export interface Observer<T, E = any> {
  /**
   * A callback function that gets called by the producer during the subscription when
   * the producer "has" the `value`. It won't be called if `error` or `complete` callback
   * functions have been called, nor after the consumer has unsubscribed.
   *
   * For more info, please refer to {@link guide/glossary-and-semantics#next this guide}.
   */
  next: (value: T) => void;
  /**
   * A callback function that gets called by the producer if and when it encountered a
   * problem of any kind. The errored value will be provided through the `err` parameter.
   * This callback can't be called more than one time, it can't be called if the
   * `complete` callback function have been called previously, nor it can't be called if
   * the consumer has unsubscribed.
   *
   * For more info, please refer to {@link guide/glossary-and-semantics#error this guide}.
   */
  error: (err: E) => void;
  /**
   * A callback function that gets called by the producer if and when it has no more
   * values to provide (by calling `next` callback function). This means that no error
   * has happened. This callback can't be called more than one time, it can't be called
   * if the `error` callback function have been called previously, nor it can't be called
   * if the consumer has unsubscribed.
   *
   * For more info, please refer to {@link guide/glossary-and-semantics#complete this guide}.
   */
  complete: () => void;
}