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
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