Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/2653#discussion_r88335539
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/api/table/functions/TableFunction.scala
---
@@ -0,0 +1,119 @@
+/*
+ * 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.flink.api.table.functions
+
+import org.apache.calcite.sql.SqlFunction
+import org.apache.flink.annotation.Internal
+import org.apache.flink.api.common.functions.InvalidTypesException
+import org.apache.flink.api.common.typeinfo.TypeInformation
+import org.apache.flink.api.java.typeutils.TypeExtractor
+import org.apache.flink.api.table.{ValidationException, FlinkTypeFactory}
+
+import scala.collection.mutable.ListBuffer
+
+/**
+ * Base class for a user-defined table function (UDTF). A user-defined
table functions works on
+ * one row as input and returns multiple rows as output.
+ *
+ * The behavior of a [[TableFunction]] can be defined by implementing a
custom evaluation
+ * method. An evaluation method must be declared publicly and named
"eval". Evaluation methods
+ * can also be overloaded by implementing multiple methods named "eval".
--- End diff --
Maybe add a short code example for `eval()` here?
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