data T a = T a (a -> Int)

data SomeT > T a

i1 = T 7 (1+)
p1 = i1 :: SomeT

i2 = T "abc" length
p2 = i2 :: SomeT

open1 (T v f) = f v

open2 :: SomeT -> _
open2 (T v f) = f v

ok1 = open1 i1
ok2 = open1 i2

ok3 = open2 p1
ok4 = open2 p2

ok5 = open2 i1
ok6 = open2 i2

--bad1 = open1 p1
--bad2 = open1 p2