我正在尝试运行《 Scala中的函数编程》一书中的代码,该书似乎是为旧版本的Scala(从此处下载)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))
}
}
引发的错误是:
$ 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))
^
我对此进行了各种代码转换,但均无济于事,由于语法过时,似乎无法正确检测到子类?
使用:paste file.scala
来粘贴内容,而不是:load file.scala
来解释每一行。
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我来说两句