SIP-69: Existential containers

[kyouko-taiga]

[jducoeur]

Comment from the peanut gallery: I agree with Haoyi that the structure of the proposal is currently a little confusing. Specifically, since it jumps directly to implementation details, it's very unobvious what the userland code would look like. I'd recommend putting a motivating example of "this is what the end result would look like" before getting into the details of the encoding of Containing.

That said, I love the meat of this proposal -- it's reifying exactly the pattern that I've had to build by hand a number of times for various systems (and which I've been occasionally teaching to my teams), so it would be a real win IMO if we had an official and concise solution.