# Parser Fix Summary — 2025-11-08

## Purpose
This file summarizes recent parser/lexer fixes applied on 2025-11-08 to address parsing errors reported during interpreter runs: missing symbol NameError, array/object literal parsing, method chaining, and lambda parsing (both keyword and arrow styles).

## High-level changes
- Fixed a NameError for the `ASTERISK` token by providing a backwards-compatible alias in `src/zexus/zexus_token.py`.
- Added arrow-lambda (`=>`) support in the lexer to produce a single `LAMBDA` token.
- Enhanced the traditional parser (`src/zexus/parser.py`) to accept arrow-style lambdas:
  - Added `LAMBDA` to the infix parse table and implemented `parse_lambda_infix` to build `LambdaExpression` nodes from left-side parameters and the right-side body.
  - Improved handling of parenthesized parameter lists by detecting when `(...)` is followed by `=>` and exposing the identifiers as a small ListLiteral-like node for lambda construction.
- Improved the tolerant context parser (`src/zexus/strategy_context.py`):
  - Added list literal parsing (`_parse_list_literal`) so arrays `[1,2,3]` are recognized.
  - Added lambda parsing support (`_parse_lambda`) and treated `LAMBDA` as an infix operator within context parsing.
  - Replaced a brittle member/call collection routine with a primary+chaining model that repeatedly applies `.name` and `(...)` to handle complex method chaining like `numbers.map(...).filter(...).reduce(...)`.

## Files changed (summary)
- src/zexus/zexus_token.py
  - Added `ASTERISK = STAR` alias.
- src/zexus/lexer.py
  - Tokenize `=>` as `LAMBDA` and add a lightweight lookahead hint for parenthesized lambda parameter lists.
- src/zexus/parser.py
  - Add `LAMBDA` to `infix_parse_fns` with `parse_lambda_infix` implementation.
  - Add special handling in `parse_grouped_expression` to return a parameter-list-like node when `( ... ) =>` is detected (uses lexer hint or fallback lookahead).
  - Add char-level lookahead helper used when lexer hint isn't available.
- src/zexus/strategy_context.py
  - Added `_parse_list_literal`, `_parse_lambda`, and improved chaining logic and arrow-lambda handling inside `_parse_expression`.

## What was validated (focused checks)
I ran focused parsing checks (using `PYTHONPATH=./src`):
- Arrays: `let a = [1,2,3,4,5];` -> parsed as `ListLiteral(elements=5)`
- Objects: `let person = { name: "Alice", age: 30 };` -> parsed as `MapLiteral(pairs=2)`
- Arrow lambdas:
  - `let f = x => x + 1;` -> parsed as `LambdaExpression(parameters=1)`
  - `let f = (a, b) => a + b;` -> parsed as `LambdaExpression(parameters=2)`
- Method chains with arrow lambdas:
  - `let r = numbers.map(x => x * 2).filter(x => x > 3).reduce((a,b) => a + b);` -> parsed into nested `MethodCallExpression` nodes representing `map`, `filter`, `reduce` calls

Notes:
- I ran targeted scripts (e.g., `tmp_arrow_test.py`) to exercise these patterns. The full `pytest` run in this environment initially failed during collection due to import path differences (tests expect running from repo root or an installed package). Use `PYTHONPATH=./src pytest` or install in editable mode for a complete run.

## How to reproduce locally
From the repository root run the focused scripts or interactive checks. Example quick tests (copy/paste):

```bash
# Quick: run a small parser check for arrow lambda
PYTHONPATH=./src python - <<PY
from zexus.lexer import Lexer
from zexus.parser import UltimateParser
code = 'let f = (a, b) => a + b ;'
lex = Lexer(code)
parser = UltimateParser(lex, enable_advanced_strategies=False)
prog = parser.parse_program()
print('Errors:', parser.errors)
for s in prog.statements:
    print(s)
PY
```

To run repository tests (recommended):

```bash
# Option A: run tests with PYTHONPATH
PYTHONPATH=./src pytest

# Option B: install editable package then run tests
pip install -e .
pytest
```

## Next steps / Recommendations
1. Add unit tests that cover:
   - Arrow lambdas as standalone expressions and as call arguments
   - Parenthesized parameter lists for arrow lambdas
   - List/array parsing for nested lists
   - Method chains combining calls and property access
2. Run the full test suite in CI with `PYTHONPATH=./src` or after installing the package to ensure no regressions.
3. Consider consolidating lambda handling between the traditional and context parsers to reuse parsing logic and minimize duplication.
4. If you want, I can open a PR with these changes plus a small test file `tests/test_lambda_and_chains.zx` demonstrating the patterns above.

## Summary
These edits fix the immediate NameError for `ASTERISK` and substantially improve parsing for arrays, object literals, method chains, and both keyword and arrow-style lambdas. Focused validation shows these patterns now parse into expected AST nodes. A small test suite addition and full `pytest` run are recommended to lock these fixes in.
