erlang: er-to-sx / er-of-sx term marshalling (+23 runtime tests)

plans/erlang-on-sx.md

@@ -114,7 +114,7 @@ Replace today's hardcoded BIF dispatch (`er-apply-bif`/`er-apply-remote-bif` in
 
 - [x] BIF registry: `er-bif-registry` global dict keyed by `"Module/Name/Arity"`, with `er-register-bif!`/`er-register-pure-bif!`/`er-lookup-bif`/`er-list-bifs`/`er-bif-registry-reset!` helpers — **+18 runtime tests** (600/600 total). Entries are `{:module :name :arity :fn :pure?}`. Arity is part of the key so `m:f/1` and `m:f/2` are independent. Re-registering the same key replaces the previous entry; reset clears.
 - [x] Migrate existing local + remote BIFs (length/hd/tl/lists:*/io:format/ets:*/etc.) onto the registry; delete the giant `cond` dispatch in `er-apply-bif`/`er-apply-remote-bif`. Conformance held at **600/600** after migration (baseline was 600, not the plan-text's 530 — the text was authored before Phase 7 work added rows). 67 builtin registrations across `erlang`/`lists`/`io`/`ets`/`code` modules; multi-arity BIFs (`is_function`, `spawn`, `exit`, `io:format`, `lists:seq`, `ets:delete`) register once per arity, all pointing at the same impl which dispatches on `(len vs)` internally. The four per-module cond dispatchers (`er-apply-lists-bif`, `er-apply-io-bif`, `er-apply-ets-bif`, `er-apply-code-bif`) are deleted. `er-apply-bif` and `er-apply-remote-bif` are now ~5-line registry lookups; user modules still win precedence over the registry.
-- [ ] Term-marshalling helpers: `er-of-sx` and `er-to-sx` (atom ↔ symbol, tuple ↔ list-of-elements, Erlang list ↔ SX list, binary ↔ string, dict ↔ map). Round-trip tests
+- [x] Term-marshalling helpers: `er-of-sx` (SX → Erlang) and `er-to-sx` (Erlang → SX). atom ↔ symbol, nil ↔ `()`, cons → list, tuple → list (one-way; tuples flatten), binary ↔ SX string, integer / float / boolean passthrough. **+23 runtime tests** (623/623 total). Erlang maps (`dict ↔ map`) deferred — Erlang map term not implemented in this port; will land when `#{}` syntax does. Pids, refs, funs pass through unchanged. SX strings on the way back become Erlang binaries (most useful FFI return shape).
 - [ ] `crypto:hash/2` — `sha256`, `sha512`, `blake3`; takes a binary, returns a binary. Uses the SX-host hash primitive
 - [ ] `cid:from_bytes/1`, `cid:to_string/1` — content-address an arbitrary binary
 - [ ] `file:read_file/1`, `file:write_file/2`, `file:list_dir/1`, `file:delete/1` — sync filesystem ops returning `{ok, Bin}` / `{error, Reason}`
@@ -126,6 +126,8 @@ Replace today's hardcoded BIF dispatch (`er-apply-bif`/`er-apply-remote-bif` in
 
 _Newest first._
 
+- **2026-05-14 term-marshalling helpers landed** — `er-to-sx` (Erlang term → SX-native) and `er-of-sx` (SX-native → Erlang term) plus internal helper `er-cons-to-sx-list` (recursive cons-chain walker). All three live in `runtime.sx` next to the BIF registry. Conversion table: atom ↔ symbol via `make-symbol`/`er-mk-atom`; nil ↔ `()`; cons-chain → SX list (recursive marshal of each head); tuple → SX list (one-way — tuples flatten and can't be reconstructed without a tag); binary ↔ SX string (bytes ↔ char codes via `char->integer`/`integer->char`); integer / float / boolean passthrough; opaque types (pid, ref, fun) passthrough. SX strings on the way back become Erlang binaries — the natural FFI return shape. Empty SX list (`type-of` `"nil"`) marshals back to `er-mk-nil`. Edit gotchas during implementation: SX has no `while`, `string-ref`, or `string-length` primitive — used `(map char->integer (string->list s))` for byte extraction and a recursive helper for cons-walking. 23 new runtime tests in `tests/runtime.sx`: 10 covering `er-to-sx` (atom/atom-is-symbol, nil, int / float / bool passthrough, binary→string, cons→list, tuple→list, nested), 8 covering `er-of-sx` (symbol→atom, atom-tag, string→binary, byte content, int passthrough, empty-list→nil, list→cons length, head field), 4 round-trips (int, atom, binary bytes, list length), 1 negative documenting that tuple round-trip flattens to cons. Total **623/623** (+23 runtime).
+
 - **2026-05-14 BIF registry migration complete — cond chains gone** — `er-register-builtin-bifs!` at the end of `runtime.sx` populates the registry with all 67 built-in BIFs in five module namespaces. Pure ops (`length`, `hd`, `tl`, `element`, predicates, arithmetic, list/atom/integer conversions, all of `lists`) registered via `er-register-pure-bif!`; side-effecting ops (`spawn`, `self`, `exit`, `link`/`monitor`/`register`, `process_flag`, `make_ref`, `throw`/`error`, `io:format`, all of `ets`, all of `code`) via `er-register-bif!`. Multi-arity entries: `is_function/1`/`/2`, `spawn/1`/`/3`, `exit/1`/`/2`, `io:format/1`/`/2`, `lists:seq/2`/`/3`, `ets:delete/1`/`/2` — six pairs, twelve registrations, all pointing at the existing arity-dispatching impl. `throw` and `error` are registered with a tiny inline `(fn (vs) (raise ...))` lambda because the original code chained directly through `raise` inside the cond instead of an `er-bif-*` helper. `er-apply-bif` shrinks from a 44-line cond chain to a 5-line registry lookup. `er-apply-remote-bif` becomes a 7-line dispatcher (user-modules-first → registry → error). All four per-module dispatchers (`er-apply-lists-bif`, `er-apply-io-bif`, `er-apply-ets-bif`, `er-apply-code-bif`) deleted — net reduction ~110 lines of cond machinery. One subtle wrinkle: `tests/runtime.sx` calls `er-bif-registry-reset!` near the end of its BIF-registry tests, which would have left subsequent test files (ring, ping-pong, etc.) unable to call `length`/`spawn`/etc. Fix: re-call `er-register-builtin-bifs!` at the bottom of `tests/runtime.sx` to repopulate. Total **600/600** unchanged.
 
 - **2026-05-14 Phase 8 BIF registry foundation** — `lib/erlang/runtime.sx` gains `er-bif-registry` (a `(list {})` mutable cell, same shape as `er-modules`) and five helpers: `er-bif-registry-get`/`er-bif-registry-reset!` (access + reset), `er-bif-key` (format `"Module/Name/Arity"`), `er-register-bif!` and `er-register-pure-bif!` (both upsert; differ only in the `:pure?` flag — pure ones are safe to inline, side-effecting ones go through normal IO), `er-lookup-bif` (returns the entry dict or nil), `er-list-bifs` (registered keys). Entries are `{:module :name :arity :fn :pure?}`. Lookup miss → nil; arity is part of the key so `m:f/1` and `m:f/2` are distinct; re-registering the same key replaces in-place (count stays the same); reset clears. Registry sits alongside `er-modules` in runtime.sx so any other piece of the system can register BIFs without touching the dispatcher — the migration onto this registry (the next checkbox) will rip out the giant cond chains in `er-apply-bif`/`er-apply-remote-bif`. 18 new runtime tests in `tests/runtime.sx`: empty-state, lookup-miss, register-grows-count, lookup-hit-fields (module/name/arity/pure?), fn-invocable, re-register-replaces, pure-flag-true, arity-disambiguation (3 entries for `fake:echo/1`, `fake:echo/2`, `fake:pure/2`), reset-clears, reset-lookup-nil. Total **600/600** (+18 runtime).