需要两个隐式参数之一的Scala方法 [英] Scala method that needs either one of two implicit parameters

问题描述

我对是否可以创建具有类似想法的方法很感兴趣:

I'm interested if I can create method with similar idea:

def myMethod[T](param: T)(implicit oneOf: Either[TypeClass1[T], TypeClass2[T]]) = oneOf match ...

我尝试使用默认参数(我在akka中看到了类似的东西):

I've tried to use default parameters (I've seen somethin similar in akka):

def myMethod[T](param: T)(implicit t1: TypeClass1[T] = null, t2: TypeClass2[T] = null) = 
  if (t1 == null) ...

但是，这样一来，我无法强制Scala编译器找到其中至少一个.

However, that way I cannot force scala compiler to find at least one of them.

此外，我已经实现了从TypeClass1[T]到Left[TypeClass1[T], TypeClass2[T]]以及从TC2到Right的隐式转换，但是Scala编译器忽略了这种转换.

Also, I've implemented implicit conversion from TypeClass1[T] to Left[TypeClass1[T], TypeClass2[T]] and from TC2 to Right, however Scala compiler ignores this conversions.

有什么办法做这样的事情吗?

Is there any way to do something like this?

推荐答案

显而易见的解决方案是创建可以使用TypeClass1或TypeClass2构造的新类型类.新的类型类实现了myMethod所共有的功能，并将其映射到TypeClass1或TypeClass2上的适当方法.

The obvious solution is to create a new typeclass that can be constructed using either TypeClass1 or TypeClass2. The new typeclass implements the functionality used by myMethod that is common to both and maps it to the appropriate methods on TypeClass1 or TypeClass2.

这里是一个例子:

  trait TypeClass1[T] {
    def showOne = println("Typeclass 1")
  }

  trait TypeClass2[T] {
    def showTwo = println("Typeclass 2")
  }

  trait UnionTypeClass[T] {
    def show
  }

  object UnionTypeClass {
    implicit def t1[T](implicit ev: TypeClass1[T]) = new UnionTypeClass[T] {
      def show = ev.showOne
    }

    implicit def t2[T](implicit ev: TypeClass2[T]) = new UnionTypeClass[T] {
      def show = ev.showTwo
    }
  }


  implicit object IntClass extends TypeClass1[Int]
  implicit object StringClass extends TypeClass2[String]


  def myMethod[T](param: T)(implicit ev: UnionTypeClass[T]) = {
    ev.show
  }

  myMethod(0)
  myMethod("hello")

这将打印

Typeclass 1
Typeclass 2

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