"Real world" example needed for ...

[Steffen Mazanek]

Hello,

I am looking for a real world example of a type constructor, that expects
another type constructor as an argument in the context of subtyping,
e.g., one could define

--not Haskell98
data T a b c = F (a b c)
a::T (->) Pt Pt
a = F id
b::T (->) CPt CPt
b = F id
l = [a,b]

Thereby CPt is assumed to be a subtype of Pt. From my expectations
l should be typable [T (->) CPt Pt], because of contravariance of (->).
Using c::T (Either) Pt Pt and d::T (Either) CPt CPt one would gain
[c,d]::T (Either) Pt Pt.

I need this "real world" example for the sake of motivation, that variance
can be expressed by Haskell kinds, e.g., in a kind system the kind of
T could be (a->b->*)->a->b->*. Thereby the "kind variables" a and b
can be instantiated by "special kinds" + and -, that represent co- and
contravariance, respectively. So one would have T (->)::- -> + -> *
and T (Either)::+ -> + -> * and so on...

Any ideas are very appreciated.
Ciao,
Steffen