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    import java.util.List;
018    
019    /**
020     * A flow is a a functional interface for working with an ordered collection of elements.
021     * A given Flow contains only elements of a particular type. Standard operations allow for
022     * filtering the flow, or appending elements to the Flow. Since flows are immutable, all operations
023     * on flows return new immutable flows. Flows are thread safe (to the extent that the {@link Mapper} , {@link Predicate}
024     * , {@link Worker} and {@link Reducer} objects applied to the flow are).
025     * Flows are <em>lazy</em>: filtering, mapping, and concatenating flows will do so with no, or a
026     * minimum, of evaluation. However, converting a Flow into a {@link List} (or other collection) will
027     * force a realization of the entire flow.
028     * <p>
029     * In some cases, a flow may be an infinite, lazily evaluated sequence. Operations that iterate over all elements (such
030     * as {@link #count()} or {@link #reduce(Reducer, Object)}) may become infinite loops.
031     * <p>
032     * Using flows allows for a very fluid interface.
033     * <p>
034     * Flows are initially created using {@link F#flow(java.util.Collection)}, {@link F#flow(Object...)} or
035     * {@link F#flow(Iterable)}.
036     * 
037     * @since 5.2.0
038     * @see F#lazy(LazyFunction)
039     */
040    public interface Flow<T> extends FlowOperations<T, Flow<T>>
041    {
042        /** Maps a Flow into a new Flow with different type values. Mapping is a lazy operation. */
043        <X> Flow<X> map(Mapper<T, X> mapper);
044    
045        /**
046         * Combines two Flows using a two-parameter Mapper. Each element of
047         * this Flow, and the corresponding element of the other flow are passed through the Mapper
048         * to provide the elements of the output Flow. The length of the result Flow is
049         * the smaller of the lengths of the two input Flows. Mapping is a lazy operation.
050         */
051        <X, Y> Flow<Y> map(Mapper2<T, X, Y> mapper, Flow<? extends X> flow);
052    
053        /**
054         * Given a {@link Mapper} that maps a T to a Flow<X>, this method will lazily concatenate
055         * all the output flows into a single Flow<X>.
056         */
057        <X> Flow<X> mapcat(Mapper<T, Flow<X>> mapper);
058    
059        /**
060         * Converts the Flow into an array of values (due to type erasure, you have to remind the Flow
061         * about the type).
062         */
063        T[] toArray(Class<T> type);
064    
065        /**
066         * Returns a new Flow with the other Flow's elements appended to this Flow's. This is a lazy
067         * operation.
068         */
069        Flow<T> concat(Flow<? extends T> other);
070    
071        /**
072         * Appends any number of type compatible values to the end of this Flow. This is a lazy
073         * operation.
074         */
075        <V extends T> Flow<T> append(V... values);
076    
077        /**
078         * Sorts this Flow, forming a new Flow. This is a non-lazy operation; it will fully realize the
079         * values of the Flow.
080         * 
081         * @throws ClassCastException
082         *             if type <T> does not extend {@link Comparable}
083         */
084        Flow<T> sort();
085    
086        /**
087         * Zips this Flow together with another flow to form a Flow of {@link Tuple}s. The resulting
088         * flow is the length of the shorter of the two input flows. Zipping flows together is a lazy
089         * operation.
090         * <p>
091         * The elements of this flow become the {@linkplain Tuple#first} value in each Tuple, the elements of the other flow
092         * become the {@linkplain Tuple#second} value in each Tuple.
093         * 
094         * @param <X>
095         *            type of element stored in the other flow
096         * @param otherFlow
097         *            contains elements to match with elements in this flow
098         * @return flow of tuples combining values from this flow with values form the other flow
099         * @since 5.3
100         */
101        <X> ZippedFlow<T, X> zipWith(Flow<X> otherFlow);
102    
103        /**
104         * "Stripes" together a group of flows. The output flow contains the first value from this flow, then the first
105         * value from each of the other flows, in turn, then the second value from this flow, etc. The resulting flow ends
106         * when this or any of the other flows runs out of values.
107         * 
108         * @return combined flow
109         */
110        Flow<T> interleave(Flow<T>... otherFlows);
111    }