Scala 子类模式匹配

Question

我正在尝试运行本书 "Functional Programming in Scala" 中的代码，它似乎是为旧版本的 scala（从 here 下载）编写的。 tmp.scala:

sealed trait Option[+A]
case class Some[+A](get: A) extends Option[A] 
case object None extends Option[Nothing]

trait Option[+A] {
    /* returns None if None, or function applied to the some object */
    def map[B](f: A => B): Option[B] = this match {
      case None => None
      case Some(a) => Some(f(a))
    }
}

抛出的错误是：

$ scala
Welcome to Scala 2.12.0-20161021-070700-8684ae8 (OpenJDK 64-Bit Server VM, Java 1.8.0_112).
scala> :load tmp.scala

tmp.scala:17: error: pattern type is incompatible with expected type;
 found   : None.type
 required: Option[A]
             case None => None
                  ^
tmp.scala:17: error: type mismatch;
 found   : None.type
 required: Option[B]
             case None => None
                          ^
tmp.scala:18: error: constructor cannot be instantiated to expected type;
 found   : Some[A(in class Some)]
 required: Option[A(in trait Option)]
             case Some(a) => Some(f(a))
                  ^
tmp.scala:18: error: type mismatch;
 found   : Some[B]
 required: Option[B]
             case Some(a) => Some(f(a))
                                 ^

我尝试了各种codefu，但无济于事，似乎由于过时的语法而无法正确检测子类？

Answer 1

使用粘贴内容的 :paste file.scala，而不是解释每一行的 :load file.scala。

Scala 子类模式匹配

Scala subclass pattern match

scala

functional-programming

subclassing