# Step 4 goal — Composite types (saved, to implement tomorrow)

## Scope (Phase 3 of the plan)

- `ARRAY`: bounds, indexing (incl. multi-dimensional), scaled address
  arithmetic, indexed loads/stores (`0EH`/`1EH`), open-array parameters.
- `RECORD`: field layout with offsets, `.field` selection, `WITH`.
- `SET`: add operators (`+` union, `-` difference, `*` intersection);
  literals/`IN`/`=`/`#` already work since step 2.
- `POINTER`: `NEW`/`DISPOSE`, dereference, `NIL` handling.
- Strings: string variables as `ARRAY OF CHAR`.

## Design deltas (vs step 3)

- SymTab: byte sizes on descriptors, field/element offsets,
  per-variable sizes (frame + globals); new queries (`TypeSize`,
  `FieldOffset`, …).
- MGen: scaled indexing, `copy_block` for composite assignment.
- Policy to fix at start: range checking (checked vs unchecked),
  `NIL` dereference (runtime trap, no static check).
- `NEW` semantics: **recommended** — VM `ALLOCATE` (`0x40 05`),
  already implemented by the interpreter. Alternatives (e.g.
  compiler-managed bump region) to be decided if ALLOCATE
  misbehaves under test.

## Acceptance (same bar)

~15 run + ~10 rejection tests, all green, no cascades; full
49-test regression still passing (→ ~74 total).

## Suggested first tests

Matrix multiply or sieve (arrays); nested records + `WITH`;
set ops with ranges; linked list via `NEW`/`DISPOSE` (combines
pointers with step-3 recursion).

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# Session checkpoint (saved, not committed)

- Tags `step1`, `step2`, `step2.1`, `step3` exist locally
  (`step3` = `c0f1924`, suite 49/49 + mc64 24/24 green).
- Pushing (commits + tags) is done by the user (credentials).
- Uncommitted on disk: `summary_step1/2/2.1/3.md` (docs commit
  `0e07f06` was undone per user request), this `goal_step4.md`,
  plus pre-existing leftovers never touched (`m2comp/coco/`
  deletions, `gnu-m2-grammar.txt`, chatgpt folder, instructions
  file, `STAK`/`DEP.MC4`).
- Next action: implement the step-4 goal above.