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+# Session summary — 2026-09-28 (c): variant records, UString, IOChan/Files
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+
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+Suite **135/135**; self-hosting fixpoint **OK**
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+(`bootstrap/fixpoint.sh`, image **2,142,363 bytes**). `master`, tree
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+clean except the user's uncommitted `compiler/toto.mod`.
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+
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+This session landed the three items left open at the previous save,
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+then showcased them.
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+
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+## Steps, tags and headline commits
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+
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+| Step | Tag | Commit | What |
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+| --- | --- | --- | --- |
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+| Variant records | `v3-variant-records` | `252d7dd` | `RECORD … ; CASE tag : T OF … END` |
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+| UString + Files | `v3-ustring-files` | `36e8dcf` | `a := U"…"` content copy; `Files` on `IOChan` |
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+| Showcase | `v3-showcase16` | `36e8dcf` | the three features in one program |
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+
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+## 1. Variant records
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+
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+```modula-2
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+Shape = RECORD
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+ id : INTEGER;
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+ CASE k : Kind OF
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+ circle : radius : INTEGER;
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+ | square : side : INTEGER;
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+ | rect : w, h : INTEGER;
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+ END;
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+END
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+```
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+
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+- Grammar stays LL(1): a variant part starts with the `CASE` keyword,
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+ so `RecItem` is unambiguously `RecField | CaseField`. Labels are
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+ parsed but not interpreted; a non-ordinal tag is error **224**.
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+- Layout: fields declared before the tag pack normally, the tag takes
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+ the next slot, and **every** branch field overlays in the region
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+ after it. Size = plain + tag + max(branch fields).
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+- `SymTab`: `SetVariantTag`/`MarkVariant`/`IsVariant`/`ResetFields`/
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+ `IsOrdinal`; `ComputeOffsets` and `TypeSizeD` handle the overlay.
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+- Whole-record equality/assignment for a variant record is not
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+ offered (overlapping storage); the existing strictness gives 213.
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+- `WITH` over variant fields works (verified).
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+- Docs: `summary_variant-records.md`.
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+
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+## 2. UString whole-string assignment
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+
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+- `a := U"…"` into a 1-D `ARRAY OF UCHAR` is a **content copy**
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+ (count-clamped, `0` terminator), mirroring `CHAR` strings.
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+- `SymTab.IsUCharArray`, `QbeGen.UAssign`, shim `m2uassign`.
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+ The descriptor is `{ l count, w codepoints… }`, so the body copies
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+ 32-bit elements off a 64-bit header.
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+- `UCHAR` arrays now reserve a terminator slot (`z (n+1)*4`), heap
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+ allocation and bounds checks included.
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+- A `U"…"` literal and a `UCHAR` array both pass to an open
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+ `VAR ARRAY OF UCHAR` formal.
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+- Docs: `summary_ustring-files.md`.
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+
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+## 3. Files layered on IOChan
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+
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+- `IOChan` gained `Handle(cid): ADDRESS` / `MakeChan(h): ChanId` so a
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+ library can wrap a channel.
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+- `Files.FileRec` holds an `IOChan.ChanId`; **all** I/O delegates to
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+ `IOChan`, with `NIL` guards. `Files` gained `ReadString` (one line).
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+- The public `Files` API is unchanged apart from that addition.
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+
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+## 4. Showcase
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+
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+`compiler/tests/showcase16.mod` (`Showcase16`, exit 42): a `Shape`
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+variant record, a `U"ok✓"` content copy, and a write/read round-trip
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+through a `Files` handle.
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+
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+```
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+showcase16: id=3 total=42
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+```
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+
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+## Tests (135, was 132)
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+
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+New: `t_variant` (31), `t_ustrassign` (63), `t_ustrformal` (3),
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+`Showcase16` (42). `FilesProg`/`TBadFile`/`MixProg`/`Showcase11` file
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+lists now include `SysShim` + `IOChan`.
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+
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+## Resume
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+
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+```sh
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+cd .../m2compiler-V3
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+./bootstrap/fixpoint.sh # FIXPOINT OK
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+cd compiler && ./build.sh && ./run_tests.sh # 135/135
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+```
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+
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+## Remaining polish
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+
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+- Clarion `CLASS` lowering (declared + checked only).
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+- `IOChan` position/seek; ISO `ConvResults` enumerations (now
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+ expressible); UString concatenation/`Length`.
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