code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * 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.reactivex.rxjava3.core;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  
  /**
   * Interface to compose {@link Maybe}s.
   *
   * @param <Upstream> the upstream value type
   * @param <Downstream> the downstream value type
   */
  @FunctionalInterface
  public interface MaybeTransformer<@NonNull Upstream, @NonNull Downstream> {
      /**
       * Applies a function to the upstream {@link Maybe} and returns a {@link MaybeSource} with
       * optionally different element type.
       * @param upstream the upstream {@code Maybe} instance
       * @return the transformed {@code MaybeSource} instance
       */
      @NonNull
      MaybeSource<Downstream> apply(@NonNull Maybe<Upstream> upstream);
  }
  