问题描述
我正在尝试使用一系列依赖元组对一个依赖映射进行编码。 这是我无法使用的内容:
class DTuple[Key,ValueMap[_ <: Key]](val first: Key)(val second: ValueMap[first.type])
type DKey = "Tag" | "Versions" | "Author"
type DMapping[X <: DKey] = X match {
case "Tag" => String
case "Versions" => Array[String]
case "Author" => String
}
def mkString(d: DTuple[DKey,DMapping]) = d.first match {
case _: "Tag" => "#" + d.second
case _: "Versions" => d.second.mkString(",")
case _: "Author" => "@" + d.second
}
我所能得到的
[error] -- [E008] Not Found Error: Main.scala:21:35
[error] 21 | case _: "Versions" => d.second.mkString(",")
[error] | ^^^^^^^^^^^^^^^^^
[error] | value mkString is not a member of Main.DMapping[(d.first : Main.DKey)]
我想不出一种模式匹配d.second
的好方法,因此其类型取决于d.first。我可以添加.asInstanceOf[Array[String]]
和.asInstanceOf[String]
,但这不是这里的目标,我正在尝试对代码进行类型检查。
解决方法
也许有更好,更轻松的方法来做到这一点,但是:
import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter.ISO_ZONED_DATE_TIME
// This is a case case solely for the unapply method,you could implement it on your own
case class DTuple[Key,ValueMap[_ <: Key]](first: Key)(val second: ValueMap[first.type])
type DKey = "Tag" | "Versions" | "Author" | "BuildTime"
type DMapping[X <: DKey] = X match {
case "Tag" => String
case "Versions" => Array[String]
case "Author" => String
case "BuildTime" => ZonedDateTime
}
// the DTuple("<value>") is used at runtime to check the string (DKey) value
// the DTuple["<value>",DMapping] type hint makes dotty see the `d` value as the correct type,hence infering the type of d.second too
def mkString(dt: DTuple[DKey,DMapping]): String = dt match {
// this would fail at runtime as e.g DTuple("Tag") would enter this case (the `DTuple["BuildTime",DMapping]` is unchecked at runtime
// case d: DTuple["BuildTime",DMapping] => d.second.format(ISO_ZONED_DATE_TIME)
// this doesn't compile because `d.second`'s type is still 'DMapping[(d.first : DKey)]',not 'DMapping["BuildTime"]'
// case d@DTuple("BuildTime") => d.second.format(ISO_ZONED_DATE_TIME)
case d@DTuple("Tag"): DTuple["Tag",DMapping] => d.second
case d@DTuple("Versions"): DTuple["Versions",DMapping] => d.second.mkString(",")
case d@DTuple("Author"): DTuple["Author",DMapping] => d.second.toString
case d@DTuple("BuildTime"): DTuple["BuildTime",DMapping] => d.second.format(ISO_ZONED_DATE_TIME)
}
object Main extends App {
List(
DTuple[DKey,DMapping]("Versions")(Array("1.0","2.0")),DTuple[DKey,DMapping]("Tag")("env=SO"),DMapping]("Author")("MK"),DMapping]("BuildTime")(ZonedDateTime.now())
).foreach { dt =>
println(mkString(dt))
}
}
打印
1.0,2.0
env=SO
MK
2020-10-23T21:04:06.696+02:00[Europe/Paris]