Question

假设我有类似以下的类型层次结构：

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，该方法包含所有Red或全部Green的元素，但不能同时包含两者？

例如，在以下代码中：

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

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

[T]应该是什么，以便上面不进行类型检查？

修改

原始问题如下：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的元素将是结合（AND），而Seq的元素将是析取（OR）。我不想将Simple与ComplexQueries混合在一起，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))))

它不理想（名称错误，没有@implicitNotFound注释）。

基本思想是我们为基类创建模糊的含义，但不为子类创建含糊不清的含义。由于隐式解析规则，这将仅针对子类进行编译，但不针对基类进行编译。

清理版：

@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 - 仅接受相同子类型的元素

1 个答案: