Improving the UI of generics

[anandabits (Matthew Johnson)]

@DevAndArtist

I still need to think through how any would apply to the design of Type<T> (or whatever we would call it). IIRC @brentdax told me back then that a metatype is like a struct or class , that's why we dropped the Metatype name.

I wonder if we should build on the any and some prefix syntax and use a prefix like meta (as a strawman):

• any T is the associated existential
• meta T is the metatype of T
• any meta T is the metatype of the associated existential
• any P is the existential for the protocol
• meta P is the protocol metatype
• any meta P is the metatype of the protocol existential

I realized after posting it that my approach to solving the nesting issue depended on the fact that the name of the existential type matches the name of the protocol (or the unbound name of a generic type for concrete type existentials). This is what made the P.NestedType or Array.NestedType syntax work.

With this prefix approach to existentials that would become (any P).NestedType or (any Array).NestedType which is not what we want when it comes to using types and protocols as namespaces. Maybe one option would be to use some on concrete types:

// We don't know what the type of `Element` is so any static members put in this
// extension are available using `Array.NestedType`, `Array.constant`, etc syntax.
// (as well as being available as `Array<Int>.NestedType`, etc
extension some Array {}