howdy/console

Values a running app hands to an interactive console.

A console such as GSH can attach to a running Erlang node and run Gleam code inside it. That code can call any module, but the database connection, auth configuration and other values main built exist only as local variables there. expose publishes them under a key so the console can get the very ones the server is using, rather than building a second set that might be configured differently.

Declare each key once, as a function with its type written out, and use it from both sides:

import howdy/console

pub type Services {
  Services(db: Repo, identity: auth.Auth)
}

pub fn services() -> console.Key(Services) {
  console.key("my_app")
}

pub fn main() {
  let services = Services(db: database.connect(), identity: ...)
  console.expose(services(), services)
  ...
}

In the console, console.get(my_app.services()) returns the running server’s Services.

Values are kept in persistent_term: reading one is as cheap as reading a constant, and exposing one again replaces it. Replacing makes the VM scan every process, so expose values once at startup, not per request.

pub opaque type Key(a)

pub fn expose(key: Key(a), value: a) -> Nil

pub fn get(key: Key(a)) -> Result(a, Nil)

pub fn key(name: String) -> Key(a)