Either.right =右，Either.Left =左？

Question

在下一个站点：

https://typelevel.org/cats/datatypes/either.html

显示：

object EitherStyle {
  def parse(s: String): Either[Exception, Int] =
    if (s.matches("-?[0-9]+")) Either.right(s.toInt)
    else Either.left(new NumberFormatException(s"${s} is not a valid integer."))

  def reciprocal(i: Int): Either[Exception, Double] =
    if (i == 0) Either.left(new IllegalArgumentException("Cannot take reciprocal of 0."))
    else Either.right(1.0 / i)

  def stringify(d: Double): String = d.toString
}

但是，我得到了错误：

[error] /application/learningSBT/hello-world/src/main/scala/Main.scala:16:39: value right is not a member of object scala.util.Either
[error]     if (s.matches("-?[0-9]+")) Either.right(s.toInt)
[error]                                       ^
[error] /application/learningSBT/hello-world/src/main/scala/Main.scala:17:17: value left is not a member of object scala.util.Either
[error]     else Either.left(new NumberFormatException(s"${s} is not a valid integer."))
[error]                 ^
[error] /application/learningSBT/hello-world/src/main/scala/Main.scala:21:14: value left is not a member of object scala.util.Either
[error]       Either.left(new IllegalArgumentException("Cannot take reciprocal of 0."))
[error]              ^
[error] /application/learningSBT/hello-world/src/main/scala/Main.scala:22:17: value right is not a member of object scala.util.Either
[error]     else Either.right(1.0 / i)
[error]                 ^
[error] four errors found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 2 s, completed Feb 12, 2020 10:25:02 AM

但是，当我用Right替换Either.right并用Left替换Either.left时，我得到了这段代码：

object EitherStyle {
  def parse(s: String): Either[Exception, Int] =
    if (s.matches("-?[0-9]+")) Right(s.toInt)
    else Left(new NumberFormatException(s"${s} is not a valid integer."))

  def reciprocal(i: Int): Either[Exception, Double] =
    if (i == 0)
      Left(new IllegalArgumentException("Cannot take reciprocal of 0."))
    else Right(1.0 / i)

  def stringify(d: Double): String = d.toString
}

所以，我想知道是什么导致这种情况发生。

Answer 1

这是猫对标准Either对象的扩展。

为此导入cats.syntax.either._。