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