Theia-Modula-2/grammars/P2

P2 Pascal validator pilot

p2_pascal.atg describes Wirth/Ammann Pascal-P2 (1972-73) as implemented by pcom. It was authored from the P4 pilot skeleton and corrected against some 60 oracle probes — every construct below was checked against the real compiler, whose parser (pcomp.pas) is the ground truth for the language. (The Grammars/P2/p2_pascal.atg file in the reference collection is a mislabeled Oberon grammar, unrelated.)

Oracle notes

pcom was built with fpc -Miso from the ISO-7185-modified sources (Pascal-P2-master/source/), needing a one-character fix in a /tmp copy (duplicate '1' in an identifier-scan set that fpc rejects). Behaviors the harness must respect, all verified:

• Uppercase only (CDC charset): lowercase never assigns the scanner symbol and the compiler ping-pongs forever. Test sources are uppercase; the validator is case-sensitive and rejects lowercase cleanly (oracle-consistent, never a false positive).
• Trailing newline required: files without one spin at EOF (EOF ENCOUNTERED flood). The harness appends \n.
• Spin on sync-loss: illegal characters (_ & { @ # → error 399) and some shapes never recover — the compiler loops instead of listing. Harness rule: timeout (10s; valid programs finish instantly) counts as oracle-reject. Every accept-case was verified to terminate.
• Verdicts: **** ^NNN markers in the listing (NNN is Wirth's error number); absence = accept. Semantic markers (undeclared identifiers etc.) also count as reject, so the battery uses only well-formed programs.
• PROGRAM T; BEGIN END. (no VAR) and PROGRAM HELLO(OUTPUT) both terminate cleanly — used as harness sanity checks.

Key probed P2 facts (vs P4): AND/OR/NOT words but no &; DIV/MOD/IN; CASE with no ELSE; (* *) comments that do not nest, no {}; numeric-only labels and GOTO; reals strictly digits.digits[E...] (no 1., no .5); consts are [sign] literal (no expressions); enums without = N; set elements without .. ranges; untagged variants rejected (CASE tag: type required); TEXT/FILE types absent (FILE OF → 399 unimplemented); PACK/UNPACK/PRED/SUCC parse but unconditionally mark 399 (stubs — kept out of the battery); NEW exists but DISPOSE does not (RELEASE does); no ROUND/HALT/EXIT/RETURN; no procedural types (the compiler crashes on them); no EXTERN; FORWARD exists; WRITELN/READLN/READ/EOF/etc. require parentheses (WRITELN; is error 9) while user routines allow bare calls; EOLN/READ take file args; PACKED, multi-dim arrays, subrange/const bounds, CHAR indices, pointer forwards, OTHERWISE-less everything.

Grammar notes

Single-statement if/while/for branches (standard Pascal, avoids ;-capture across case items), begin..end compounds as statements, trailing-;-tolerant sections/fields/case-items, order-free repeatable declaration sections (as in pcom's BLOCK), mandatory (...) on the 13+16 predeclared routines (StdProc/StdFunc, which is what rejects bare WRITELN;), write-width params, PROGRAM name[(files)] headings, strict EOF check (patched into Parse by build.sh, which also fails on degenerate IF/WHILE FALSE generation).

Lenient acceptances (documented misses, never false positives): DISPOSE/ROUND/user-TEXT parse as ordinary identifier calls/ types although pcom marks them; lowercase is rejected while fpc would accept (P2 direction wins — pcom is the oracle here).

Build

./build.sh        # -> build/P2 (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 (P2 vs pcom)

Accept battery (23 cases: all statements, NEW/deref/NIL, ORD/CHR, reals, strings, sets, ODD/TRUNC, trig, EOF(INPUT))

• tests/deep.pas (labels/goto, nested routines with var params, records, enums, subranges, pointers, all statements): 0 false positives. Broken mutants (13, incl. spin-cases &, {}, @, unterminated comment, ELSE, ident labels, 1.5.2, bare WRITELN): 0 missed, positions close. Real P2 corpus (Pascal-P2-master/sample_programs, p2/Examples): 5/5 agree (qsort needed the trailing-newline normalization).