    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}