gregfelice opened a new pull request, #2360:
URL: https://github.com/apache/age/pull/2360
## Summary
Enables bare graph patterns as boolean expressions in WHERE clauses (issue
#1577):
```cypher
-- Now valid (equivalent to EXISTS):
MATCH (a:Person), (b:Person)
WHERE (a)-[:KNOWS]->(b)
RETURN a.name, b.name
-- Also works with NOT, AND, OR:
WHERE NOT (a)-[:OWNS]->(:Asset)
WHERE (a)-[:KNOWS]->(b) AND a.name = 'Alice'
WHERE (a)-[:KNOWS]->(b) OR (a)-[:WORKS_WITH]->(b)
```
This is standard openCypher syntax, used extensively in Neo4j, and was the
most frequently cited migration blocker (open since 2024, no prior PR).
## Approach: GLR Parser
The fundamental challenge is that `(a)` is valid as both a parenthesized
expression and a graph node pattern. LALR(1) cannot distinguish them with one
token of lookahead.
**Solution:** Switch from LALR(1) to Bison's GLR mode. When the parser
encounters the ambiguity, it forks the parse stack and tries both
interpretations. The failing path is discarded. `%dprec` annotations resolve
cases where both paths succeed (bare `(a)` prefers the expression
interpretation).
**Changes:**
- `%glr-parser` directive + `YYRHSLOC` fix for GLR location compatibility
- `anonymous_path` added as `expr_atom` alternative with `%dprec 1`
- `'(' expr ')'` annotated with `%dprec 2` (higher priority)
- `expr_var` / `var_name_opt` rules annotated with `%dprec` to resolve
variable reduce/reduce conflicts
- Extracted `make_exists_pattern_sublink()` helper (shared by
`EXISTS(pattern)` and bare pattern rules)
**Conflict budget:** 7 shift/reduce (path extension vs arithmetic), 3
reduce/reduce (expr_var vs var_name_opt). All expected and correctly handled by
GLR forking + `%dprec`.
**Performance:** GLR adds ~20% to generated parser size. For non-ambiguous
queries (the common case), overhead is negligible — conflict states are only
reached when patterns appear in expression context. No fork occurs when
`%dprec` priorities differ.
## Regression Tests
New `pattern_expression` test (15 queries):
- Basic `WHERE (a)-[:REL]->(b)` patterns
- `NOT` pattern negation
- Labeled first node `(a:Person)-[:REL]->(b)`
- `AND` / `OR` boolean combinations
- Left-directed patterns `(a)<-[:REL]-(b)`
- Anonymous node patterns `(a)-[:REL]->()`
- Multi-hop patterns `(a)-[:R1]->()-[:R2]->(c)`
- `EXISTS()` backward compatibility verification
- Equivalence check: bare pattern produces same results as `EXISTS(pattern)`
- Non-pattern expression regression (`RETURN (1 + 2)`, `RETURN (n.name)`)
All 32 regression tests pass (31 existing + 1 new).
## Test plan
- [x] All 32 existing + new regression tests pass
- [x] Verified `WHERE (a)-[:KNOWS]->(b)` returns correct results
- [x] Verified `WHERE NOT (a)-[:KNOWS]->(:Person)` returns correct results
- [x] Verified backward compatibility with `EXISTS(pattern)` syntax
- [x] Verified non-pattern expressions `(1 + 2)` still work
- [ ] Reviewer: verify GLR conflict counts match expectations
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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