# m2comp step 5 — separate compilation (tag: `m2comp-step5`)

Single-image separate compilation, v1 step-11 style: `M2comp lib.def
lib.mod prog.mod` compiles definitions, then implementations, then
exactly one program into one `<Prog>.MC4` (depCount 0). Definition
and implementation share their basename (`toto.def` / `toto.mod`)
and hence one `.LST` (the implementation listing overwrites the
definition's); one `.LST` per basename otherwise, fail-fast. Suite
55/55 (41 inherited + 7 run + 7 reject); all single-file images
byte-identical to step 4 (no drift).

## Session model

- Grammar: `M2comp = Unit "."`, `Unit = "DEFINITION" DefUnit |
  "IMPLEMENTATION" ImplUnit | Module`. New keywords DEFINITION /
  IMPLEMENTATION (token numbers shifted; `Msg` regenerated from
  `.err` automatically, no test depends on numbers).
- `DEFINITION MODULE L` — CONST (integer-family only, else 230) /
  TYPE / VAR + procedure HEADINGS (no bodies). All top-level names
  auto-exported at `END` (parameters excluded).
- `IMPLEMENTATION MODULE L` — needs L defined (201) and not yet
  implemented (200); private CONST/TYPE/VAR + bodies + optional
  `BEGIN` init body. Every heading needs its body at `END` (231).
- Program `MODULE` — `IMPORT L` / `FROM L IMPORT x` need L defined
  AND implemented (201). Unknown-module imports stay unchecked
  stubs (legacy behavior, unchanged).
- Order inside a session is free except program-last (grammar checks
  enforce per-library order); a second program file fails the
  session with `Incorrect source` (no `.LST` mark). A session
  without a program emits nothing (`Incorrect source`).

## What was built

- `src/SymTab`: session state (no per-file `Init`; driver inits
  once) — `SetUnit/UnitKind`, `BeginDef/EndDef` (auto-export +
  def registry), `DefExists/ImplDone`, `OpenImpl/EndImpl` (fresh
  scope + interface materialization), `EnterHeading`,
  `BeginBody/EndBodyHeader` (body headers re-parse into the heading
  entry: signature snapshot compare, 231 on mismatch/double-body),
  `EnterAlias/SetProcNum` (FROM-import materialization),
  `ExpConstVal` + const-value export snapshot (integer family).
  231 message already existed in the driver (`compiler.frm`).
- Grammar: `DefDecl/ProcHeading`, `ImplUnit/ImplDecl/ImplProcBody`,
  rewritten `Import` (second `ax` attribute carries alias decl
  nodes) + `ImpName` (201 unknown / 200 duplicate / 230
  unsupported), `InProc` guard (session FROM-import inside a
  PROCEDURE is 230), `InDef` guard (REAL/STRING CONST in a
  definition is 230). `ConstFold` accepts qualified `Lib.C` (bounds
  across files).
- Driver (`compiler.frm`): `SymTab.Init` once, per-file
  `ResetErrors` (the `firstErr` list never reset — stale errors
  leaked into later files' listings), unit-root collection, impl→def
  merge by module name, ONE `MGen.EmitModule` over the session
  root (program decls chained after the library units; zero units
  reuses the program root verbatim).
- `src/MGen` (3 small changes): `EmitDecls` shares the library
  global slot for tag-stamped alias `nkVar`s (no double slots);
  `ConstFindMod` + tag-aware `PushConstName`/`ConstTextOf` with an
  export-table fallback (plain `IMPORT` + `Lib.C` needs no
  materialization); `EmitExpr` routes qualified const designators
  (`aux = KindConst`) to the const path.
- `run_tests.sh`: `expect_run_files` / `expect_fail_files` /
  `expect_fail_multi` (MC4-aware, timeout-guarded like the rest).

## Tests — 55/55

New `tests/d_*` (hand-computed ExitCodes): `DBasic` 60 (FROM +
qualified, var/proc), `DFrom` 35 (two libs, const/var/proc,
cross-def import), `DMulti` 68 (qualified VAR actual), `DInit` 7
(impl `BEGIN` runs: preset 2 vs zeroed 0), `DFunc` 81 (nested
function calls), `DType` 40 (`FROM` array type + qualified
`Lib.Point` record). Showcase `tests/showcase5.mod` (+ `s5lib.def`
/ `s5lib.mod`) tours every interface kind in one program and
prints 46. Rejections: unknown definition (201),
signature mismatch (231), missing body (231), unimplemented
import (201), duplicate definition (200), private access (201),
second program file (driver-level).

## Bugs found (all fixed, all covered)

- `Unit` double-consumed `MODULE` (every file rejected) — `Module`
  keeps its own keyword.
- `ProcHeading` ate the `;` its caller also expected (def files
  rejected with cascade `';' expected`).
- Driver stage machine rejected def-after-impl; relaxed to
  program-last (v1 interleaves per lib too).
- Test-only: `CONST dbl = step + step` doesn't fold (`ConstFold`
  is literal/negation/name-only, pre-existing single-file limit) —
  test lib uses a literal + a bound-via-imported-const instead.

## Limits (documented, next steps)

- One program per session; def/impl file order must respect
  dependencies (imports need completed implementations).
- Interface consts are integer-family only (CHAR/BOOLEAN/ordinal
  included); REAL/STRING consts in definitions are 230.
- Bodies are define-before-use inside an implementation (V2 has no
  FORWARD, same discipline as everywhere else).
- Unknown-module imports unchecked (legacy); proc/type/var/const
  exports only (no opaque or procedure types yet).
- Next candidates: opaque + procedure types, `WITH`/`CASE`,
  `LONGINT`/`LONGREAL` quads, multi-`.MC4` depCount emission.