Generic container types should really have two parameters: one type that you're guaranteed to be able to put in, and one that you're guaranteed to get when you

Generic container types should really have two parameters: one type that you're guaranteed to be able to put in, and one that you're guaranteed to get when you take it out. Then a Container<emits:A,accepts:B> is-a Container<emits:C,accepts:D> if A<C and B>D. So, e.g., HarvestTomatoes() can return a Basket<Tomato>, which is shorthand for Basket<emits:Tomato,accepts:Tomato>, and MakeJuice can take a Basket<emits:Fruit,accepts:EMPTY_TYPE>, and it's safe to feed the former into something that wants the latter. (Of course actual container objects will typically be of type Container<emits:T,accepts:T> for some specific T.)