EarlyReturnHack.java

/**
 * Example:
 *
 * ```java
 * withEarlyReturn(Result.class, (c) -> {
 *   var value = switch (someAction()) {
 *     case Success(var value) -> {
 *       return value;
 *     }
 *     case Failure(var error) -> {
 *       throw c.earlyReturn(error);
 *     }
 *   }
 *   ...
 * })
 * ```
 *
 * ```java
 * var resetRequest =
 *     userRepository
 *         .findByEmail(email)
 *         .orElseThrow(() -> c.earlyReturn(new UserNotFound()));
 * ```
 */
public class EarlyReturn {
  @SuppressWarnings("unchecked")
  public static <T> T withEarlyReturn(Class<T> clazz, EarlyReturnBlock<T> block) {
    try {
      return block.run(new EarlyReturnController());
    } catch (EarlyReturnException e) {
      if (clazz.isInstance(e.value)) {
        return (T) e.value;
      } else {
        // for nested withEarlyReturn
        throw e;
      }
    }
  }

  @FunctionalInterface
  public interface EarlyReturnBlock<T> {
    T run(EarlyReturnController k);
  }

  public static class EarlyReturnException extends RuntimeException {
    Object value;

    private EarlyReturnException(Object value) {
      this.value = value;
    }
  }

  public static class EarlyReturnController {
    public <T> EarlyReturnException earlyReturn(T value) {
      throw new EarlyReturnException(value);
    }
  }
}