# Session summary — 2026-09-28 (c): variant records, UString, IOChan/Files

Suite **135/135**; self-hosting fixpoint **OK**
(`bootstrap/fixpoint.sh`, image **2,142,363 bytes**). `master`, tree
clean except the user's uncommitted `compiler/toto.mod`.

This session landed the three items left open at the previous save,
then showcased them.

## Steps, tags and headline commits

| Step | Tag | Commit | What |
| --- | --- | --- | --- |
| Variant records | `v3-variant-records` | `252d7dd` | `RECORD … ; CASE tag : T OF … END` |
| UString + Files | `v3-ustring-files` | `36e8dcf` | `a := U"…"` content copy; `Files` on `IOChan` |
| Showcase | `v3-showcase16` | `36e8dcf` | the three features in one program |

## 1. Variant records

```modula-2
Shape = RECORD
  id : INTEGER;
  CASE k : Kind OF
    circle : radius : INTEGER;
  | square : side : INTEGER;
  | rect   : w, h : INTEGER;
  END;
END
```

- Grammar stays LL(1): a variant part starts with the `CASE` keyword,
  so `RecItem` is unambiguously `RecField | CaseField`. Labels are
  parsed but not interpreted; a non-ordinal tag is error **224**.
- Layout: fields declared before the tag pack normally, the tag takes
  the next slot, and **every** branch field overlays in the region
  after it. Size = plain + tag + max(branch fields).
- `SymTab`: `SetVariantTag`/`MarkVariant`/`IsVariant`/`ResetFields`/
  `IsOrdinal`; `ComputeOffsets` and `TypeSizeD` handle the overlay.
- Whole-record equality/assignment for a variant record is not
  offered (overlapping storage); the existing strictness gives 213.
- `WITH` over variant fields works (verified).
- Docs: `summary_variant-records.md`.

## 2. UString whole-string assignment

- `a := U"…"` into a 1-D `ARRAY OF UCHAR` is a **content copy**
  (count-clamped, `0` terminator), mirroring `CHAR` strings.
- `SymTab.IsUCharArray`, `QbeGen.UAssign`, shim `m2uassign`.
  The descriptor is `{ l count, w codepoints… }`, so the body copies
  32-bit elements off a 64-bit header.
- `UCHAR` arrays now reserve a terminator slot (`z (n+1)*4`), heap
  allocation and bounds checks included.
- A `U"…"` literal and a `UCHAR` array both pass to an open
  `VAR ARRAY OF UCHAR` formal.
- Docs: `summary_ustring-files.md`.

## 3. Files layered on IOChan

- `IOChan` gained `Handle(cid): ADDRESS` / `MakeChan(h): ChanId` so a
  library can wrap a channel.
- `Files.FileRec` holds an `IOChan.ChanId`; **all** I/O delegates to
  `IOChan`, with `NIL` guards. `Files` gained `ReadString` (one line).
- The public `Files` API is unchanged apart from that addition.

## 4. Showcase

`compiler/tests/showcase16.mod` (`Showcase16`, exit 42): a `Shape`
variant record, a `U"ok✓"` content copy, and a write/read round-trip
through a `Files` handle.

```
showcase16: id=3 total=42
```

## Tests (135, was 132)

New: `t_variant` (31), `t_ustrassign` (63), `t_ustrformal` (3),
`Showcase16` (42). `FilesProg`/`TBadFile`/`MixProg`/`Showcase11` file
lists now include `SysShim` + `IOChan`.

## Resume

```sh
cd .../m2compiler-V3
./bootstrap/fixpoint.sh                       # FIXPOINT OK
cd compiler && ./build.sh && ./run_tests.sh   # 135/135
```

## Remaining polish

- Clarion `CLASS` lowering (declared + checked only).
- `IOChan` position/seek; ISO `ConvResults` enumerations (now
  expressible); UString concatenation/`Length`.