    001// Copyright 2010 The Apache Software Foundation
    002//
    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
    006//
    007// http://www.apache.org/licenses/LICENSE-2.0
    008//
    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.
    014
    015package org.apache.tapestry5.func;
    016
    017/**
    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>
    024{
    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();
    033}