# Session 2026-10-23 — ISO exceptions (M1–M5)

Follows the same-day range-check/PACKEDSET session
(`summary_session-2026-10-23.md`) and the earlier design commit
`2a30d8c`.

**Branch:** `master`.  **Feature state:** `v3-exceptions` + `v3-ptrchain` + `v3-function-exception` + `v3-finally`.
**Gates at the end:** suite **283/283**, fixpoint **OK
(4,184,457 bytes)**, corpus **512/612 compile-OK, 0 crashes**.

---

## Numbers

| gate | at design start | at end |
| --- | --- | --- |
| suite (`compiler/run_tests.sh`) | 263/263 | **283/283** |
| fixpoint (`bootstrap/fixpoint.sh`) | 4,075,853 B | **4,184,457 B** |
| corpus (`tools/v3-corpus/corpus.sh`) | 511/612 | **512/612 compile-OK, 0 crashes** |

The corpus is compile-only.  After the follow-up pointer-chain fix,
`lazyunique` compiles **and runs** (`rc=0`, unique output `0, 2, 1`);
the corpus count stays 512 because it was already compile-OK.
`except2/4/5/7/8/9` still fail the strict declaration-order rule
because `ip` is declared after the procedures, like `onebyte*`.

---

## What landed

### M1 — parser/AST (`98da444`)

- `EXCEPT` and `RETRY` became reserved words.
- New AST nodes:
  - `NkExcept` — body statements plus handler statements
  - `NkRetry`
- `EXCEPT` accepted on procedure, program-module, implementation-module,
  local-module and class implementation bodies.
- `RETRY` outside a handler is error **237**
  (`RETRY outside EXCEPT`).
- Two-phase defer/reparse carries the handler body like any other
  statement sequence.
- `EXCEPT` initially reported `230` so no handler was silently ignored.

### M2 — runtime and codegen (`6e87def`)

- `runtime/syslib/shim.c`:
  - handler stack with saved/restored exception state
  - `m2_exc_push`, `m2_exc_leave`, `m2_exc_retry`,
    `m2_exc_raise_lang`, `m2_exc_raise_user`
  - `m2_exc_init`
  - basic `SIGSEGV`/`SIGFPE` to language-exception mapping
- `QbeGen`:
  - `ExcBegin`, `ExcBodyDone`, `ExcHandlerBegin`, `ExcEnd`, `ExcRetry`
  - handler pop on `RETURN`
  - `m2_exc_init` emitted at `main` entry
- `SYSTEM.THROW` is bound to `m2_exc_throw`.
- Tests: `t_except_parse`, `t_except_throw`, `t_except_retry`,
  `t_except_return`, `t_except_nested`, `t_except_module`,
  `t_except_signal`; `t_retry_bad` (237).

### M3 — libraries (`5f47b76`)

- `runtime/syslib/SysExceptions.def/.mod` — C runtime binding.
- `stdlib/exceptions.def/.mod` — ISO `EXCEPTIONS`.
- `stdlib/m2exception.def/.mod` — ISO `M2EXCEPTION`.
- `t_except_lib` covers:
  - `AllocateSource` / `RAISE`
  - `CurrentNumber` and wrong-source `exException`
  - `GetMessage`, including empty message in normal state
  - `IsCurrentSource`, `IsExceptionalExecution`
  - `IsM2Exception`, `M2Exception`
  - `M2Exception` after `SYSTEM.THROW`

### M4 — catchable language traps (`02b3763`)

- `QbeGen.TrapCode`.
- `CheckIndex` → `indexException`; `CheckRange` → `rangeException`.
- `CASE` no match with no `ELSE` → `caseSelectException`.
- Internal descriptor/copy mismatch → `sysException`.
- Unhandled exceptions still abort with **rc 134**.
- Tests: `t_except_trap`, `t_except_case`, `t_except_unhandled`;
  existing `t_trap_*` tests still pass.

### M5 — finalization

- This summary, handoff, status and feature/tag updates.
- Feature tag **`v3-exceptions`** at `02b3763` (M4).
- Session tag **`v3-session-2026-10-23-exceptions-end`** at this docs
  commit.

---

## Commits

| commit | tag | work |
| --- | --- | --- |
| `98da444` | — | M1 parser/AST for `EXCEPT`/`RETRY` |
| `6e87def` | — | M2 runtime, `sigsetjmp` codegen, `SYSTEM.THROW` |
| `5f47b76` | — | M3 `EXCEPTIONS` / `M2EXCEPTION` libraries |
| `02b3763` | `v3-exceptions` | M4 catchable index/range/CASE traps |
| `96a3080` | `v3-session-2026-10-23-exceptions-end` | M5 summary/docs |
| `e804e57` | `v3-ptrchain` | self-referential record layout / pointer chains |
| `4238f67` | `v3-function-exception` | result-function fall-through trap |
| `c0c227c` | `v3-finally` | module finalization at normal termination |

The design commit is `2a30d8c`.

### Follow-up — pointer-chain codegen

The self-referential `TYPE List = POINTER TO RECORD next: List ... END`
and `p^.next^.value` gaps were fixed by publishing an unresolved alias
for a TYPE before parsing its body, then invalidating record/class
layout when `SetTarget` completes the alias.  Regression
`t_ptrchain`; `lazyunique` now runs with rc 0.  This follow-up is
tagged `v3-ptrchain` (`e804e57`).

### Follow-up — functionException

`QbeGen.EndFunc` now traps with language code 4 when a result function
reaches its end without `RETURN`, instead of silently returning 0.
`t_except_funcret` catches the exception and `t_trap_funcret` verifies
the unhandled rc=134 path.  Tagged `v3-function-exception`
(`4238f67`).

### Follow-up — module FINALLY

Module bodies now accept `FINALLY` (with its own optional `EXCEPT`)
for program, implementation, local and class bodies.  The finalizer is
emitted as `<module>_fini`; main calls the program finalizer first, then
imported/local finalizers in reverse initialization order.  The program
body itself is emitted as `<module>_init`, so a module `RETURN` leaves
the init function and finalizers still run.  Tests: `t_finally`,
`t_finally_return`, `t_finally_except`, `t_finally_local`,
`FinallyOrder` (reverse order `P!B!A!`).  Tagged `v3-finally`
(`c0c227c`).

---

## Remaining work

- Module termination edge cases: `HALT`/unhandled aborts do not run
  finalizers; the ISO `TERMINATION` module is not implemented.
- Coroutine (`coException`) and protection (`protException`) exceptions.
- Complex-number exceptions (complex arithmetic is still a placeholder).
- Multi-dimensional open arrays, separate compilation and the dialect
  library-folder layout (see `handoff-2026-10-23-next.md` and
  `plan-separate-compilation.md`).