is of
type SimpExist c. Yet, I can't unify it with any other SimpExist c'.

It seems to me that this is something like exists x . SimpExist x, and
is similar to:

data ExistWrap = forall a . ExistWrap (forall x . x -> a)


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