this means supporting the 'exists' quantifier as creating existential types much the same way 'forall' can be used in ghc. An example would be
type Any = exists a . Typeable a => a sameType :: Any -> Any -> Bool sameType x y = typeOf x == typeOf y
When existentials appear in the contravarient position such as above, they can be desugared into rank-n types with foralls. if existentials are allowed in the covarient position, then support for FirstClassExistentials? is needed.
Jhc currently supports the existential syntax anywhere a type is accepted, but will report an error if one is used in a covariant position with the exception of when it is a component of a data type.