Scala：隱式值查找與類型類成語

Question

我試圖擴展使用值類爲類型安全的想法，這裏描述的http://docs.scala-lang.org/overviews/core/value-classes.html到一個更抽象的版本，允許隱式轉換。例如，我有一些措施特質

trait Measure { 
implicit def +[B <: Measure](other: B): Measure 
val value: Double 
} 

和一些實現

//this will compile because [_ , Meters] is not ambiguous since 
//for this example im only using one conversion. see below 


case class Meters(value: Double) extends Measure { 
    def +[B <: Measure](other: B): Meters = Meters(other.value * implicitly[Converter[_ , Meters]].factor + this.value) } 
case class Fathoms(value: Double) extends Measure { 
    def +[B <: Measure](other: B): Fathoms = Fathoms(other.value * implicitly[Converter[_ , Fathoms]].factor + this.value) } 

和轉換特徵和實施隱式查找機制找到

trait Converter[F, T]{ 
    val factor: Double 
} 
implicit object Meter2Fathom extends Converter[Meter, Fathom]{ 
    val factor: Double = .556 
} 
implicit object Fathom2Meter extends Converter[Fathom, Meter]{ 
    val factor: Double = 1.8 
} 

有沒有一種辦法在「無論B是什麼」上定義Converter實現？如果我們嘗試將兩個不存在轉換因子的度量值相加，但仍然能夠進行編譯，則需要運行時錯誤。

//will not compile of course. the idea is to somehow reify whatever B 
// is and look that implicit converter up, instead of actual B. 
case class Meters(value: Double) extends Measure { 
    def +[B <: Measure](other: B): Meters = Meters(this.value + other.value * implicitly[Converter[* B *, Meters]].factor 

Answer 1

據我瞭解你想要的是磁鐵圖案。這裏有一個廣泛的博客：http://spray.io/blog/2012-12-13-the-magnet-pattern/