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This library focuses on providing different flavors of eventsourced aggregrates

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/*
 * Copyright 2021-2024 the original author or authors.
 *
 * 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
 *
 *      https://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 dk.cloudcreate.essentials.components.eventsourced.aggregates.decider;

/**
 * {@link Decider} related interface that is responsible for handling COMMAND(s),whose side effect is either an ERROR or a List of EVENT's (can be an empty list)
 *
 * @param  The type of Commands that the {@link #handle(Object, Object)} can process
 * @param    The type of Events that can be returned by {@link #handle(Object, Object)} and applied in the {@link StateEvolver#applyEvent(Object, Object)}
 * @param    The type of Error that can be returned by the {@link #handle(Object, Object)} method
 * @param    The type of Aggregate State that this {@link Decider} works with in {@link #handle(Object, Object)}, {@link InitialStateProvider#initialState()}, {@link StateEvolver#applyEvent(Object, Object)}
 *                  and {@link IsStateFinalResolver#isFinal(Object)}
 */
@FunctionalInterface
public interface Handler {
    /**
     * The execute method is responsible for handling a COMMAND, which can either result
     * in an ERROR or a list of EVENT's (can be an empty list).

     * Note: This method is called decide in the decider pattern

     * Idempotent handling of a COMMAND will result in an empty list of EVENT's
     *
     * @param cmd   the command to handle
     * @param state the state of the aggregate
     * @return either an ERROR or a list of EVENT's.

     * Idempotent handling of a COMMAND will result in an empty list of EVENT's
     */
    HandlerResult handle(COMMAND cmd, STATE state);
}