And in the context of Haskell, non-nullary type constructors are still types. They don't happen to be the types of any values, but they can still parameterize t

And in the context of Haskell, non-nullary type constructors are still types. They don't happen to be the types of any values, but they can still parameterize types, be constrained by type equality (eg. a ~ Maybe), etc.