rose-ash/plans/erlang-on-sx.md

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

• Tokenizer: atoms (bare + single-quoted), variables (Uppercase/_-prefixed), numbers (int, float, 16#HEX), strings "...", chars $c, punct ( ) { } [ ] , ; . : :: ->
• Parser: module declarations, -module/-export/-import attributes, function clauses with head patterns + guards + body
• Expressions: literals, vars, calls, tuples {...}, lists [...|...], binaries <<...>>, if, case, receive, fun, try/catch, operators
• Unit tests in lib/erlang/tests/parse.sx

Phase 2 — sequential eval + pattern matching + BIFs

• erlang-eval-ast: evaluate sequential expressions
• Pattern matching (atoms, numbers, vars, tuples, lists, [H|T], underscore, bound-var re-match)
• Guards: is_integer, is_atom, is_list, is_tuple, comparisons, arithmetic
• 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
• 30+ tests in lib/erlang/tests/eval.sx

Phase 3 — processes + mailboxes + receive (THE SHOWCASE)

• Scheduler in runtime.sx: runnable queue, pid counter, per-process state record
• spawn/1, spawn/3, self/0
• ! (send), receive ... end with selective pattern matching
• receive ... after Ms -> ... timeout clause (use SX timer primitive)
• exit/1, basic process termination
• Classic programs in lib/erlang/tests/programs/:
  • ring.erl — N processes in a ring, pass a token around M times
  • ping_pong.erl — two processes exchanging messages
  • 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.

• (not started)

Blockers

• (none yet)