become

Signature

become : behaviour → { next-behaviour: behaviour }

Description

become new-beh returns { next-behaviour = new-beh }. The actor uses the next-behaviour field to replace the current behaviour after processing each message.

Without become, every message is handled by the initial behaviour. With become, each message can leave a different behaviour for the next — this is how state machines work.

Example

counter = count: msg:
  if msg == "inc" then
    reply (count + 1) // become (counter (count + 1))
  else if msg == "get" then
    reply count
  else
    { };

After "inc", become (counter (count + 1)) installs a new behaviour that has count + 1 closed over. The next message will call counter (count + 1).

State machine

toggle =
  state: msg:
  if msg == "flip"  then reply.right (! state) // become (toggle (! state))
  else if msg == "query" then reply.right state
  else { };

toggle-c = actor (toggle false);
(toggle-c { inbox = st "flip" "query" "flip" "query"; }).outbox.toList
# [ { right = true; } { right = true; } { right = false; } { right = false; } ]

Notes

become is only meaningful inside a behaviour returned to actor. Using it in a plain function has no special effect — it just produces an attrset.
If next-behaviour is absent, the current behaviour is reused for the next message.
become is stateless itself — it just packages a function reference into an attrset field.

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