Hi!
GHC has some support for referring to existentially quantified type
variables. For example, consider the following definition:
> data SomeList = forall a . SomeList [a]
With the ScopedTypeVariables extension, you can get hold of the concrete
type a used in a SomeList value:
> f :: SomeList -> SomeList
> f (SomeList [x :: a]) = SomeList xs where
>
> xs :: [a]
> xs = [x, x]
However, this approach does not work if there are no fields whose type
involves the existentially quantified type variable. Consider, for
example, the following definition:
> data IsListType a where
>
> IsListType :: IsListType [b]
For each type T, we can match a value of type IsListType T against the
pattern IsListType. If this succeeds, we know that T is a list type, but
we do not have access to the respective element type.
Is there a way to determine existentially quantified type variables like
the b in the above example?
All the best,
Wolfgang
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