Как устранить ошибку несоответствия типов?

Question

У меня есть следующий scala (2.13.1) код, который дает мне ошибку ниже:

import scala.{Option => _, Either => _, _}                                                                                
sealed trait Option[+A] {                                                                                                 
  def map[B](f: A => B): Option[B] = this match {                                                                         
    case None => None                                                                                                     
    case Some(_) => Some(f(_))
  }

  def flatMap[B](f: A => Option[B]): Option[B] = this match {
    case None => None
    case Some(_) => f(_)
  }

  def getOrElse[B >: A](default: => B): B = this match {
    case None => default
    case Some(x) => x
  }

  def orElse[B >: A](ob: => Option[B]): Option[B] = this match {                                                         
    case None => ob
    case _ => this
  }

  def filter(f: A => Boolean): Option[A] = this match {
    case Some(a) if f(a) => this
    case _ => None
  }

  def filter_1(f: A => Boolean): Option[A] =
    flatMap(a => if (f(a)) Some(a) else None)
}
case class Some[+A](get: A) extends Option[A]
case object None extends Option[Nothing]

Ошибка из REPL:

scala> :load Option.scala               
args: Array[String] = Array()               
Loading Option.scala...                                                    
import scala.{Option=>_, Either=>_, _}  

           case None => None
                ^
Option.scala:3: error: pattern type is incompatible with expected type;
        found   : None.type
        required: Option[A]

           case None => None
                        ^
Option.scala:3: error: type mismatch;
        found   : None.type
        required: Option[B]

           case Some(_) => Some(f(_))
                ^
Option.scala:4: error: constructor cannot be instantiated to expected type;
        found   : Some[A(in class Some)]
        required: Option[A(in trait Option)]

           case Some(_) => Some(f(_))
                               ^
Option.scala:4: error: type mismatch;
        found   : Some[A => B]
        required: Option[B]

           case None => None
                ^
Option.scala:8: error: pattern type is incompatible with expected type;
        found   : None.type
        required: Option[A]     

           case None => None
                        ^                                                  
Option.scala:8: error: type mismatch;   
        found   : None.type                                             
        required: Option[B]

           case Some(_) => f(_) 
                ^                    
Option.scala:9: error: constructor cannot be instantiated to expected type;
        found   : Some[A(in class Some)]                                    
        required: Option[A(in trait Option)]

           case Some(_) => f(_)
                            ^                                          
Option.scala:9: error: type mismatch;
        found   : A => Option[B]
        required: Option[B]

           case None => default
                ^
Option.scala:13: error: pattern type is incompatible with expected type;
        found   : None.type
        required: Option[A]

           case Some(x) => x
                ^
Option.scala:14: error: constructor cannot be instantiated to expected type;
        found   : Some[A(in class Some)]
        required: Option[A(in trait Option)]

           case None => ob
                ^
Option.scala:18: error: pattern type is incompatible with expected type;
        found   : None.type
        required: Option[A]

           case Some(a) if f(a) => this
                ^
Option.scala:23: error: constructor cannot be instantiated to expected type;
        found   : Some[A(in class Some)]
        required: Option[A(in trait Option)]

           case _ => None
                     ^
Option.scala:24: error: type mismatch;
        found   : None.type
        required: Option[A]

           flatMap(a => if (f(a)) Some(a) else None)
                                      ^
Option.scala:28: error: type mismatch;
        found   : Some[A]
        required: Option[A]

           flatMap(a => if (f(a)) Some(a) else None)
                                               ^
Option.scala:28: error: type mismatch;
        found   : None.type
        required: Option[A]

       case class Some[+A](get: A) extends Option[A]
                                           ^
Option.scala:1: error: illegal inheritance from sealed class Option

       case object None extends Option[Nothing]
                                ^
Option.scala:1: error: illegal inheritance from sealed class Option

что я делаю неправильно здесь в Scala?

Answer 1

Вы должны исправить map и flatMap. Замените case Some(_) => ... на case Some(x) => ... и затем используйте это x, как вы делаете в getOrElse.

В противном случае Some(f(_)), f(_) - это лямбды, а не Option s, как обещано в методе return наберите

Do

import scala.{Option => _, Some => _, None => _, Either => _, _} 

в REPL.

Вместо :load Option.scala используйте :paste Option.scala, тогда все будет интерпретировано вместе.