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    
015    package 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     */
023    public 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    }