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A combination of the Try monad and the Either monad, to help tame complex business flows.
There is a newer version: 10.3.0
/*
 * Copyright 2016 Liam Williams .
 *
 * This file is part of business-flows.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.github.theangrydev.businessflows;

/**
 * A {@link SadPath} is a {@link BusinessFlow} that is biased towards the result being {@link Sad}.
 */
public interface SadPath extends BusinessFlow, WithOptional {

    /**
     * Provides a {@link SadPath} view over a known {@link Sad} object.
     *
     * @param sad     The sad object to initiate the flow with
     * @param  The type of happy object the resulting {@link SadPath} may represent
     * @param    The type of sad object the resulting {@link SadPath} may represent
     * @return A {@link SadPath} that is sad on the inside
     */
    static  SadPath sadPath(Sad sad) {
        return new SadCaseSadPath<>(sad);
    }

    /**
     * Provides a {@link SadPath} view over a known {@link Happy} object.
     *
     * @param happy   The happy object to initiate the flow with
     * @param  The type of happy object the resulting {@link SadPath} may represent
     * @param    The type of sad object the resulting {@link SadPath} may represent
     * @return A {@link SadPath} that is happy on the inside
     */
    static  SadPath happyPath(Happy happy) {
        return new HappyCaseSadPath<>(happy);
    }

    /**
     * Provides a {@link SadPath} view over a known {@link Exception} object.
     *
     * @param technicalFailure The technical failure object to initiate the flow with
     * @param           The type of happy object the resulting {@link SadPath} may represent
     * @param             The type of sad object the resulting {@link SadPath} may represent
     * @return A {@link SadPath} that is a technical failure on the inside
     */
    static  SadPath technicalFailure(Exception technicalFailure) {
        return new TechnicalFailureCaseSadPath<>(technicalFailure);
    }

    /**
     * If the underlying business case is sad, then apply the given action, otherwise do nothing to the underlying case.
     *
     * @param action   The action to apply to an existing sad case
     * @param  The type of sad object that will be present after the action is applied to an existing sad object
     * @return The result of applying the action to the existing sad path, if applicable
     */
     SadPath then(Mapping> action);

    /**
     * If the underlying business case is sad, then apply the given mapping, otherwise do nothing to the underlying case.
     *
     * @param mapping  The action to apply to an existing sad case
     * @param  The type of sad object that will be present after the mapping is applied to an existing sad object
     * @return The result of applying the mapping to the existing sad path, if applicable
     */
     SadPath map(Mapping mapping);

    /**
     * If the underlying business case is sad, recover to a happy path using the given recovery mapping.
     *
     * @param recovery The recovery to apply to an existing sad case
     * @return The result of applying the recovery to the existing sad path, if applicable
     */
    HappyPath recover(Mapping recovery);

    /**
     * If the underlying business case is sad, recover to a happy path using the {@link Attempt}.
     *
     * @param recovery The recovery to apply to an existing sad case
     * @return The result of applying {@link Attempt}, if applicable
     */
    HappyPath recover(Attempt recovery);

    /**
     * Take a look at the sad case (if there really is one).
     *
     * @param peek What to do if the underlying business case is sad
     * @return The same {@link SadPath}
     */
    SadPath peek(Peek peek);

    /**
     * {@inheritDoc}
     */
    @Override
    default SadPath ifSad() {
        return this;
    }
}