scala類型別名 - 如何有一個代表多種數據類型的類型

Question

是否可以定義一個代表多個數據類型的類型別名？

package object scala { 
    type SingleDimension = Double 
    type MultiDimensionMap = Map[String, Double] 
    type MultiDimensionList = List[Tuple2[String, Double]] 
} 

例如，我需要一個suptertype讓我們說DataDimension只代表上面三種類型。所以我可以做以下事情：

trait AbstractDataWorker[T] { 

    def formula(d: Double): T 
} 


class multiDimensionWorker extends AbstractDataWorker[MultiDimensionMap] { 

    type T = MultiDimensionMap 
    override def formula(d: Double): MultiDimensionMap = { 

    Map[String, Double]() 
    } 

} 

class singleDimensionWorker extends AbstractDataWorker[SingleDimension] { 

    type T = SingleDimension 
    override def formula(d: Double): SingleDimension = { 
    2.0 
    } 

} 

但是，下面應該給編譯錯誤。目前它的工作。

class stringDimensionWorker extends AbstractDataWorker[String] { 

    type T = String 
    override def formula(d: Double): String = { 

    "hello" 
    } 

} 

Answer 1

你可以不是擴展類型別名的單一性狀他們真正的類：

sealed trait DimensionLike 
case class SingleDimension(value: Double) extends DimensionLike 
case class MultiDimensionMap(value: Map[String, Double]) extends DimensionLike 
case class MultiDinmensionList(value: List[(String, Double)]) extends DimensionLike 

abstract class AbstractDataWorker[T <: DimensionLike] { 
    def formula(d: Double): T 
} 

class MultiDimensionWorker extends AbstractDataWorker[MultiDimensionMap] { 
    // ... 
} 

或者你可以用這些類型別名隱含實現一個類型類。然後，讓AbstractDataWorker的abstract class，並添加開往這個類型類上下文其類型參數：

type SingleDimension = Double 
type MultiDimensionMap = Map[String, Double] 
type MultiDimensionList = List[(String, Double)] 

sealed trait IsDimensionLike[T] 
object IsDimensionLike { 
    implicit object singleDimension extends IsDimensionLike[SingleDimension] 
    implicit object multiDimensionMap extends IsDimensionLike[MultiDimensionMap] 
    implicit object multiDimensionList extends IsDimensionLike[MultiDimensionList] 
} 

abstract class AbstractDataWorker[T : IsDimensionLike] { 
    def formula(d: Double): T 
} 

class MultiDimensionWorker extends AbstractDataWorker[MultiDimensionMap] { 
    // ... 
}