Provide access to auto-generated greetings

[cpsievert]

As a follow up to #109, see #109 (comment)

@gadenbuie wrote:

I've been thinking about this a bit and I'd like to explore having greeting be set up as an active binding. We'd have a private$greeting that stores the greeting, and then $greeting() can be a getter/setter. If we you call $greeting() and we don't have one stored yet, we call generate_greeting(), store it and then re-use it.

I think this would eliminate the need for has_greeted and instead we can restore the greeting.

I guess this brings up the question of how we'd want to handle greetings, for which there are four options.

1. If not provided, always generate a greeting on every new session.
2. If not provided, generate on the first session and then re-use each session.
3. If not provided, don't greet.
4. If provided, always use that greeting.

Should we support all of these? If so, we could have a generate_greeting = c("once", "always", "never") argument in QueryChat$new().

[cpsievert]

I'd like to explore having greeting be set up as an active binding

Assuming we want a greeting to be streamed in if not provided (which seems critical -- we don't want to block the UI until the entire greeting is generated), I'm struggling to see how this is gonna work.

eliminate the need for has_greeted and instead we can restore the greeting.

This is also a bit tricky because the backend/frontend state of the chat isn't identical -- the backend state will have the "user greeting" prompt, whereas the frontend won't

All that said, it's still not great that we don't provide easy access to the auto-generated greeting. If we want to allow for that, we may just want to include it with the $server()/.server() result? Or we just punt on this since you can get it from the client?

[gadenbuie]

I don't think we need in-session access to the greeting -- I'm not sure what you would do with it anyway. IIRC (this might have changed recently), we generate the greeting from a forked client, so it's shown in the UI and not part of the general chat history. I think this is okay and if you want to re-use the greeting, you'd use generate_greeting = "once".

Assuming we want a greeting to be streamed in if not provided (which seems critical -- we don't want to block the UI until the entire greeting is generated), I'm struggling to see how this is gonna work.

With generate_greeting = "once", the first session that generates the greeting would store it in the querychat object and re-use it in subsequent sessions. Since we control that and multiple sessions might start in the greeting-needed window, we'd just store the first generated greeting, and only re-use it in sessions where the greeting was already available.

This would also let users store or modify the global greeting by updating the querychat object (in R and Python).

So reading the active binding would not need to trigger the generation, it would just be a getter/setter. (Or maybe in R we won't need an active binding, this could just be a field/property?)