## Session summary — parsing fixes and tests

Date: 2025-11-08

Summary
-------
This document records the recent work to resolve several parsing issues in the Zexus interpreter repository. The primary goals were:

- Fix missing token alias (`ASTERISK`).
- Add support for arrow-style lambdas (`=>`) in the lexer and parser.
- Improve parsing of list (array) literals and map/object literals.
- Improve method/property chaining parsing and tolerant parsing behavior.
- Reduce token fragmentation produced by the structural analyzer so the tolerant/context parser receives larger blocks.

Actions performed
-----------------

1. Token changes
   - Added `ASTERISK = STAR` alias in `src/zexus/zexus_token.py` to avoid NameError when older code references `ASTERISK`.
   - Lexer updated to emit a single `LAMBDA` token for `=>` and to provide a small parenthesis hint for `( ... ) =>` detection. File: `src/zexus/lexer.py`.

2. Parser changes
   - Extended `src/zexus/parser.py` (the deterministic recursive-descent parser) to accept `LAMBDA` as an infix form and handle arrow-lambda groupings.
   - Implemented lookahead / grouped-expression special-casing so parenthesized parameter lists followed by `=>` are parsed as lambda parameter lists.

3. Tolerant/context parser changes
   - Implemented list-literal parsing and keyword-lambda parsing logic in `src/zexus/strategy_context.py`.
   - Reworked the chaining logic to parse a primary expression then iteratively apply chained operations (method calls and property access) instead of brittle token-pair heuristics.
   - Added a fallback grouping for runs of tokens so the context parser can parse larger runs and avoid producing many tiny, empty `BlockStatement`s.

4. Structural analyzer changes
   - Adjusted `src/zexus/strategy_structural.py` heuristics to group consecutive non-starter tokens into larger statement blocks; this substantially reduced token fragmentation and improved the tolerant parser's input.

5. Tests and validation
   - Added a focused test script `tests/test_lambda_and_chains.py` to exercise:
     - arrow lambdas (single-param and parenthesized),
     - keyword lambdas,
     - list (array) literals,
     - object (map) literals,
     - nested arrays,
     - method chaining and property chains.
   - Validation command used locally in the dev container:

```bash
PYTHONPATH=./src python tests/test_lambda_and_chains.py
```

Test run summary
----------------
- The focused tests executed and reported the following:
  - All arrow-lambda cases parsed with no errors (both base parser and tolerant parser for the tested expressions).
  - List and object literal parsing succeeded (ListLiteral / MapLiteral nodes created).
  - Method chains (e.g., `numbers.map(...).filter(...).reduce(...)`) parsed into nested `MethodCallExpression` nodes.
  - The tolerant/context parser now receives larger token blocks and produces populated `BlockStatement`s instead of many single-token blocks.
  - Minor remaining item: the base parser printed a warning for the keyword-style lambda using a colon (`lambda(x): x + 1`) — it constructed a LambdaExpression but also emitted an "Unexpected token ':'" warning. This is small and localized; the tolerant parser handles keyword-style lambdas without errors.

Files modified (high-level)
--------------------------
- `src/zexus/zexus_token.py` — token alias `ASTERISK = STAR`
- `src/zexus/lexer.py` — `LAMBDA` token for `=>` and paren-lambda hint
- `src/zexus/parser.py` — arrow lambda infix handling and grouped-expression lookahead
- `src/zexus/strategy_context.py` — list literal, keyword-lambda parsing, robust chaining, fallback grouping
- `src/zexus/strategy_structural.py` — grouping heuristics for statement blocks
- `tests/test_lambda_and_chains.py` — added focused test script

Next recommended steps
----------------------
1. Convert `tests/test_lambda_and_chains.py` into assert-driven pytest test cases so the CI can catch regressions automatically.
2. Fix the small colon (`:`) handling warning for keyword-style lambdas in the base parser (`src/zexus/parser.py`) to remove the non-fatal syntax warning.
3. Refactor duplicate lambda parsing code between `parser.py` and `strategy_context.py` into shared helpers to reduce duplication.
4. Run full test-suite under `PYTHONPATH=./src` or install the package in editable mode and run `pytest` in CI to validate no regressions exist outside the focused cases.

How to view the saved report
---------------------------
Open the file in the repository at `docs/session_summary.md`.

If you want, I can convert the focused test script into pytest tests now and run the full test-suite locally (it will use `PYTHONPATH=./src`), or I can implement the small base-parser colon fix first — tell me which you prefer and I'll proceed.

— Automated session report generated by the development assistant
