content: /*
   *  Licensed to the Apache Software Foundation (ASF) under one
   *  or more contributor license agreements.  See the NOTICE file
   *  distributed with this work for additional information
   *  regarding copyright ownership.  The ASF licenses this file
   *  to you 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 org.apache.causeway.commons.functional;
  
  import java.io.Serializable;
  import java.util.Optional;
  import java.util.function.BiFunction;
  import java.util.function.Consumer;
  import java.util.function.Function;
  
  import org.jspecify.annotations.NonNull;
  
  /**
   * The {@link Either} type represents a value of one of two possible types (a disjoint union),
   * referred to by {@code left} or {@code right}.
   * <p>
   * Factory methods {@link Either#left(Object)} and {@link Either#right(Object)}
   * correspond to the two possible values.
   *
   * @since 2.0 {@index}
   */
  public sealed interface Either<L, R>
  permits Either.Left, Either.Right {
  
      // -- FACTORIES
  
      public static <L, R> Either<L, R> left(final @NonNull L left) {
          return new Left<>(left);
      }
  
      public static <L, R> Either<L, R> right(final @NonNull R right) {
          return new Right<>(right);
      }
  
      // -- ACCESSORS
  
      Optional<L> left();
      Optional<R> right();
  
      L leftIfAny();
      R rightIfAny();
  
      // -- PREDICATES
  
      boolean isLeft();
      boolean isRight();
  
      // -- MAPPING
  
      <T> Either<T, R> mapLeft(final @NonNull Function<L, T> leftMapper);
      <T> Either<L, T> mapRight(final @NonNull Function<R, T> rightMapper);
  
      <X, Y> Either<X, Y> map(
              final @NonNull Function<L, X> leftMapper,
              final @NonNull Function<R, Y> rightMapper);
  
      // -- FOLDING
  
      <T> T fold(@NonNull BiFunction<L, R, T> biMapper);
  
      <T> T fold(
              @NonNull Function<L, T> leftMapper,
              @NonNull Function<R, T> rightMapper);
  
      // -- TERMINALS
  
      void accept(
              @NonNull Consumer<L> leftConsumer,
              @NonNull Consumer<R> rightConsumer);
  
      // -- LEFT
  
      static record Left<L, R>(L _left) implements Either<L, R>, Serializable {
  
          @Override public Optional<L> left() { return Optional.ofNullable(_left); }
          @Override public Optional<R> right() { return Optional.empty(); }
  
          @Override public L leftIfAny() { return _left; }
          @Override public R rightIfAny() { return null; }
  
          @Override public final boolean isLeft() { return true; }
          @Override public final boolean isRight() { return false; }
  
          @Override public final <T> Either<T, R> mapLeft(final @NonNull Function<L, T> leftMapper){
              return Either.left(leftMapper.apply(_left)); }
          @Override public final <T> Either<L, T> mapRight(final @NonNull Function<R, T> rightMapper){
              return Either.left(_left); }
  
          @Override
          public final <X, Y> Either<X, Y> map(
                  final @NonNull Function<L, X> leftMapper,
                  final @NonNull Function<R, Y> rightMapper){
              return Either.left(leftMapper.apply(_left));
          }
  
          @Override
          public final <T> T fold(final @NonNull BiFunction<L, R, T> biMapper){
              return biMapper.apply(_left, null);
          }
  
          @Override
          public final <T> T fold(
                  final @NonNull Function<L, T> leftMapper,
                  final @NonNull Function<R, T> rightMapper){
              return leftMapper.apply(_left);
          }
  
          @Override
          public final void accept(
                  final @NonNull Consumer<L> leftConsumer,
                  final @NonNull Consumer<R> rightConsumer) {
              leftConsumer.accept(_left);
          }
  
      }
  
      // -- RIGHT
  
      static record Right<L, R>(R _right) implements Either<L, R>, Serializable {
  
          @Override public Optional<L> left() { return Optional.empty(); }
          @Override public Optional<R> right() { return Optional.ofNullable(_right); }
  
          @Override public L leftIfAny() { return null; }
          @Override public R rightIfAny() { return _right; }
  
          @Override public final boolean isLeft() { return false; }
          @Override public final boolean isRight() { return true; }
  
          @Override public final <T> Either<T, R> mapLeft(final @NonNull Function<L, T> leftMapper){
              return Either.right(_right); }
          @Override public final <T> Either<L, T> mapRight(final @NonNull Function<R, T> rightMapper){
              return Either.right(rightMapper.apply(_right)); }
  
          @Override
          public final <X, Y> Either<X, Y> map(
                  final @NonNull Function<L, X> leftMapper,
                  final @NonNull Function<R, Y> rightMapper){
              return Either.right(rightMapper.apply(_right));
          }
  
          @Override
          public final <T> T fold(final @NonNull BiFunction<L, R, T> biMapper){
              return biMapper.apply(null, _right);
          }
  
          @Override
          public final <T> T fold(
                  final @NonNull Function<L, T> leftMapper,
                  final @NonNull Function<R, T> rightMapper){
              return rightMapper.apply(_right);
          }
  
          @Override
          public final void accept(
                  final @NonNull Consumer<L> leftConsumer,
                  final @NonNull Consumer<R> rightConsumer) {
              rightConsumer.accept(_right);
          }
  
      }
  
  //    // -- TYPE COMPOSITION
  //
  //    @FunctionalInterface
  //    public static interface HasEither<L, R> extends Either<L, R> {
  //
  //        Either<L, R> getEither();
  //
  //        @Override default Optional<L> left() { return getEither().left(); }
  //        @Override default Optional<R> right() { return getEither().right(); }
  //
  //        @Override default L leftIfAny() { return getEither().leftIfAny(); }
  //        @Override default R rightIfAny() { return getEither().rightIfAny(); }
  //
  //        @Override default boolean isLeft() { return getEither().isLeft(); }
  //        @Override default boolean isRight() { return getEither().isRight(); }
  //
  //        @Override default <T> Either<T, R> mapLeft(final @NonNull Function<L, T> leftMapper){
  //            return getEither().mapLeft(leftMapper); }
  //        @Override default <T> Either<L, T> mapRight(final @NonNull Function<R, T> rightMapper){
  //            return getEither().mapRight(rightMapper); }
  //
  //        @Override default <X, Y> Either<X, Y> map(
  //                final @NonNull Function<L, X> leftMapper,
  //                final @NonNull Function<R, Y> rightMapper){
  //            return getEither().map(leftMapper, rightMapper);
  //        }
  //
  //        @Override default <T> T fold(final @NonNull BiFunction<L, R, T> biMapper){
  //            return getEither().fold(biMapper);
  //        }
  //
  //        @Override default <T> T fold(
  //                final @NonNull Function<L, T> leftMapper,
  //                final @NonNull Function<R, T> rightMapper){
  //            return getEither().fold(leftMapper, rightMapper);
  //        }
  //
  //        @Override default void accept(
  //                final @NonNull Consumer<L> leftConsumer,
  //                final @NonNull Consumer<R> rightConsumer) {
  //            getEither().accept(leftConsumer, rightConsumer);
  //        }
  //
  //    }
  
  }
  