comment_form_final_reducer.kt

fun reduce(event: TextChangedEvent): Reducer<CommentFormData> {
  // implementation
}

fun reduce(event: Lce<Comment>): Reducer<CommentFormData> {
  // implementation  
}

 fun formState(
    textChangedEvents: Observable<TextChangedEvent>,
    submitEvents: Observable<SubmitCommentEvent>
  ) : Observable<CommentFormData> {
   val textChangedReducer: Observable<Reducer<CommentFormData>> = textChangedEvents.map { textEvent -> reduce(textEvent) }
   
    // Explicitly declaring the type, for clarity sake
    val requestStateReducer: Observable<Reducer<CommentFormData>> = submitEvents
      .switchMap { event -> commentService.submit(event.comment) }
      .map { lceEvent -> reduce(lceEvent) }
    
    // We now have a clear common type (which happens to be a lambda)
    val reducers: Observable<Reducer<CommentFormData>> = Observable.merge(textChangedReducer, requestStateReducer)
    
    // And to finish this
    return reducers.scan(initialState) { reducer, state -> reducer.invoke(state) } 
  }