001// Copyright 2010 The Apache Software Foundation
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
007// http://www.apache.org/licenses/LICENSE-2.0
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
015package org.apache.tapestry5.func;
018 * A lazy function is used to populate a {@link Flow} incrementally. Lazy functions allow calculations to be
019 * deferred. They also support infinitely large Flows, where successive values are computed just as needed.
020 * 
021 * @since 5.2.0
022 */
023public interface LazyFunction<T>
025    /**
026     * Calculates the next value for the function. The return value is tricky: it combines the next value in the
027     * {@link Flow} with a function to compute the value after that as a {@link LazyContinuation}. Alternately, a
028     * LazyFunction can return null to indicate that it has returned all the values it can.
029     * 
030     * @return continuation containing next value and next function, or null when no more values can be produced
031     */
032    LazyContinuation<T> next();