Skip to content
dnzl

Multi-Output Cycles

Named outputs

Section titled “Named outputs”

A cycle-c can return any attrset. Return multiple named streams instead of a single outbox:

flowchart LR
  inbox["inbox [5,-1,3,0,7]"]
  v["validator-c"]
  inbox --> v
  v -->|"outbox.right"| vals["values\n[5, 3, 7]"]
  v -->|"outbox.left"| errs["errors\n["non-positive: -1", …]"]
validate-c =
  { inbox }:
  let a = validator-c { inherit inbox; };
  in {
    values = a.outbox.right;
    errors = a.outbox.left;
  };


result = validate-c { inbox = st 5 (-1) 3 0 7; };
result.values.toList
# [ 5 3 7 ]
result.errors.toList
# [ "non-positive: -1" "non-positive: 0" ]

Callers access named outputs directly without knowing about Either internals.

Side-channel state with states.fields

Section titled “Side-channel state with states.fields”

Behaviours can set extra fields that don’t appear in outbox. The states stream carries all per-message state, and states.fields "key" extracts any field:

flowchart LR
  inbox["inbox ["inc","get","inc","get"]"]
  lc["logged-counter-c"]
  inbox --> lc
  lc -->|"outbox"| ob["outbox [1,1,2,2]"]
  lc -->|"states.fields "kind"\n(st.filter "mutation")"| mut["mutations ["mutation","mutation"]"]
  lc -->|"states.fields "kind"\n(st.filter "query")"| qry["queries ["query","query"]"]
logged-counter =
  count: msg:
  if msg == "inc" then
    reply.right (count + 1) // { kind = "mutation"; } // become (logged-counter (count + 1))
  else if msg == "get" then
    reply.right count // { kind = "query"; }
  else
    { };


lc = actor (logged-counter 0);


lc-c =
  { inbox }:
  let a = lc { inherit inbox; };
  in {
    mutations = a.states.fields "kind" (st.filter (k: k == "mutation"));
    queries   = a.states.fields "kind" (st.filter (k: k == "query"));
    outbox    = a.outbox;
  };


result = lc-c { inbox = st "inc" "get" "inc" "get"; };
result.mutations.toList  # [ "mutation" "mutation" ]
result.queries.toList    # [ "query" "query" ]

states.fields "kind" extracts the kind field from each state into a ST. Applying st.filter to it filters by value.

Audit log pattern

Section titled “Audit log pattern”

Use an extra field to build an audit log alongside normal output:

logged-div =
  msg:
  if msg.divisor == 0 then
    { reply = { left = "div-by-zero"; }; log = "error: div-by-zero"; }
  else
    { reply = { right = msg.dividend / msg.divisor; }; log = "ok: ${toString (msg.dividend / msg.divisor)}"; };


a = (actor logged-div) { inbox = st { dividend = 10; divisor = 2; }
                                     { dividend = 5;  divisor = 0; }; };


a.outbox.toList                  # [ { right = 5; } { left = "div-by-zero"; } ]
(a.states.fields "log").toList   # [ "ok: 5" "error: div-by-zero" ]
Contribute Community Sponsor