Github user tillrohrmann commented on a diff in the pull request:
https://github.com/apache/flink/pull/1605#discussion_r52445522
--- Diff:
flink-core/src/main/java/org/apache/flink/api/common/state/FoldingStateDescriptor.java
---
@@ -0,0 +1,108 @@
+/*
+ * 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.common.state;
+
+import org.apache.flink.api.common.functions.FoldFunction;
+import org.apache.flink.api.common.functions.RichFunction;
+import org.apache.flink.api.common.typeinfo.TypeInformation;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+
+import static java.util.Objects.requireNonNull;
+
+/**
+ * {@link StateDescriptor} for {@link ReducingState}. This can be used to
create partitioned
+ * folding state.
+ *
+ * @param <T> Type of the values folded int othe state
+ * @param <ACC> Type of the value in the state
+ */
+public class FoldingStateDescriptor<T, ACC> extends
StateDescriptor<FoldingState<T, ACC>, ACC> {
+ private static final long serialVersionUID = 1L;
+
+
+ private final FoldFunction<T, ACC> foldFunction;
+
+ /**
+ * Creates a new {@code FoldingStateDescriptor} with the given name,
type, and initial value.
+ *
+ * <p>If this constructor fails (because it is not possible to describe
the type via a class),
+ * consider using the {@link #FoldingStateDescriptor(String, ACC,
FoldFunction, TypeInformation)} constructor.
+ *
+ * @param name The (unique) name for the state.
+ * @param initialValue The initial value of the fold.
+ * @param foldFunction The {@code FoldFunction} used to aggregate the
state.
+ * @param typeClass The type of the values in the state.
+ */
+ public FoldingStateDescriptor(String name, ACC initialValue,
FoldFunction<T, ACC> foldFunction, Class<ACC> typeClass) {
+ super(name, typeClass, initialValue);
+ this.foldFunction = requireNonNull(foldFunction);
+
+ if (foldFunction instanceof RichFunction) {
+ throw new UnsupportedOperationException("FoldFunction
of FoldingState can not be a RichFunction.");
+ }
+ }
+
+ /**
+ * Creates a new {@code ReducingStateDescriptor} with the given name
and default value.
--- End diff --
FoldingStateDescriptor
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