Context bounds with two generic parameters

问题

In Scala, I can use context bounds:

def sort[T : Ordered](t: Seq[T])

To mean the same thing as:

def sort[T](t: Seq[T])(implicit def Ordered[T])

What if I have a class with two generic parameters. I.e. I want to be able to ensure that I have a Writer[T, String]. Is there a syntax where I can use context bounds (T : ...) or do I need to have the implicit explicitly (that was fun to write).

回答1:

Yes, it's possible! But not really very pretty:

trait Writer[T, O] {
  def write(t: T): O
}

def writeToString[T: ({ type L[x] = Writer[x, String] })#L](t: T) =
  implicitly[Writer[T, String]].write(t)

implicit object intToStringWriter extends Writer[Int, String] {
  def write(t: Int) = t.toString
}

And then:

scala> writeToString(1)
res0: String = 1

The ({ type L[x] = Writer[x, String] })#L thing is called a type lambda. Sometimes they're very convenient (if buggy), but this definitely isn't one of those times. You're much better off with the explicit implicit.

来源：https://stackoverflow.com/questions/20131589/context-bounds-with-two-generic-parameters