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
017import 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 */
040public 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}