gregfelice commented on code in PR #2360:
URL: https://github.com/apache/age/pull/2360#discussion_r3089620914
##########
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:
Addressed — `%expect` / `%expect-rr` have been removed. The grammar now uses
`-Wno-conflicts-sr -Wno-conflicts-rr` via `BISONFLAGS` in the Makefile (see
Makefile:166 and the comment block at cypher_gram.y:74). A block comment
documents what the 7 s/r and 3 r/r conflicts are and why GLR + `%dprec` handles
them correctly. This keeps builds clean across Bison versions without pinning.
##########
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:
Good catch — this is intentional, and the PR description has been updated to
reflect it. Adding `anonymous_path` to `expr_atom` legalizes pattern
expressions in every expression context (RETURN, SET, CASE, WITH, boolean
combinators), which matches openCypher semantics and what Neo4j users expect.
Rather than restrict to a WHERE-only nonterminal, the PR now explicitly
embraces the broader surface area and covers it with regression tests — see
regress/sql/pattern_expression.sql lines 156-183 (RETURN projection, mixed
projections, CASE WHEN) plus the SET/WITH/AND/OR examples in the PR
description. See also the "Note on scope" block at the top of the description.
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