The dumbest (line-based, char-based would need some escaping) multiplexing layer I can think of is: every line (in & out) starts by a number followed by space and the actual line. 0 is reserved for creation/destruction of in & out pairs.

I'm a bit confused following this, what is multiplexing?

@dominicm https://en.wikipedia.org/wiki/Multiplexing?wprov=sfsi1 here channeling concurrent repl sessions over a single connection.

@cgrand Ah, tangential to what I was asking about previously. By "repl session" do you mean the multiple isolated calls to unrepl/launch (or what it actually is)? Or something else?

Multiple isolated repls (plain repl, unrepl, whatever)

And the use case is parallelism?

No. It's reusing an established connection rather than creating a new one.

It only matters in contexts where new connections can't be established.

Why would you need to establish another repl?

One for the user interactions in the repl UI. One for other tools (eg completion etc)

Why can't it all be done via a single repl & group ids? Possibly with a few new commands for background evaluations?

It could but then it would barely qualifies as a REPL. A REPL is a single thread model by definition. I think commands should not execute in the background.

Okay. Is there a specific reason to adhere to that? Or mostly simplicity reasons

Simplicity (of model). One connection = serial execution. Want concurrency? Then use several connections.

The fun thing is that since embarking on this project a fortnight ago my understanding of what is a REPL has been refined a lot. Some points made by Rich when he added the socket repl only made sense recently to me.

@cgrand care to elaborate?

The serial execution model for once. (See above). The session multiplexing being orthogonal to the repl and marginally needed too.

The "fun" is that it's usually a feeling I get when teaching.

This design is really good. It fits really well with Vim's crappy async model. The more I think about it, the more I like it & can see it bringing a lot of useful features to Vim that aren't possible currently.