Actor

Actor module: messaging helpers for the actor system.

cast/2 is fire-and-forget (async): the message is enqueued and the caller continues immediately without waiting for a response.

call/3 is synchronous: it sends the given message to the target actor and blocks until the actor calls Actor.reply(reply_to, result). Returns Ok(result), or Err(reason) if no reply arrives.

call protocol: Pass a zero-arg sentinel constructor as the call message. Its tag selects which handler receives the call, and the runtime injects the caller (the "reply channel") as that handler's first argument. The handler must call Actor.reply(reply_to, result) to unblock the caller; if it never replies, call returns Err("no reply ...").

So the call handler must be declared FIRST in the actor (tag 0), and
the sentinel must have a name distinct from that handler to avoid a
constructor-name clash.

Example:

type GetReq = GetReq          -- zero-arg sentinel for the sync call

actor Counter do
  state { count : Int }
  init  { count: 0 }

  -- First handler (tag 0) = the call handler; reply_to is the caller.
  on GetCount(reply_to) do
    Actor.reply(reply_to, state.count)
    state
  end

  on Inc(n : Int) do
    { state with count: state.count + n }
  end
end

fn main() do
  let pid = spawn(Counter)
  Actor.cast(pid, Inc(1))
  run_until_idle()
  match Actor.call(pid, GetReq, 5000) do
    Ok(n)  -> println("count = " ++ int_to_string(n))
    Err(e) -> println("error: " ++ e)
  end
end