Map with path-dependent value type?

问题

I know that Scala has path-dependent types, so for example if I have a class within an inner class I can constrain one argument of a method to be an instance of the inner class of the other argument:

class Outer { class Inner( x:Int ) }
val o1 = new Outer
val o2 = new Outer
val i11 = new o1.Inner(11)
val i12 = new o1.Inner(12)
val i21 = new o2.Inner(21)
def f[ A <: Outer ]( a:A )( i:a.Inner ) = (a,i)
f(o1)(i11)  // works
f(o1)(i21)  // type mismatch; i21 is from o2, not o1

And I can create a Map from an Outer to an Inner using a type projection:

var m = Map[Outer,Outer#Inner]()

But that would allow entries like o1 -> i21, and I don't want that to be allowed. Is there any type magic to require that a value be an instance of its key's inner class? That is, I want to say something like

var m = Map[Outer,$1.Inner]()  // this doesn't work, of course

回答1:

No.

Path dependent types are scoped to an object instance, as in the example you gave:

def f[ A <: Outer ]( a:A )( i:a.Inner ) = (a,i)

Here the path-dependent type relies on the object reference "a", which is in scope for the declaration of the second parameter list.

The map definition looks a bit like:

trait Map[A,+B] {
  def get(key: A): Option[B]
  def + [B1 >: B](kv: (A, B1)): This
}

There is no object of type A in scope here for you to reference when defining B.

As others have hinted, you could define a new similar Map class which fits this requirement:

trait Map2[A <: Outer] {
  def get(key: A): Option[key.Inner]
  def put(key: A)(value: key.Inner): this.type
}

来源：https://stackoverflow.com/questions/21558555/map-with-path-dependent-value-type