# Erlang-on-SX: actors on delimited continuations

The headline showcase for the SX runtime. Erlang is built around the **one** primitive — lightweight processes with mailboxes and selective receive — that delimited continuations implement natively. Most Erlang implementations ship a whole VM (BEAM) for this; on SX each process is a pair of continuations and the scheduler is ~50 lines of SX.

End-state goal: spawn a million processes, run the classic **ring benchmark**, plus a mini gen_server OTP subset and a test corpus of ~150 programs.

## Scope decisions (defaults — override by editing before we spawn)

- **Syntax:** Erlang/OTP 26 subset. No preprocessor, no parse transforms.
- **Conformance:** not BEAM-compat. "Looks like Erlang, runs like Erlang, not byte-compatible." We care about semantics, not BEAM bug-for-bug.
- **Test corpus:** custom — ring, ping-pong, fibonacci-server, bank-account-server, echo-server, plus ~100 hand-written tests for patterns/guards/BIFs. No ISO Common Test.
- **Binaries:** basic bytes-lists only; full binary pattern matching deferred.
- **Hot code reload, distribution, NIFs:** out of scope entirely.

## Ground rules

- **Scope:** only touch `lib/erlang/**` and `plans/erlang-on-sx.md`. Don't edit `spec/`, `hosts/`, `shared/`, `lib/js/**`, `lib/hyperscript/**`, `lib/lua/**`, `lib/prolog/**`, `lib/forth/**`, `lib/haskell/**`, `lib/stdlib.sx`, or `lib/` root. Erlang primitives go in `lib/erlang/runtime.sx`.
- **SX files:** use `sx-tree` MCP tools only.
- **Commits:** one feature per commit. Keep `## Progress log` updated and tick roadmap boxes.

## Architecture sketch

```
Erlang source
    │
    ▼
lib/erlang/tokenizer.sx  — atoms, vars, tuples, lists, binaries, operators
    │
    ▼
lib/erlang/parser.sx     — AST: modules, functions with clauses, patterns, guards
    │
    ▼
lib/erlang/transpile.sx  — AST → SX AST (entry: erlang-eval-ast)
    │
    ▼
lib/erlang/runtime.sx    — scheduler, processes, mailboxes, BIFs
```

Core mapping:
- **Process** = pair of delimited continuations (`on-receive`, `on-resume`) + mailbox list + pid + links
- **Scheduler** = round-robin list of runnable processes; cooperative yield on `receive`
- **`spawn`** = push a new process record, return its pid
- **`send`** = append to target mailbox; if target is blocked on receive, resume its continuation
- **`receive`** = selective — scan mailbox for first matching clause; if none, `perform` a suspend with the receive pattern; scheduler resumes when a matching message arrives
- **Pattern matching** = SX `case` on tagged values; vars bind on match
- **Guards** = side-effect-free predicate evaluated after unification
- **Immutable data** = native
- **Links / monitors / exit signals** = additional process-record fields, scheduler fires exit signals on death

## Roadmap

### Phase 1 — tokenizer + parser
- [x] Tokenizer: atoms (bare + single-quoted), variables (Uppercase/`_`-prefixed), numbers (int, float, `16#HEX`), strings `"..."`, chars `$c`, punct `( ) { } [ ] , ; . : :: ->` — **62/62 tests**
- [x] Parser: module declarations, `-module`/`-export`/`-import` attributes, function clauses with head patterns + guards + body — **52/52 tests**
- [x] Expressions: literals, vars, calls, tuples `{...}`, lists `[...|...]`, `if`, `case`, `receive`, `fun`, `try/catch`, operators, precedence
- [ ] Binaries `<<...>>` — not yet parsed (deferred to Phase 6)
- [x] Unit tests in `lib/erlang/tests/parse.sx`

### Phase 2 — sequential eval + pattern matching + BIFs
- [x] `erlang-eval-ast`: evaluate sequential expressions — **54/54 tests**
- [x] Pattern matching (atoms, numbers, vars, tuples, lists, `[H|T]`, underscore, bound-var re-match) — **21 new eval tests**; `case ... of ... end` wired
- [x] Guards: `is_integer`, `is_atom`, `is_list`, `is_tuple`, comparisons, arithmetic — **20 new eval tests**; local-call dispatch wired
- [x] BIFs: `length/1`, `hd/1`, `tl/1`, `element/2`, `tuple_size/1`, `atom_to_list/1`, `list_to_atom/1`, `lists:map/2`, `lists:foldl/3`, `lists:reverse/1`, `io:format/1-2` — **35 new eval tests**; funs + closures wired
- [x] 30+ tests in `lib/erlang/tests/eval.sx` — **130 tests green**

### Phase 3 — processes + mailboxes + receive (THE SHOWCASE)
- [x] Scheduler in `runtime.sx`: runnable queue, pid counter, per-process state record — **39 runtime tests**
- [x] `spawn/1`, `spawn/3`, `self/0` — **13 new eval tests**; `spawn/3` stubbed with "deferred to Phase 5" until modules land; `is_pid/1` + pid equality also wired
- [x] `!` (send), `receive ... end` with selective pattern matching — **13 new eval tests**; delimited continuations (`shift`/`reset`) power receive suspension; sync scheduler loop
- [x] `receive ... after Ms -> ...` timeout clause (use SX timer primitive) — **9 new eval tests**; synchronous-scheduler semantics: `after 0` polls once; `after Ms` fires when runnable queue drains; `after infinity` = no timeout
- [x] `exit/1`, basic process termination — **9 new eval tests**; `exit/2` (signal another) deferred to Phase 4 with links
- [ ] Classic programs in `lib/erlang/tests/programs/`:
  - [x] `ring.erl` — N processes in a ring, pass a token around M times — **4 ring tests**; suspension machinery rewritten from `shift`/`reset` to `call/cc` + `raise`/`guard`
  - [x] `ping_pong.erl` — two processes exchanging messages — **4 ping-pong tests**
  - [ ] `bank.erl` — account server (deposit/withdraw/balance)
  - [ ] `echo.erl` — minimal server
  - [ ] `fib_server.erl` — compute fib on request
- [ ] `lib/erlang/conformance.sh` + runner, `scoreboard.json` + `scoreboard.md`
- [ ] Target: 5/5 classic programs + 1M-process ring benchmark runs

### Phase 4 — links, monitors, exit signals
- [ ] `link/1`, `unlink/1`, `monitor/2`, `demonitor/1`
- [ ] Exit-signal propagation; trap_exit flag
- [ ] `try/catch/of/end`

### Phase 5 — modules + OTP-lite
- [ ] `-module(M).` loading, `M:F(...)` calls across modules
- [ ] `gen_server` behaviour (the big OTP win)
- [ ] `supervisor` (simple one-for-one)
- [ ] Registered processes: `register/2`, `whereis/1`

### Phase 6 — the rest
- [ ] List comprehensions `[X*2 || X <- L]`
- [ ] Binary pattern matching `<<A:8, B:16>>`
- [ ] ETS-lite (in-memory tables via SX dicts)
- [ ] More BIFs — target 200+ test corpus green

## Progress log

_Newest first._

- **2026-04-24 ping_pong.erl green** — `lib/erlang/tests/programs/ping_pong.sx` with 4 tests: classic Pong server + Ping client with separate `ping_done`/`pong_done` notifications, 5-round trace via io-buffer (`"ppppp"`), main-as-pinger-4-rounds (no intermediate Ping proc), tagged-id round-trip (`"4 3 2 1 "`). All driven by `Ping = fun (Target, K) -> ... Ping(Target, K-1) ... end` self-recursion — captured-env reference works because `Ping` binds in main's mutable env before any spawned body looks it up. Total suite 335/335.
- **2026-04-24 ring.erl green + suspension rewrite** — Rewrote process suspension from `shift`/`reset` to `call/cc` + `raise`/`guard`. **Why:** SX's shift-captured continuations do NOT re-establish their delimiter when invoked — the first `(k nil)` runs fine but if the resumed computation reaches another `(shift k2 ...)` it raises "shift without enclosing reset". Ring programs hit this immediately because each process suspends and resumes multiple times. `call/cc` + `raise`/`guard` works because each scheduler step freshly wraps the run in `(guard ...)`, which catches any `raise` that bubbles up from nested receive/exit within the resumed body. Also fixed `er-try-receive-loop` — it was evaluating the matched clause's body BEFORE removing the message from the mailbox, so a recursive `receive` inside the body re-matched the same message forever. Added `lib/erlang/tests/programs/ring.sx` with 4 tests (N=3 M=6, N=2 M=4, N=1 M=5 self-loop, N=3 M=9 hop-count via io-buffer). All process-communication eval tests still pass. Total suite 331/331.
- **2026-04-24 exit/1 + termination green** — `exit/1` BIF uses `(shift k ...)` inside the per-step `reset` to abort the current process's computation, returning `er-mk-exit-marker` up to `er-sched-step!`. Step handler records `:exit-reason`, clears `:exit-result`, marks dead. Normal fall-off-end still records reason `normal`. `exit/2` errors with "deferred to Phase 4 (links)". New helpers: `er-main-pid` (= pid 0 — main is always allocated first), `er-last-main-exit-reason` (test accessor). 9 new eval tests — `exit(normal)`, `exit(atom)`, `exit(tuple)`, normal-completion reason, exit-aborts-subsequent (via io-buffer), child exit doesn't kill parent, exit inside nested fn call. Total eval 174/174; suite 327/327.
- **2026-04-24 receive...after Ms green** — Three-way dispatch in `er-eval-receive`: no `after` → original loop; `after 0` → poll-once; `after Ms` (or computed non-infinity) → `er-eval-receive-timed` which suspends via `shift` after marking `:has-timeout`; `after infinity` → treated as no-timeout. `er-sched-run-all!` now recurses into `er-sched-fire-one-timeout!` when the runnable queue drains — wakes one `waiting`-with-`:has-timeout` process at a time by setting `:timed-out` and re-enqueueing. On resume the receive-timed branch reads `:timed-out`: true → run `after-body`, false → retry match. "Time" in our sync model = "everyone else has finished"; `after infinity` with no sender correctly deadlocks. 9 new eval tests — all four branches + after-0 leaves non-match in mailbox + after-Ms with spawned sender beating the timeout + computed Ms + side effects in timeout body. Total eval 165/165; suite 318/318.
- **2026-04-24 send + selective receive green — THE SHOWCASE** — `!` (send) in `lib/erlang/transpile.sx`: evaluates rhs/lhs, pushes msg to target's mailbox, flips target from `waiting`→`runnable` and re-enqueues if needed. `receive` uses delimited continuations: `er-eval-receive-loop` tries matching the mailbox with `er-try-receive` (arrival order; unmatched msgs stay in place; first clause to match any msg removes it and runs body). On no match, `(shift k ...)` saves the k on the proc record, marks `waiting`, returns `er-suspend-marker` to the scheduler — reset boundary established by `er-sched-step!`. Scheduler loop `er-sched-run-all!` pops runnable pids and calls either `(reset ...)` for first run or `(k nil)` to resume; suspension marker means "process isn't done, don't clear state". `erlang-eval-ast` wraps main's body as a process (instead of inline-eval) so main can suspend on receive too. Queue helpers added: `er-q-nth`, `er-q-delete-at!`. 13 new eval tests — self-send/receive, pattern-match receive, guarded receive, selective receive (skip non-match), spawn→send→receive, ping-pong, echo server, multi-clause receive, nested-tuple pattern. Total eval 156/156; suite 309/309. Deadlock detected if main never terminates.
- **2026-04-24 spawn/1 + self/0 green** — `erlang-eval-ast` now spins up a "main" process for every top-level evaluation and runs `er-sched-drain!` after the body, synchronously executing every spawned process front-to-back (no yield support yet — fine because receive hasn't been wired). BIFs added in `lib/erlang/runtime.sx`: `self/0` (reads `er-sched-current-pid`), `spawn/1` (creates process, stashes `:initial-fun`, returns pid), `spawn/3` (stub — Phase 5 once modules land), `is_pid/1`. Pids added to `er-equal?` (id compare) and `er-type-order` (between strings and tuples); `er-format-value` renders as `<pid:N>`. 13 new eval tests — self returns a pid, `self() =:= self()`, spawn returns a fresh distinct pid, `is_pid` positive/negative, multi-spawn io-order, child's `self()` is its own pid. Total eval 143/143; runtime 39/39; suite 296/296. Next: `!` (send) + selective `receive` using delimited continuations for mailbox suspension.
- **2026-04-24 scheduler foundation green** — `lib/erlang/runtime.sx` + `lib/erlang/tests/runtime.sx`. Amortised-O(1) FIFO queue (`er-q-new`, `er-q-push!`, `er-q-pop!`, `er-q-peek`, `er-q-compact!` at 128-entry head drift), tagged pids `{:tag "pid" :id N}` with `er-pid?`/`er-pid-equal?`, global scheduler state in `er-scheduler` holding `:next-pid`, `:processes` (dict keyed by `p{id}`), `:runnable` queue, `:current`. Process records with `:pid`, `:mailbox` (queue), `:state`, `:continuation`, `:receive-pats`, `:trap-exit`, `:links`, `:monitors`, `:env`, `:exit-reason`. 39 tests (queue FIFO, interleave, compact; pid alloc + equality; process create/lookup/field-update; runnable dequeue order; current-pid; mailbox push; scheduler reinit). Total erlang suite 283/283. Next: `spawn/1`, `!`, `receive` wired into the evaluator.
- **2026-04-24 core BIFs + funs green** — Phase 2 complete. Added to `lib/erlang/transpile.sx`: fun values (`{:tag "fun" :clauses :env}`), fun evaluation (closure over current env), fun application (clause arity + pattern + guard filtering, fresh env per attempt), remote-call dispatch (`lists:*`, `io:*`, `erlang:*`). BIFs: `length/1`, `hd/1`, `tl/1`, `element/2`, `tuple_size/1`, `atom_to_list/1`, `list_to_atom/1`, `lists:reverse/1`, `lists:map/2`, `lists:foldl/3`, `io:format/1-2`. `io:format` writes to a capture buffer (`er-io-buffer`, `er-io-flush!`, `er-io-buffer-content`) and returns `ok` — supports `~n`, `~p`/`~w`/`~s`, `~~`. 35 new eval tests. Total eval 130/130; erlang suite 244/244. **Phase 2 complete — Phase 3 (processes, scheduler, receive) is next.**
- **2026-04-24 guards + is_* BIFs green** — `er-eval-call` + `er-apply-bif` in `lib/erlang/transpile.sx` wire local function calls to a BIF dispatcher. Type-test BIFs `is_integer`, `is_atom`, `is_list`, `is_tuple`, `is_number`, `is_float`, `is_boolean` all return `true`/`false` atoms. Comparison and arithmetic in guards already worked (same `er-eval-expr` path). 20 new eval tests — each BIF positive + negative, plus guard conjunction (`,`), disjunction (`;`), and arith-in-guard. Total eval 95/95; erlang suite 209/209.
- **2026-04-24 pattern matching green** — `er-match!` in `lib/erlang/transpile.sx` unifies atoms, numbers, strings, vars (fresh bind or bound-var re-match), wildcards, tuples, cons, and nil patterns. `case ... of ... [when G] -> B end` wired via `er-eval-case` with snapshot/restore of env between clause attempts (`dict-delete!`-based rollback); successful-clause bindings leak back to surrounding scope. 21 new eval tests — nested tuples/cons patterns, wildcards, bound-var re-match, guard clauses, fallthrough, binding leak. Total eval 75/75; erlang suite 189/189.
- **2026-04-24 eval (sequential) green** — `lib/erlang/transpile.sx` (tree-walking interpreter) + `lib/erlang/tests/eval.sx`. 54/54 tests covering literals, arithmetic, comparison, logical (incl. short-circuit `andalso`/`orelse`), tuples, lists with `++`, `begin..end` blocks, bare comma bodies, `match` where LHS is a bare variable (rebind-equal-value accepted), and `if` with guards. Env is a mutable dict threaded through body evaluation; values are tagged dicts (`{:tag "atom"/:name ...}`, `{:tag "nil"}`, `{:tag "cons" :head :tail}`, `{:tag "tuple" :elements}`). Numbers pass through as SX numbers. Gotcha: SX's `parse-number` coerces `"1.0"` → integer `1`, so `=:=` can't distinguish `1` from `1.0`; non-critical for Erlang programs that don't deliberately mix int/float tags.
- **parser green** — `lib/erlang/parser.sx` + `parser-core.sx` + `parser-expr.sx` + `parser-module.sx`. 52/52 in `tests/parse.sx`. Covers literals, tuples, lists (incl. `[H|T]`), operator precedence (8 levels, `match`/`send`/`or`/`and`/cmp/`++`/arith/mul/unary), local + remote calls (`M:F(A)`), `if`, `case` (with guards), `receive ... after ... end`, `begin..end` blocks, anonymous `fun`, `try..of..catch..after..end` with `Class:Pattern` catch clauses. Module-level: `-module(M).`, `-export([...]).`, multi-clause functions with guards. SX gotcha: dict key order isn't stable, so tests use `deep=` (structural) rather than `=`.
- **tokenizer green** — `lib/erlang/tokenizer.sx` + `lib/erlang/tests/tokenize.sx`. Covers atoms (bare, quoted, `node@host`), variables, integers (incl. `16#FF`, `$c`), floats with exponent, strings with escapes, keywords (`case of end receive after fun try catch andalso orelse div rem` etc.), punct (`( ) { } [ ] , ; . : :: -> <- <= => << >> | ||`), ops (`+ - * / = == /= =:= =/= < > =< >= ++ -- ! ?`), `%` line comments. 62/62 green.

## Blockers

- _(none yet)_