Copilot commented on code in PR #2360:
URL: https://github.com/apache/age/pull/2360#discussion_r2996979904
##########
src/backend/parser/cypher_gram.y:
##########
@@ -49,6 +49,30 @@
%locations
%name-prefix="cypher_yy"
%pure-parser
+/*
+ * GLR mode handles the ambiguity between parenthesized expressions and
+ * graph patterns. For example, WHERE (a)-[:KNOWS]->(b) starts with (a)
+ * which is valid as both an expression and a path_node. The parser forks
+ * at the conflict point and discards the failing path. %dprec annotations
+ * on expr_var/var_name_opt and '(' expr ')'/anonymous_path resolve cases
+ * where both paths succeed (bare (a) prefers the expression interpretation).
+ */
+%glr-parser
+/*
+ * Conflict budget for the GLR parser. Update these counts if grammar
+ * rules change.
+ *
+ * %expect 7 (shift/reduce) -- All arise from the ambiguity between
+ * path extension ('-' '[' ... ']' '-' '>') and arithmetic operators
+ * on '-' and '<'. GLR forks at these points and discards the
+ * failing alternative.
+ *
+ * %expect-rr 3 (reduce/reduce) -- From the overlap between expr_var
+ * and var_name_opt on ')' / '}' / '='. Resolved by %dprec
+ * annotations that prefer the expression interpretation.
+ */
+%expect 7
+%expect-rr 3
Review Comment:
The new `%expect 7` / `%expect-rr 3` conflict budget is sensitive to the
exact Bison conflict counts. Since CI installs `bison` from `apt` without
pinning a version (see `.github/workflows/installcheck.yaml`), a future Bison
upgrade (or differences across distros) could change the reported conflict
counts and reintroduce parser-generation warnings/failures. Consider pinning
the Bison version in CI and/or documenting the minimum supported Bison version
for GLR + `%expect-rr`, so builds stay reproducible across environments.
##########
src/backend/parser/cypher_gram.y:
##########
@@ -1953,6 +1966,17 @@ expr_atom:
}
$$ = n;
}
+ | anonymous_path %dprec 1
+ {
+ /*
+ * Bare pattern in expression context is semantically
+ * equivalent to EXISTS(pattern). Example:
+ * WHERE (a)-[:KNOWS]->(b)
+ * becomes
+ * WHERE EXISTS((a)-[:KNOWS]->(b))
+ */
+ $$ = make_exists_pattern_sublink($1, @1);
+ }
Review Comment:
This adds `anonymous_path` as an `expr_atom`, which makes bare pattern
expressions legal anywhere `expr` is accepted (e.g., `RETURN (a)-[:R]->(b)` /
`SET n.flag = (a)-[:R]->(b)`), not just in `WHERE`. If the intent is strictly
“pattern expressions in WHERE”, consider restricting this production to a
boolean/WHERE-specific expression nonterminal, or update the PR
description/tests to reflect the broader surface area.
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