writeonce/docs/plan/oop-vm/00-wob-format.md

The .wob format v2 — normative reference

Copied verbatim from the normative section of docs/superpowers/plans/2026-08-01-wob-format-and-vm-core.md (plan 1 of the approved spec 2026-08-01-oop-compiler-vm-design.md). The machine-readable twin is runtime/src/wob.h — constants there and prose here must never disagree. The test-side assembler runtime/test/wob_build.c is a second, independent encoding; builder/loader disagreements surface as test failures.

All integers little-endian; offsets are absolute file offsets.

Header (44 bytes): magic "WOB1", version 2, then offset/count u32 pairs for the constant pool, class table, interface section, and method table, then a u32 entry-method index (all-ones = none).

Constant pool — sequential entries: one tag byte; tag 0 = i64 follows; tag 1 = text (u32 length + bytes, no NUL).

Class table — per class: name constant index, flags u32 (bit0 = instances are @gc), field count, then one kind byte per field padded to a 4-byte boundary, then three u32 arrays of per-field metadata (v2), one entry per field each, in declaration order:

1. field_names[i] — constant index of the field's name, or all-ones for "not recorded" (what a hand-built test image writes).
2. field_class[i] — the class id the field refers to: its own class for an OWNED/GCREF field, its element's class for a container of records; 0xFFFFFFFE marks a json.Value field, whose Text holds a raw JSON slice; all-ones for none.
3. field_elem[i] — a container field's element kinds: a MULTI's element kind, or a MAP's key kind in the low nibble and value kind in the next; 0 otherwise.

Field kinds: 0 SCALAR, 1 OWNED, 2 GCREF, 3 TEXT, 4 MULTI, 5 MAP. Runtime object layout: 16-byte header then one 8-byte slot per field, in declaration order.

The metadata exists for exactly one reason: json.encode/json.decode are runtime services driven by class metadata (runtime/src/json.c) rather than per-type generated code, so the names a JSON object needs and the shapes a decode has to build must live in the image. Every other part of the runtime ignores it.

Interface section — per interface: name constant index, method count. Global slot ids are assigned sequentially across interfaces in declaration order. Then a vtable row count and rows: class id, interface id, one method index per interface method.

Method table — per method: name constant index, class id (all-ones = free fn), arg count u8, register count u8, reserved u16, code length in bytes (multiple of 4), the u32 instructions, a line table (count + ascending pc→line pairs), and a drop table (count + ascending entries of pc, owned-register bitmask u64, gc-register bitmask u64). Drop-table lookup = last entry with pc ≤ current pc; no entry means nothing live.

Instructions — fixed 32-bit, Lua-style fields: opcode byte, A byte, then either B and C bytes or a 16-bit Bx (signed jumps encode as Bx − 32768).

op	name	semantics (in words)
0	NOP	nothing
1	LOADK A Bx	register A = constant Bx (int inline; text = pointer to interned const string)
2	MOVE A B	copy register; for owned values this IS the move — compiler guarantees the source is dead
3–7	ADD/SUB/MUL/DIV/NEG	i64 arithmetic, two's-complement wrapping (no signed-overflow UB); DIV traps on zero divisor and on INT64_MIN ÷ −1
8	CONCAT A B C	new owned text from two texts
9–12	EQ/LT/LE/EQS	i64 compares and text-content equality, result 0/1
13–14	JMP / JZ	relative jump (JZ when register A is zero)
15	CALL A Bx	call method Bx; callee's register window starts at caller base + A (register-window overlap, Lua-style); args sit at A, A+1, …; return value lands back in slot A
16	ICALL A Bx	interface call by global slot id Bx; receiver in A; vtable lookup by the receiver's class
17–18	RET A / RET0	return value from register A (or zero), pop frame
19	NEW A Bx	new zeroed instance of class Bx
20–21	GETF / SETF	field read/write with runtime null/native/bounds checks (trap T_BOUNDS); overwriting a non-scalar field does NOT auto-drop the old value — the compiler emits the drop
22	DROP A	recursively drop the owned value in A per its class drop plan, null the register
23–26	BORROW_S/BORROW_X/RELEASE_S/RELEASE_X	borrow-word ops on the object in A; violation traps T_BORROW
27–28	RC_INC / RC_DEC	refcount ops on the @gc object in A
29	BUILTIN A B C	register A = builtin C applied to args starting at register B (fixed arity per builtin; multi_new/map_new carry kind immediates in B instead)
30	DB_STUB	trap T_DB "engine not linked" (spec: SQL-layer statements in milestone 1)
31	TRAP Bx	explicit trap with code Bx
32	TRY A sBx	push a catch frame for this frame and window: handler at pc + sBx, error record register A (haxe-parity Task 5)
33	ENDTRY	pop the innermost catch frame — the try region completed without trapping

try/catch (Task 5). A trap raised while a catch frame is live unwinds every frame above the catching one exactly as an uncaught trap does (drop maps run, registers null), then releases what the try region owned in the catching frame — the difference between the drop entry at the trapping instruction and the one at the handler pc — and resumes at the handler instead of leaving the VM. A frame that returns takes its still-open catch frames with it, so a return out of a try region cannot leave a handler pointing at a dead window. With no catch frame live, a trap behaves byte-for-byte as it did before v2. The catch arm's error record is an ordinary compiler-allocated object filled by the err_fill builtin (field order: 0 code, 1 line, 2 method, 3 msg).

Builtins: now (ms), print (text), print_int, words (whitespace token count), multi_new/multi_push/multi_get/count/latest, map_new/map_set/map_get/map_has, int_to_text (haxe-parity Task 2), variant_tag (haxe-parity Task 4 — see "Enum payload variants" below), err_fill (Task 5's catch record), then the systems stdlib:

• text/containers — len, byte_at, print_err, starts_with, ends_with, index_of, last_index_of, substr, trim, to_lower, char_of, parse_int, split, split_ws, join, slice, pop, shift, sort, reverse, remove, key_at, val_at, multi_set. Ids 16–39; runtime/src/builtin.c.
• the OS half — fs.exists/list/stat/read_all/read_at/append, time.sleep/local/iso, env.get/stopping, net.listen/accept/read/write/close, proc.run. Ids 40–56; runtime/src/sysio.c. A member that returns a record takes that record's class id as its last argument, so the VM allocates what it fills without knowing any source type name.
• json — encode (value + the value's static kind), decode (text + the class id to build). Ids 57–58; runtime/src/json.c. Decode yields the zero word on malformed input rather than trapping, which is what makes json.decode(t) as T a checked decode.

?T and nil. A heap-shaped optional (?Text, ?Rec, ?multi, ?map, ?@gc) stores what T stores and spells nil as the zero word — every per-kind drop plan already ignores a zero slot, so ?T's field kind is T's. A nullable scalar (?Int, ?Bool, ?Timestamp, ?Id) cannot: 0 is a perfectly good Int, and real programs store it in a ?Int. Its nil is therefore WO_NIL_SCALAR = −2^62 (not INT64_MIN: the compiler's own integers are 63-bit, so that value is not expressible on the emitting side). Such a field is marked WOB_FIELD_NIL_SCALAR in field_class[i], which is how the runtime knows to write that word where it must produce absence itself — today only json.decode leaving a key absent, and parse_int on unparseable input.

EQS accepts a nil operand for the same reason: two ?Text values compare with it, and the answer is "both absent is equal, one absent is not". A non-nil operand must still be a real Text.

Trap codes: DIV0, BORROW, STACK, OOM, DB, BOUNDS, KEY, EXPLICIT, IO (a syscall the source cannot prevent said no — errno's message rides in the error record).

Enum payload variants (haxe-parity compiler Task 4)

.wob v1 is unchanged — no new section, no new header field, no version bump. A union with at least one payload variant (type Status = Pending | Failed(reason: Text)) compiles to one ordinary class-table entry per variant, named "<Union>.<Variant>" in the constant pool (source identifiers can never contain a dot, so the composite name cannot collide with a declared class — the same convention the method table already uses for "Class.method"). A variant's payload fields are the entry's fields, declaration order, ordinary kind bytes — so a variant object is dropped, masked, and cycle-scanned exactly like any other instance, including recursive payload frees, with zero collector changes.

The variant tag IS the class-table index, carried by the object header's existing class_id field — nothing new is stored and NEW needs no change. The one VM addition is builtin 14 variant_tag: register A = the header class_id of the object in register B, so a switch over a payload union reads the tag once and compares it against LOADK-ed class-id constants — no per-arm allocation. It traps T_BOUNDS on a null receiver or a native (WO_CLS_*) class id, the same defense ICALL keeps; a non-pointer register stays the compiler's to prevent (untyped registers, the residual-check doctrine). variant_tag is compiler-internal: it is not a source-callable name and does not appear in 08-builtin-surface.md.

An all-bare union (type CronResult = Ok | ErrorFinal | Miss) never reaches this file's format at all: its values are plain integer ordinals (0, 1, 2 … in declaration order) in WO_K_SCALAR positions, compared with EQ — no class entries, no heap objects, no variant_tag.

Payload move-out (Task 4 fix rounds 1–2): a switch arm that yields its own payload binding as the switch's value (case Boxed(b): b;) MOVES the payload out of the variant object — pointer-kind fields only (OWNED/GCREF/TEXT/MULTI/MAP). The convention needs no format or collector change: the compiler emits a SETF writing zero into the moved field right after the value lands in its new owner's register, and the shell's ordinary recursive drop plan — which already skips zero slots for every kind (runtime/src/gc.c wo_drop_kind) — thereby frees the shell only. Escaping a SCALAR field (Int/Bool/Timestamp/Id/ref, a bare-union tag) is a plain COPY: no ownership moves and the field is left intact — nulling it would corrupt the subject with a value indistinguishable from a legitimate 0. A DISCARDED yield (statement-position switch) does not null either: the shell keeps the payload and frees it as usual. Re-reading a moved-out payload is nil: the field holds the zero word, so a later switch over the same subject GETFs 0 into the binding and any use of it traps T_BOUNDS ("null receiver") — memory-safe and defined, the residual-check doctrine's direction; a later task may promote this to a compile-time partial-move diagnostic (WO-E301 family). One companion rule on the caller side: an owned heap temporary passed as a borrow argument — a record/class constructor literal, a variant construction, or an owned-returning call (peek(Pay{}), get(Boxed(Pay{}))) — is copied to a stable register below the call window and DROPped by the caller once the call returns (take arguments are the callee's to drop; places are their scope's; @gc and Text temporaries are excluded — the rc system's and the Copy-aliasing story's, respectively). Recursive drop is correct both ways, because a payload the callee moved out left the field nulled.

Typedef records (typedef Name = { ... }) are ordinary class-table entries too, with one compiler-side convention the loader never sees: two records with the same shape (same ordered fields, same types, same defaults) share a single entry — structural aliasing decided entirely at emit time.

Single-binary trailer (woc build, plan 3 Task 6)

This section is not part of the .wob format above — .wob v1 is unchanged. It documents the wrapper a deployable executable carries: woc build <dir> -o app makes app by copying the wovm runtime binary and appending the compiled .wob image plus a small fixed-size trailer. wovm's own startup (runtime/src/main.c) looks for this trailer in its own executable (/proc/self/exe) before falling back to the classic wovm file.wob argv contract, so the result runs standalone with no separate .wob file. Writer: compiler/bin/main.ml. Reader: runtime/src/main.c's load_self_embedded. Append-based only, deliberately — no ELF section manipulation.

Layout — the trailer is the fixed last 20 bytes of the file, all integers little-endian, found by seeking from the end (no scanning):

byte offset from EOF   size   field
  -20                   8     payload_off  -- absolute file offset where the embedded .wob image starts
  -12                   8     payload_len  -- length in bytes of the embedded .wob image
   -4                   4     magic        -- 0x31544257 ("WBT1" read as LE u32, mirrors WOB_MAGIC's "WOB1")

[ wovm runtime bytes (payload_off bytes) ][ .wob image (payload_len bytes) ][ trailer: payload_off | payload_len | magic ]
^ byte 0                                  ^ byte payload_off                ^ byte payload_off+payload_len == file_size-20
                                                                                                              file_size ^

Reader algorithm (load_self_embedded): open /proc/self/exe; if the file is shorter than 20 bytes, or its last 4 bytes don't equal the magic, there is no trailer — fall back to the argv .wob path unchanged. If the magic matches, payload_off and payload_len are validated to account for every trailing byte exactly (payload_off + payload_len == file_size - 20, checked via a bounds-safe subtraction so a corrupt/huge value can't wrap the arithmetic and slip past); any mismatch is reported as a clear "corrupt trailer" error (exit 2) rather than a crash or silent misbehavior. On success, the executable is mmap'd and wo_load_buf parses the embedded region exactly as wo_load_file parses a standalone .wob today — argv is never consulted.

Runtime location (writer side): --runtime <path> wins when given; otherwise the default is runtime/wovm resolved relative to the current working directory (the same repo-root-relative assumption every other just/build-tooling entry point in this repo already makes). A missing runtime binary is a build-time error naming the recipe: make -C runtime wovm. woc build never invokes or inspects the runtime binary beyond reading its bytes — it does not need to be executable as run by woc, only as run by whoever runs the produced artifact.

Edge cases decided for woc build (each implemented deliberately, not left to fall out accidentally):

• Output path already exists: overwritten, but atomically — the new binary is assembled in a temp file (<out>.woc-build.tmp, freshly created with mode 0755 each time so a stale temp file's permissions can never leak through) next to -o, then renamed over it. A failed build (bad compile, missing runtime, disk-full mid-write) never clobbers a previously-working binary with a partial one.
• A directory with no main: unlike --emit (where a .wob with no entry method is a legitimate, already-specified artifact), build's entire purpose is something runnable, so a clean compile with no zero-argument free fn named main is a build-time error, no output written — not deferred to wovm's own "module has no entry method" message at run time. Detected by reading the compiled image's own entry field (WOB_OFF_ENTRY, offset 40) rather than plumbing a new return value through the emitter.
• --runtime itself already carries a trailer (rebuilding from a previously-built single binary): its embedded payload is stripped before copying — the writer recognizes its own trailer on the input runtime binary the same way the C reader does, and keeps only the pristine runtime prefix (payload_off bytes). This makes woc build ... --runtime already-built-app -o new-app produce a binary byte-identical in size to building fresh from runtime/wovm directly, instead of chaining stale payloads and bloating on every rebuild. Any input that doesn't unambiguously look like our own trailer (wrong magic, or offsets that don't exactly account for every trailing byte) is left untouched and copied as-is — the safe default when it's not certain.