I don't know what you consider reasonable, but you can express this in Haskell with type classes and existential types: data P = forall a . Parent a => P a

I don't know what you consider reasonable, but you can express this in Haskell with type classes and existential types: data P = forall a . Parent a => P a type PList = [P] where Parent is a type class. You have to be a bit more explicit when using a Child in the position of a Parent when adding to the list and you have to use (fully polymorphic!) pattern matches to extract elements from the list, but personally I consider this a good thing as it's more explicit. Aside: Existential types is what OOP interfaces are -- and interfaces are often encoded as such in language semantics; see e.g. Types and Programming Languages (Pierce). EDIT: Typos in code -- unfortunately my Haskell is a little rusty :(.