Scala 的要么元组为 Right

Question

假设我有以下代码:

val either: Either[String, (Int, Int)] = Right((1,2))
for {
  (a, b) <- either.right
} yield a + b

当我在 REPL 中评估它时，我得到

:13: error: constructor cannot be instantiated to expected type; found : (T1, T2) required: scala.util.Either[Nothing,(Double, Double)] (a, b) <- a.right ^ :14: error: not found: value a } yield a + b ^

为什么我有这样的错误？我不能对来自“Either's Right”的元组进行模式匹配吗？

Answer 1

问题似乎是 Scala 错误 https://issues.scala-lang.org/browse/SI-7222 .将 for comprehension 转换为 flatMap/map 符号似乎有效。

val either: Either[String, (Int, Int)] = Right((1, 2))
either.right.map {
  case (a, b) =>
    a + b
}

either: Either[String,(Int, Int)] = Right((1,2))
res0: Serializable with Product with scala.util.Either[String,Int] = Right(3)