Github user StephanEwen commented on a diff in the pull request:
https://github.com/apache/flink/pull/1371#discussion_r45248610
--- Diff:
flink-java/src/main/java/org/apache/flink/api/java/typeutils/Either.java ---
@@ -0,0 +1,147 @@
+/*
+ * 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.java.typeutils;
+
+/**
+ * This type represents a value of one two possible types, Left or Right
+ * (a disjoint union), inspired by Scala's Either type.
+ *
+ * @param <L> the type of Left
+ * @param <R> the type of Right
+ */
+public abstract class Either<L, R> {
+
+ /**
+ * Create a Left value of Either
+ */
+ public static <L, R> Either<L, R> left(L value) {
--- End diff --
I think it is better to return the specific type here `public static <L, R>
Left<L, R> left(L value)`. More specific types allow people to assign them to
more specific variables.
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