[Proposal] Create new Registry functions to allow arbitrary PID registration

[Noah Betzen]

Erlang's process group module allows adding arbitrary PIDs to a group, which can be used for things like PubSub: https://www.erlang.org/doc/man/pg.html

Registry's register function currently always registers the current process; you cannot register an arbitrary process: https://hexdocs.pm/elixir/1.14.5/Registry.html#register/3

However, there are ways to work around this with Registry; see attached Livebook markdown for an example.

I propose new functions:

• Registry.register/4
  • register(pid, registry, key, value)
    
• Registry.unregister/3
  • unregister(pid, registry, key)
    
• Registry.unregister_match/5
  • unregister_match(pid, registry, key, pattern, guards \\ [])
    
• Registry.update_value/4
  • update_value(pid, registry, key, callback)
    

Another (potentially simpler) option might be to have the pid argument be at the end and make it default to self().

Thoughts? 😄 

[Noah Betzen]

I made a PR that implements this: https://github.com/elixir-lang/elixir/pull/12609

[Noah Betzen]

Disregard, José explained that this isn't possible because of the two ETS tables that Registry uses under the hood. Oh well. 😄