# Two-phase parse (design) — eliminating the forward mechanism

## Goal

Delete `SymTab.FwdVarRef`/`FwdVarName`/`FwdVarResolve`, `Check.NoteForward`/
`FlushForward` and the `Design` forward branch, plus the `Op=1` marker —
by binding names before they are checked.

## Why post-parse checking is not enough

`Design` defers a name it cannot find **inside a procedure** because it
may be a *module-level* declaration that appears **later in the file**.
The resolve/check pass happens after the parse, so it cannot help with
*binding*: binding happens during the parse, and the declaration simply
has not been read yet.  The only fix is to read the declarations before
the bodies — a two-phase (or two-pass) *parse*.

## Design A (recommended): module-level body deferral + re-scan

The grammar/parser stays almost entirely as-is; only the *order* in which
the parser visits text changes.

1. **Skip module-level procedure bodies.**  When `ProcDecl` sees a
   module-level `PROCEDURE`, record the body's source span and consume
   its tokens without parsing them, then carry on with the remaining
   declarations.  Result: every module-level name is in `SymTab` once the
   module's declarations are done.
2. **Re-scan and parse the bodies.**  At the end of the module, for each
   recorded span: seek the scanner to its start and parse the body with
   the existing `Block` production.  Every module-level name now
   resolves, so no forward reference is ever created.

Consequences:
- `Design`'s forward branch never fires → the whole `FwdVar*` path is
  dead and removable.
- The body productions are **reused unchanged** (both the skip and the
  re-parse reuse existing machinery), unlike Design B below.

### Pieces

1. **Scanner seek** (`scanner.frm`, and expose it in `M2S.def`'s
   template): `PROCEDURE SeekTo (p: INT32)` — set `bp0`/`bp`/`nextPos` to
   `p` and recompute `line`/`col` (scan from offset 0 counting `CHR(10)`;
   `CharAt` already indexes the in-memory buffer, so this is O(p)).
2. **Token-level body skipper** (grammar helper): starting at the
   `PROCEDURE` keyword, consume tokens counting block openers
   `{PROCEDURE, IF, CASE, WHILE, FOR, LOOP, WITH, RECORD}`; stop when the
   count returns to 0 — that `END` closes the procedure.  `BEGIN` is
   **not** an opener (a block's `BEGIN…END` shares the declaration's
   `END`), and `REPEAT…UNTIL`/`EXIT`/`RETURN` are not counted.
   Verify afterwards that the token is `END` and the following identifier
   equals the procedure name (guards a mis-skip).
3. **Deferral** in `ProcDecl` (module level only): skip + record the
   span; the declaration node is a placeholder `NkProcDecl`.
4. **Body re-parse**: a new grammar entry (reusing `Block`) driven at the
   end of `Unit`, walking the recorded spans.
5. **Flag**: all of this behind `-twophase` (default off) until the suite
   and the fixpoint are byte-identical, then flip.

## Design B (rejected): syntax-only bodies + binding walk

Make every expression/statement action resolution-free and bind/type/
check in `Check`, walking with scopes mirrored from the AST.  This is the
"true" two-phase and is much larger: it touches every body action, and
parse decisions that need types (typed brace literals, method calls,
`L.x` qualified names, `HIGH`/`LEN`) would have to be deferred too.

## Risks / open questions

- **Skipper exactness.** A mis-count mis-skips to the wrong `END`.  The
  post-skip check (`END` + name) turns this into a clear error rather
  than silent corruption.
- **Scanner state after a seek** (`line`, `col`, lookahead) and error
  positions inside re-parsed bodies.
- **AST/`Lower` ordering.** Bodies are parsed after the declarations, so
  node allocation order changes.  `Lower` walks by structure (order-
  independent), but any parse-time ordinal that assumes file order must
  be re-checked.
- **Nested procedures** are inside the deferred span, so they are
  re-parsed with the same mechanism (their own locals are declared
  before their statements, so they need no deferral themselves).

## Milestones (each with the fixpoint green)

1. `SeekTo` + the skipper behind the flag; inert.
2. Module-level deferral + body re-parse; byte-compare against the
   default across the suite.
3. Flip the default; delete `FwdVarRef`/`FwdVarName`/`FwdVarResolve`
   and `Check.NoteForward`/`FlushForward`, the `Design` forward branch,
   and the `Op=1` marker.

## Cost

Pieces 1–2 are a few hundred lines across the scanner frame, the grammar
and the driver, and are the largest single change in the resolve/check
thread — the reason `FwdVar*` could not be deleted by moving checks
alone.

## Progress

- **2026-10-15, milestone 1 (probe behind `-twophase`, inert).**
  Each outermost procedure/method body is shadow-skipped at the token
  level and the skip verified before the body parses normally:
  - `scanner.frm`: `PushPos`/`PopPos` save and restore the *full*
    scanner state (cursor + all published positions + `errors`, so a
    re-scanned bad character cannot double-count). 1-deep use, 8-deep
    stack.
  - `M2.atg`: `SetTwoPhase` switch, `procNest` counter (hooked once in
    `Block`, so `ProcDecl` and method bodies are both covered),
    `TokIs` (compares token *text*, immune to token renumbering) and
    `ProbeBody` (opener counting + `END <name>` check, silent on
    success, `twophase probe mismatch` to StdOut otherwise — never
    into the listing).
  - `compiler.frm` + `parser.frm`: `-twophase` flag plumbed to
    `M2P.SetTwoPhase` at both driver `Parse` sites.
  - Openers counted: `MODULE` (nested modules), `PROCEDURE`, `CLASS`
    (incl. `CLASS IMPLEMENTATION`), `RECORD` (incl. variant `CASE`),
    `IF`, `CASE`, `WHILE`, `FOR`, `LOOP`, `WITH`. `BEGIN` is not an
    opener; `REPEAT…UNTIL` needs none.
  - `run_tests.sh` gains a `twophase_same` gate (3 procedure-heavy
    files byte-compared flag-off vs flag-on).
  - Verified: suite **196/196** (193 + 3 probe gates); all 142
    single-file tests plus session spot-checks (incl. Library1) and
    the compiler's own sources produce **byte-identical `.ssa`** with
    the flag on and zero mismatches; **fixpoint OK (3,517,333 bytes)**.
- **Finding (V3 name resolution).** The first cut called the frame's
  `Get` from the new atg helper. gm2 accepts it (whole-scope), but the
  self-hosted V3 compiler resolves single-pass: the bare `Get`
  bound the *imported* `M2S.Get(VAR sym)` and stage 2 failed with
  233s (plus knock-on 201s at `END M2P`). Fix: the helper advances
  via `Adv` (`M2S.Get(sym); INC(errDist)` — exactly the frame's `Get`
  body for a pragma-free grammar, where the scanner never returns a
  token above `maxT`). Rule going forward: atg helpers must never
  call a later-declared frame procedure by a name that collides with
  an import.
- **Next**: milestone 2 — record body spans during the probe, add the
  body-entry production, and re-parse bodies after the declarations
  (driver-level restart), byte-compared against the default.