# Modula-2 diagnostics — step 1

This first diagnostics layer is deliberately lightweight. It does **not** replace
Coco/R. It provides immediate editor feedback for a small set of structural
errors while the real parser/LSP is being prepared.

It currently checks:

- unterminated `(* ... *)` comments;
- unterminated string literals;
- presence of the module header;
- `;` after the module header;
- presence of the final `END <ModuleName>.`;
- module-name consistency between the header and final `END`;
- the final period after `END <ModuleName>`.

The diagnostics are published through the VS Code-compatible
`DiagnosticCollection`, which Theia displays directly in the editor.

The next stage will replace the structural checks with diagnostics generated by
the Coco/R parser, while keeping the same editor diagnostic mechanism.

## Server-side semantic layer (`src/m2/analyse.ts`, source `modula2`)

In parallel, the language server runs its own checks on every open
document — always on, merged with Coco/R output when a validator is
configured:

- duplicate declarations in one scope (error, with first-declared line);
- unknown identifiers (error), with conservative skips: qualified names,
  import clauses, `WITH` regions, unverifiable FROM-imports, predefined
  identifiers, closing `END Name`;
- enumeration literals `(A, B, ...)` are treated as declared constants, so
  they resolve, complete, and appear in the outline.

Not covered (needs statement/expression parsing): assignment type
compatibility, call arity, use-before-declaration order, FROM-list membership
against the defining `.def`.

## Statement checker (`src/m2/checker.ts` + `src/m2/types.ts`, source `modula2`)

Bodies (`BEGIN..END`, recorded by the parser) are walked with a recursive
statement/expression analyser. Operator and compatibility rules were
calibrated case-by-case against `gm2`:

- assignment compatibility, including whole-number family, `REAL`↔`LONGREAL`,
  `CHAR`/string rules with `ARRAY OF CHAR` length checks, sets, pointers,
  enumerations, `NIL`;
- call arity (GNU optional parameters supported) and argument types,
  including open-array formals; ignored function results and void procedures
  used as values;
- `BOOLEAN` conditions (`IF`, `WHILE`, `ELSIF`, `UNTIL`), ordinal `CASE`
  selectors and `FOR` variables/bounds/`BY` steps;
- illegal targets (`CONST`, procedure, type, module), illegal operands
  (`NOT`/`AND`/`OR`, mixed arithmetic, `DIV`/`MOD` on reals, indexing,
  dereferencing, comparisons of arrays/records, `CARDINAL` vs `INTEGER`
  comparisons);
- `RETURN` without a value, value-returning `RETURN`s, `RETURN` outside
  procedures and values returned from void procedures.

Still out of scope: use-before-declaration order, range checking,
builtin-procedure signatures, record-field types, `WITH`-block internals.