Generics

• Swift's tool for type-level abstraction

Generics 의 한계

• they work in a way that is fundamentally in the caller's control

Opaque Type 이 필요한 이유

However, it's common to want to abstract a return type chosen by the implementationfrom the caller.

For instance, a function may produce a collection, but not want to reveal the details of exactly what kind of collection it is. This may be because the implementer wants to reserve the right to change the collection type in future versions, or because the implementation uses composed lazy  transforms and doesn't want to expose a long, brittle, confusing return type in its interface. At first, one might try to use an existential in this situation:

AnyCollection 을 사용해도 타입 관계 정보는 지워지기 때문에 value-level abstraction

implementaion이 type-level absratction 을 할수 있는 방법이 기존에는 없었다.