# flow-on-erlang — durable workflows in the fed-sx runtime

A native Erlang-on-SX port of the Scheme flow engine (`lib/flow`), so
the fed-sx kernel can fan arriving activities out into durable,
branching, multi-step business flows **in its own runtime** — no
cross-guest FFI, no marshalling, no Scheme dependency. The seed of a
real engine that can later supersede the Scheme one for substrate use.

Run the suite: `bash next/flow/conformance.sh` → engine conformance.

## Model

A **flow** is an Erlang `fun(Ctx) -> Ctx`. Combinators (`flow_spec`)
compose flows; user code stays value-level (the functions you hand to
`flow_node`/`branch`/… take and return plain values). A flow that
ignores its input is a thunk; composition *is* function composition.

```erlang
F = flow_spec:sequence([
      flow_spec:flow_node(fun(Draft) -> Draft + 1 end),
      flow_spec:branch(fun(P) -> P >= 3 end,
                       flow_spec:flow_const(ok),
                       flow_spec:flow_const(rejected))]),
flow:run(F, 2)            %% => {flow_done, ok}
```

## Durability — deterministic replay

Same semantics as the Scheme engine: a flow re-runs from the top on
every resume; effects/non-determinism go through `flow:suspend/1`,
whose resolved values are logged; an already-resolved suspend replays
its logged value, and the first unresolved suspend short-circuits back
to the driver. The persisted state is the **replay log** — plain
`[{Tag, Value}]` data — so nothing live (no continuation, no process)
is ever serialized; an instance survives restart by re-driving its
named flow against its log.

```erlang
flow_store:register_flow(publish, F),
{ok, Id, R} = flow_store:start(publish, Draft),  %% R = {flow_suspended, Tag} | {flow_done, V}
%% ... driver performs the effect for Tag, then:
flow_store:resume(Id, EffectResult)              %% re-drives; completes or suspends again
```

## Why railway threading instead of call/cc + a global

The Scheme engine uses an escape-only `call/cc` plus a mutable global
replay log. This Erlang-on-SX runtime can't do either, and has a third
sharp edge:

- **No re-enterable continuation** — but suspend only needs to *escape*,
  which Erlang `throw` could do …
- **… except a blocking `receive` / `gen_server:call` inside a `try`
  deadlocks** the cooperative scheduler. So `suspend` must not consult
  the log via a registry process while inside a `try`.
- **No process dictionary** — so there is no ambient per-process slot to
  stash the replay log in.

The resolution: thread the replay log through a railway-style **context**
and make `suspend` *short-circuit* (like a `fail` value) rather than
throw. No ambient state, no throw, no gen_server in the hot path —
purely functional, which sidesteps all three constraints. The driver
(`flow_store`) is the only stateful part, and it calls the pure
`flow:drive/3` from inside `handle_call`, never wrapping a blocking
receive.

A `Ctx` is `{flow_cont, Value, Log}` (running) or `{flow_susp, Tag,
Log}` (short-circuited); every combinator passes a suspended context
straight through.

## Modules

| Module | Role |
|---|---|
| `flow.erl` | pure replay driver: `drive/3`, `run/2`, `suspend/1`, the `Ctx` constructors/accessors |
| `flow_spec.erl` | combinator algebra: leaves, `sequence`/`parallel`/`map_flow`, `flow_while`/`flow_until`, `branch`, railway `fail`/`recover`/`attempt`, `tap`, `try_catch`/`retry` |
| `flow_store.erl` | durable gen_server: named-flow registry + instance table + `start`/`resume`/`status` |

## Consumed by

The fed-sx kernel's trigger fan-out (`pipeline.erl` + `flow_dispatch`)
starts named flows from arriving activities; see
`plans/fed-sx-host-types.md` and the triggers phases.

## Not yet (later layers)

- Persisting instance logs to the kernel's durable on-disk log (the
  data shape is already restart-ready; only the backing is in-memory).
- `parallel` with multiple independent suspends resolving concurrently
  (current `parallel` is sequential under one shared log).
- Full parity with the Scheme engine's distributed/remote nodes.
