dotty seq映射到联合

Question

我无法用最新的dotty (0.9.0-RC1)编译以下代码，乍一看，它看起来应该.

object UnionMapping {

  private def parse(string: String): Int | Double = {
    if(string.contains("."))
      string.toDouble
    else
      string.toInt
  }

  def test_number = {
    val strings: Seq[String] = Seq("123", "2.0", "42")
    // Works
    val asdf: Seq[AnyVal] = strings.map(parse(_))
    // Fails to compile
    val union: Seq[Int | Double] = strings.map(parse(_))
  }

}

有谁能洞察其失败的原因，以及它的预期能否奏效？

Answer 1

目前，类型推理几乎从不推断合并类型，因为它通常不是最佳选择。在您的特定示例中，我同意这样做更有意义，所以我打开了https://github.com/lampepfl/dotty/issues/4867来跟踪它。同时，通过手动指定类型参数，可以让它编译：

val union = strings.map[Int | Double, Seq[Int | Double]](parse(_))

或者将联合隐藏在类型别名后面：

object UnionMapping {

  type Or[A, B] = A | B

  private def parse(string: String): Or[Int, Double] = {
    if(string.contains("."))
      string.toDouble
    else
      string.toInt
  }

  def test_number = {
    val strings: Seq[String] = Seq("123", "2.0", "42")
    val union = strings.map(parse(_))
      // infered type for union is Seq[Or[Int, Double]]
  }

}