# P4 Pascal validator pilot

`p4_pascal.atg` describes Wirth's Pascal-P4 (1977) compiler input. It
needed a full repair pass to build with Coco/R V1.53 (`CR`) and GNU
Modula-2 (`gm2 -fiso`), all diagnosed against `fpc -Miso` as oracle:

1. Bare `;` is not a Coco/R literal — every statement separator is
   written `";"` (this alone was 5 of the original errors; the same
   applies inside `(...)` groups).
2. The `Statement` production was missing its terminating `.`.
3. `EmptyStatement = ""` is illegal — empty positions use the
   `[ X { ";" [ X ] } ]` list shape instead (progress-guaranteed, so
   the generated parser cannot hang on empty matches).
4. `ident` cannot be both a `TOKENS` class and a production (renamed
   the literal tokens to `pnumber`/`pstring`); `EOF` is reserved
   (dropped the junk line with it); the start production must match
   the `COMPILER` name.
5. Left-recursive `Primary "^"` rewritten as a postfix loop.
6. Undefined nonterminals filled in: element productions extracted
   from the `...List` wrappers (`ConstDecl`, `TypeDecl`, `VarDecl`),
   `ProcDecl` (nested procedures/functions), `Value`, `PointerType`,
   `noQuote1`, `ParList`→`ParamList`, `Stmts`→`Statements`,
   `CaseStatement`/`WhileStatement`/`...`→actual names,
   `Var`→`VarIdent`.
7. Name collisions in generated `CASE` statements fixed by merging:
   duplicate `for..to/downto` branches, duplicate `Type/StructType`
   entries (`ArrayType`/`SetType`/`ProcType` reachable twice), and
   ident-led `Assignment` vs `ProcCall` (single `IdentTail` dispatch
   on `:=` vs `(`).
8. Grammar gaps closed to oracle level: `TypeIdent` (named types —
   without it every `VAR x: MyType` failed), `Compound` program bodies,
   trailing-`;`-tolerant sections/fields/case-items, `{}` comments,
   subscripts/field selectors in statements and expressions,
   `and`/`or` words alongside P4 `&`, bare parameterless calls,
   statement labels + `goto` (+`LabelSection`), `and`-less `or`...,
   formal parameter groups (`name: type`, `var`-prefixed) for
   declarations (call sites keep expression lists), subrange array
   indices (`1..10`), const expressions as `Value`, `CASE` branches
   take a single statement (also fixes a `;`-separator competition),
   multi-routine `{ ProcDecl }` blocks.

Two benign LL(1) warning families remain (dangling-`ELSE`, loop-exit
`";"` greediness — both resolve correctly).

Floats (`1.5E-3`), enums, subranges (`0..255`, `N..M`, dotted names)
and `Label = pnumber | ident` (enum case labels; statement labels stay
numeric-only or `i := 1` breaks on the `:=`) were added to oracle
level afterwards.

`if`/`while`/`for` branches take a single `Statement` (standard Pascal
— plural branches greedily swallow the `;` separating `case` items,
so `if..then begin..end;` inside a `case` branch broke), and
`begin..end` compounds are statements in their own right.

The generated parser gets a strict EOF check
(`IF sym # 0 THEN SynError(0)` after the start symbol in `Parse`,
patched in by `build.sh` with a loud assertion): without it, the
Coco/R driver never scans past the program's final `.`, silently
ignoring trailing garbage — and worse, masking any failure that
leaves a `.` behind. This makes the validator marginally stricter
than `fpc`, which accepts trailing junk after `END.` — degenerate
input only, documented here.

## Build

```sh
./build.sh        # -> build/P4 (build/ is git-ignored)
```

Needs `CR` (Coco/R), `gm2 -fiso`, the `parser.frm`/`scanner.frm`
frames and `FileIO.def/.mod` — all resolved from the environment with
sensible defaults (`CRFRAMES`, `M2LIB`, `CR`, `GM2`).

## Differential results (P4 vs `fpc -Miso`)

Accept battery + `tests/deep.pas` (labels/goto, nested routines with
`var` params, records, enums, subranges, sets, pointers, all
statements, hex-free literals): **0 false positives**. Broken
mutants: **0 missed**, positions close. P4-era shapes (`&`, `#`,
`unit` sections, `return`) are accepted leniently although `fpc
-Miso` rejects them (direction is accept-more, so these can only ever
be missed rejections, never false positives).