# V3 step 3.1 — arrays: bounded/open, strings, whole assign (done 2026-09-20)

First of four phased composite substeps (arrays → records → sets →
pointers). Suite 42/42 (16 run — incl. 2 trap-aborts — + 26 reject),
LL(1)-clean (only the known-benign TypeBlock warning).

## Type syntax (`M2.atg`)

- `Type` gains `allowOpen` and `ArrayType`: `"ARRAY" "OF" Type`
  (open formal; 230 elsewhere incl. named open aliases) and
  `"ARRAY" "[" bounds "]" "OF" Type` (multi-index sugar nests
  inside-out via `NestArray`). Bounds are folded int/char literals
  (`ConstInt`, inverted → 230); bare-type indices wait for enum
  ordinals; `VarDecl` takes full `Type<FALSE>` (anonymous arrays
  in `VAR`); formals take `Type<TRUE>`; composite function results
  are 230 (V2 parity).
- Coco/R V1.53 cannot parse dotted types (`SymTab.BoundsArr`) in
  *attribute* declarations (generates garbage), so bounds accumulate
  SymTab-side (`BoundBegin/BoundAdd/NestArray`, max 8).
- `Design` grows index suffixes (`[i, j]` descends nested levels;
  217/218) plus an `sfx` out-flag; `Fact` loads indexed elements
  (`ElemLoad`) and lowers string literals to descriptors
  (1-char stays CHAR, `''` works); `Assign` copies whole arrays
  (`CopyArray`) or stores elements (`ElemStore`, with `ConvIR`
  for `r[i] := 3`); array/string `=` is 213 (strict Wirth; done in
  `EqCheck`, so CASE labels agree).

## Layout (locked rule, now real)

`DeclArr` emits `data $a = { l n, … }` with CHAR→b, int/bool/enum→w,
REAL→d, nested/pointer→l, all zero; nested levels are separate
`$a_i` objects linked by address (probes proved `l $sym` in data,
`copy $sym` addressing, `loadub`, `blit`-class ops). Indexes widen
to LONGCARD (`extsw`), check `lo..hi` (static, or `0..count-1`
for open via `OpenHi`), trap via `call $abort()` + `hlt`
(rc=134; interim until `runtime/syslib/Trap`).

## Backend lessons (all probed, not guessed)

- QBE `blit` needs a *constant* length → whole copies use
  `call $memcpy(l dst, l src, l n)` (runtime length, uniform for
  fixed/open/nested; nested recurses through a runtime loop).
- QBE `call` needs per-argument classes (`call $f(l a, …)`);
  bare `$abort()` is fine.
- Single-letter `%p` temporaries mislink — always `%tN` (already
  the convention; now recorded).
- String `data` must be top-level: literals buffer during the body
  and flush at `EndModule` (a `data` line mid-function is a QBE
  syntax error — found the hard way).

## SymTab (additive)

Bounded/open descriptors (`NewArrayB/OpenArray`, `alo/ahi`),
`IsOpenArray/ArrayLo/Hi/Len/Depth`, `ConstInt` reuse;
`Assignable` tightened (array↔array by `SameType`,
string→1-D-CHAR-array only); `EqCheck` rejects array/string `=`.

## Tests

`t_array` → 108 (named + anonymous arrays, nested `m[i,j]`,
whole + nested copies, strings, char compare); `t_bad_index`
(218), `t_bad_bounds` (inverted → 230), `t_openformal`/`t_bad_open`
(open rules); new `expect_trap` harness (rc=134): index breach
and `"abcd"`-into-3 count mismatch.

Deferred: `HIGH`/`LEN` (need calls, step 4), string `=`/`+`,
string CONSTs, enum/char-subrange bounds as index *types*,
`CHAR`-bound subtleties, open-formal *reads* (parse-only now).