8 Dec
2005
8 Dec
'05
9:04 a.m.
Ben,
data State = Start
| Stop | forall a . (Show a, Eq a) => State a
But this is a rank-2 type, not an existentially quantified type?
No, this one really is an existentially quantified type. An example of a data constructor with a rank-2 type would be, f.i.: data State = ... | State (forall a . (Show a, Eq a) => a) Here we have State :: (forall a . (Show a, Eq a) => a) -> State while in the existentially quantified case we had State :: forall a . (Show a, Eq a) => a -> State which has rank 1. (The rank-2 example does not make much sense, however, for the only value that can be passed to State is bottom. But, hey, it's just an example.) Regards, Stefan