[Pitch] Primary Associated Types in the Standard Library

[Ben_Cohen]

@lorentey

In the specific case of LazySequenceProtocol / LazyCollectionProtocol , this recommendation still leads to choosing Elements as the primary associated type rather than Element .

I agree it should be Elements. Elements is kind of what I refer to as Base in this post on the review of primary associated types:

SE-0346: Lightweight same-type requirements for primary associated types

Not with Lazy in its current form. You still need to surface enough information to drive the conditional conformance. In the case of LazyMapCollection, that conditional conformance is driven by a generic parameter Base, the collection the lazy wrapper wraps. When that wrapped collection is bidirectional, so is the lazy collection. If you just returned an opaque some Collection<Element> that happened to be a LazyMapCollection<[Int]> then the information to drive the conditional conformance is n…

I am not actually sure Elements will actually prove useful to anyone as a primary associated type, but it probably won't do any harm.