imbajin commented on code in PR #3259:
URL: https://github.com/apache/hugegraph/pull/3259#discussion_r4276498779


##########
hugegraph-server/hugegraph-api/src/main/java/org/apache/hugegraph/api/cypher/CypherErrorMapper.java:
##########
@@ -0,0 +1,85 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hugegraph.api.cypher;
+
+import java.util.Locale;
+import java.util.regex.Pattern;
+
+/**
+ * Classify cypher execution failures into stable error codes and attach
+ * actionable hints for the most common, cryptic translator messages.
+ */
+public final class CypherErrorMapper {
+
+    public static final String SYNTAX_ERROR = "HugeGraph.Cypher.SyntaxError";
+    public static final String UNSUPPORTED_FEATURE =
+            "HugeGraph.Cypher.UnsupportedFeature";
+    public static final String EXECUTION_ERROR = 
"HugeGraph.Cypher.ExecutionError";
+
+    private static final Pattern UNDEFINED_VARIABLE =
+            Pattern.compile("variable `[^`]+` not defined");
+
+    private static final Pattern PARSER_POSITION =
+            Pattern.compile("\\(line \\d+, column \\d+ \\(offset: 
\\d+\\)\\)$");
+
+    private CypherErrorMapper() {
+    }
+
+    public static CypherModel.CypherError map(Throwable e) {
+        String message = e.getMessage() != null ? e.getMessage() : 
e.toString();
+        // strip noisy exception class prefixes, e.g. 
"...driver.exception.ResponseException: "
+        message = message.replaceFirst(
+                "^([a-zA-Z0-9_]+\\.)+[A-Za-z0-9_]+(Exception|Error):\\s*", "");
+        String lower = message.toLowerCase(Locale.ROOT);
+
+        if (lower.contains("undefined vertex label")
+                || lower.contains("undefined edge label")
+                || lower.contains("undefined property key")
+                || lower.contains("undefined index label")) {
+            return new CypherModel.CypherError(EXECUTION_ERROR, message,
+                    "Create the schema element via the schema API before " +
+                    "running the query");
+        }
+        if (UNDEFINED_VARIABLE.matcher(lower).find()) {
+            return new CypherModel.CypherError(SYNTAX_ERROR, message,
+                    "Declare the variable in a MATCH, UNWIND or WITH clause " +
+                    "before referencing it");
+        }
+        if (lower.contains("invalid input") || lower.contains("unknown 
function") ||
+            PARSER_POSITION.matcher(lower).find()) {
+            return new CypherModel.CypherError(SYNTAX_ERROR, message,
+                    "Check the query syntax and function names supported by " +
+                    "the built-in Cypher translator");
+        }
+        if (lower.contains("expected exactly one statement")) {
+            return new CypherModel.CypherError(SYNTAX_ERROR, message,
+                    "Submit exactly one Cypher statement per request");
+        }
+        if (lower.startsWith("unsupported clause:") ||

Review Comment:
   The user has now explicitly accepted the documented limitation for this PR, 
with complete translation-stage classification deferred to a separate 
follow-up. [The current 
documentation](https://github.com/apache/hugegraph/blob/3d448393fe875d9b1cc3ed9524189d8e5ecefee9/docs/cypher-api.md#L94-L102)
 states that unrecognized translation failures, including reduce and semantic 
rejections without a recognized position suffix, can return ExecutionError with 
an empty hint; that code does not prove traversal execution began, and the 
original message is preserved.
   
   This is an accepted scope boundary, not an implementation fix. The mapper 
and processor remain unchanged, and the requested additional UnsupportedFeature 
coverage has not been implemented. The full-classification work is recorded 
separately, and I am leaving this Important thread open.



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