Shared variables and functions

[xiaq]

Persistent variables and persistent functions are shared across multiple sessions and preserved on shell exits. They are useful for passing data or code between concurrent sessions without (explicit) serialization/deserialization. In particular, they are ideal for configuration variables and functions and may serve as a more advanced clipboard facility.

The ideas are taken from fish, which has universal variable and autoload functions, with quite different implementations. All universal variables are stored into one file, and fish instances talk to an agent, fishd, to access them. In the case of autoload functions, however, each function is stored into a separate file and there is no agent to coordinate the access. Users are encouraged to manage autoload functions manually.

Open questions:

1. Is unifying persistent variables and functions necessarily a good idea?
2. How to statically check persistent variables and functions?
3. Is an agent needed to coordinate access? If so, how should the protocol be? (Note: The elvished package already includes a prototype of the agent.)

[xiaq]

I have settled on the terminology of "shared" variable and function. Updates to the design:

shared: requires mandatory qualification

Shared variables (including functions which are also variables) reside in the special shared: namespace (as per #8), which is deliberately not in the scoping rule of unqualified variable access, since it is hard (impossible?) to check access to the shared namespace statically - both the existence of a variable and its static type cannot be determined statically if variables can be added and deleted from shared: at any shell session.

This means that all accesses to shared: must be qualified. They are still suitable for storing global configuration and as a clipboard, but the case with autoload functions is now more difficult.

Storage mechanism ✔

There will be no agents; all clients access the SQLite database directly. According to the FAQ, SQLite handles process-level concurrency well.

[xiaq]

We can implement a predefined shared module.

[tfga]

Was this shared module ever implemented?
How can I define my own shared variables?

[krader1961]

Was this shared module ever implemented?

Yes, see commit 92fb6e5 that appears as a link directly above your question. It was removed two years later in commit c1b433d.

[tfga]

@krader1961 Is there any alternative today?

[xiaq]

Shared variables have more complex semantics than simple variables. Notably, reading a shared variable can fail while reading a normal variable never fails.

The underlying store API is still there. This is in scope for #1032.