Scala Seq - 只接受相同子类型的元素

Question

假设我有如下类型层次结构：

trait Color
case class Red(r: String) extends Color
case class Green(g: String) extends Color

是否可以创建一个接受 Seq[Color] 的方法，该 Seq[Color] 包含全部 Red 或全部 Green 的元素，但不能同时包含两者？

例如在下面的代码中：

def process[T](colors: Seq[T]) = colors.size

process(Seq(Red("a"), Green("g")))

[T] 应该怎么做才能使上面的代码不进行类型检查？

编辑

最初的问题如下：我正在尝试为嵌套查询设计 JSON API。我想出了以下设计：

trait QueryUnit
case class SimpleQuery(query: String, metadata: Metadata)
case class ComplexQuery(Map[String, Seq[SimpleQuery])

case class API(
  query: Map[String, Seq[QueryUnit]]
)

Map 的元素将是连词 (AND)，而 Seq 的元素将是析取 (OR)。我不想将 Simple 与 ComplexQueries 混合使用，我打算在 Seq[QueryUnit] 上进行模式匹配。

Answer 1

试试这个（部分基于 this blog post from Miles Sabin）：

// Your domain
trait QueryUnit
case class SimpleQuery(query: String, metadata: AnyRef) extends QueryUnit
case class ComplexQuery(map: Map[String, Seq[SimpleQuery]]) extends QueryUnit
// End of your domain

// Something which has type parameter, so we can add QueryUnit, ...
trait WrongArg[T]

// Create ambiguous implicits for QueryUnit
implicit def v0: WrongArg[QueryUnit] = ???
implicit def v2: WrongArg[QueryUnit] = ???

// And valid values for the concrete subclasses
implicit val simpleQWrongArg: WrongArg[SimpleQuery] = new WrongArg[SimpleQuery] {}
implicit val complexQWrongArg: WrongArg[ComplexQuery] = new WrongArg[ComplexQuery] {}

case class API[QU <: QueryUnit](
                query: Map[String, Seq[QU]]
     // Require an evidence that we are getting the correct type
                               )(implicit w: WrongArg[QU]) {
}

API/*[SimpleQuery]*/(Map("" -> Seq(SimpleQuery("", ""))))

API/*[ComplexQuery]*/(Map("" -> Seq(ComplexQuery(Map.empty))))

// Fails to compile because of ambiguous implicits
//API(Map("s" -> Seq(SimpleQuery("", "")), "c" -> Seq(ComplexQuery(Map.empty))))

不理想（名字错误，没有@implicitNotFound注释）。

基本思想是我们为基 class 创建模糊的隐式，但不为子 class es。由于隐式解析规则，这将仅针对子classes 进行编译，而不会针对基class.

进行编译

清理后的版本：

@annotation.implicitNotFound("Base QueryUnit is not supported, you cannot mix SimpleQuery and ComplexQuery")
trait Handler[T] extends (T => Unit)

object API {
  implicit def baseQueryUnitIsNotSupported0: Handler[QueryUnit] = ???

  implicit def baseQueryUnitIsNotSupported1: Handler[QueryUnit] = ???

  implicit val simpleQWrongArg: Handler[SimpleQuery] = new Handler[SimpleQuery] {
    override def apply(s: SimpleQuery): Unit = {}
  }
  implicit val complexQWrongArg: Handler[ComplexQuery] = new Handler[ComplexQuery] {
    override def apply(s: ComplexQuery): Unit = {}
  }
}

case class API[QU <: QueryUnit](query: Map[String, Seq[QU]])(
  implicit handler: Handler[QU]) {
  // Do something with handler for each input
}

// Usage

import API._

import scala.annotation.implicitNotFound
API/*[SimpleQuery]*/(Map("" -> Seq(SimpleQuery("", ""))))

API/*[ComplexQuery]*/(Map("" -> Seq(ComplexQuery(Map.empty))))

// Error:(56, 71) Base QueryUnit is not supported, you cannot mix SimpleQuery and ComplexQuery
//API(Map("s" -> Seq(SimpleQuery("", "")), "c" -> Seq(ComplexQuery(Map.empty))))

或者使用类型边界和 implicitly:

case class API[QU <: QueryUnit: Handler](query: Map[String, Seq[QU]]) {
  def doSomething: Unit = for {(_, vs) <- query
       v <- vs} {
    val handler: Handler[QU] = implicitly[Handler[QU]]
    handler(v)
  }
}

Scala Seq - 只接受相同子类型的元素

Scala Seq - accept only elements of the same subtype

scala

scala-collections

type-systems