actor

Signature

actor : behaviour → cycle-c
# where
#   behaviour = state → msg → { reply?, next-behaviour?, ...extras }
#   cycle-c   = { inbox: ST } → { outbox: ST, states: ST }

Description

actor initial-behaviour returns a cycle-c — a function that takes { inbox } and returns { outbox, states }.

flowchart LR
  beh["initial-behaviour\n(Nix function)"]
  actor["actor"]
  cyclec["cycle-c\n{ inbox } → { outbox, states }"]
  beh --> actor --> cyclec

Internally, actor runs scanl over the inbox stream. Each message is passed to the current behaviour; the result is used to:

Extract next-behaviour (if present) as the behaviour for the next message
Carry all other result fields into the accumulated state

outbox is states.fields "reply" — the reply field from each state, flattened into a stream.

Parameters

Parameter	Type	Description
`initial-behaviour`	`state → msg → attrset`	The first behaviour to run. Subsequent behaviours come from `become`.
`inbox`	`ST`	Stream of messages to process.

Return value

Field	Type	Description
`outbox`	`ST`	Stream of `reply` values from each message that produced one.
`states`	`ST`	Full scanl output — one state per message plus the initial state.

Behaviour return fields

Field	Effect
`reply`	Value placed in `outbox`. May be a plain value or a `ST` (flattened).
`next-behaviour`	Replaces the current behaviour for the next message.
Any other field	Passes through to `states`. Accessible via `states.fields "fieldname"`.

Examples

Stateless actor:

echo-c = actor (msg: reply msg);
(echo-c { inbox = st "a" "b"; }).outbox.toList
# [ "a" "b" ]

Stateful actor:

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

counter-c = actor (counter 0);
(counter-c { inbox = st "inc" "inc" "get"; }).outbox.toList
# → [ { right = 1; } { right = 2; } { right = 2; } ]

Fan-out (ST reply):

expander-c = actor (msg: reply (st msg msg));
(expander-c { inbox = st "a" "b"; }).outbox.toList
# [ "a" "a" "b" "b" ]

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