Proposal for joined and joined(separator:)

[FranzBusch]

This adds a proposal for the joined and joined(separator:) algorithms

@phausler Could you check if there is anything missing. Also is it fine to have you as the author of the proposal since you implemented the algo?

Read the full proposal here

[phausler]

That looks fine to me, if you want to put it on the forums and manage the thread associated with feedback for it please feel free to post.

[twittemb]

Hi,

What is the difference between joined and flatMap { $0 } ?

[kperryua]

What is the difference between joined and flatMap { $0 } ?

They're semantically identical. However the theory is that knowing in advance that the incoming element is already an AsyncSequence and that no transformation will be happening on it will allow for a more optimized implementation.

It's worth noting that the stdlib also has both joined() and flatMap() on Sequence, although perhaps for different reasons than we do here. Sequence.joined() returns a FlattenSequence which lazily enumerates the constituent sequences, while flatMap() does so greedily.