# Pascal-S validator pilot

`pascals.atg` describes Wirth Pascal-S (1976) as implemented by
`PascalS0` (Turbo-Pascal batch interpreter). It was authored from the
P2 pilot skeleton and corrected against some 90 oracle probes —
every construct below was checked against the running interpreter
(no fpc involved anywhere).

## Oracle notes

`psc` was built with `fpc -Mtp` from `Pascals/` plus a `/tmp`-only
`TrmInOut` stub (the shipped unit needs DOS `REGISTERS`/`MsDos`;
only console setup, irrelevant to parse verdicts). Behaviors the
harness respects, all verified:

- Verdicts come from the `.lst` listing: `******* ^ NNN` markers
  (full error table printed in `PascalS0.pas:errormsg`); absence =
  accept. Feeding is via `psc src.lst` args with stdin `</dev/null`
  (renders blocking on `read` harmless); 15s timeout (valid programs
  finish instantly).
- Lowercase is accepted (Turbo scanner), so the validator uses
  `IGNORE CASE` — unlike the P2 pilot.
- Mandatory program heading with file list:
  `program t (input, output);` (bare `program t;` is error 9).
  `input`/`output` exist for `eof`/`eoln` only.

Key probed Pascal-S facts: `and`/`or`/`not`/`div`/`mod` words;
`if`/`while`/`repeat`/`for to/downto`/`case` (no `ELSE`, no
`OTHERWISE`); `(* *)` comments that do not nest, plus `{}`;
`const`/`type`/`var`/nested routines with `var` params; integer,
real, char, boolean (no string/enum/subrange/pointer/set/file
types); multi-dim arrays with const bounds; records without
variants; `read`/`readln`/`write`/`writeln` (bare `writeln;` is
legal!); write-widths; `odd`/`abs`/`ord`/`chr`; `eof`/`eoln` with
mandatory file arg; unary signs only at expression start
(`a + +b` is error 58, but `-a` is fine); `NOT` anywhere;
single-statement branches; empty statements; char/enum-less case
labels (`pnumber`/`ident`/`pstring`).

Absent (all probed): labels/`goto`, `forward`, `packed`, sets,
pointers (`^` is error 24), files, `with`, procedural params,
`break`/`exit`/`halt`/`return`, `nil`, `_` in identifiers (error
24), `//` comments, `OTHERWISE`, `max`, const expressions
(`3 + 4` is error 6), multi-char const strings (error 50),
`1.`/`.5` reals, `$`/`#` literals, `IN`, chained assignment,
function-calls-as-statements (error 45).

## Grammar notes

Single-statement branches (avoids `;`-capture), `begin..end` as
statements, trailing-`;`-tolerant sections/fields/case-items,
order-free declaration sections, no label section at all,
`EofCall` with mandatory `(file)` (which is what rejects bare
`eof`), strict EOF check (patched into `Parse` by `build.sh`,
which also fails on degenerate `IF/WHILE FALSE` generation).

Lenient acceptances (documented, never false positives):
multi-char const strings (a `pchar` token breaks V1.53
determinism — probe H2 shows the oracle's error 50); bare
builtin calls (`odd;` parses although only `eof`/`eoln` strictness
was probed); `TYPE`-level constructs the oracle would mark
semantically are out of scope for a syntax validator.

## Build

```sh
./build.sh        # -> build/PascalS (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 (PascalS vs `psc`)

Accept battery (22 cases: all statements, arrays, records,
`ord`/`chr`, reals, strings, `odd`, write-widths, `eof(input)`):
**0 false positives**. Broken mutants (12: truncation, bad
factors, unterminated comment, `@`, missing `TO`, `ELSE`,
`goto`, `_`, `1..3`, set constructor): **0 missed**, positions
close. Example corpus (`Pascals/eg0`–`eg7`): agree on all 8
(`eg0`/`eg1` are intentionally broken and both sides reject them).
`tests/deep.pas` (nested routines with `var` params, records,
all statements, reals): clean on both sides.