路径相关类型和类型投影

Question

As I understand the def bar(x: X#Inner) signature, it simply requires any instance of Inner to compile successfully. Whereas, foo(x: this.Inner) will only compile with the same Outer.Inner class instance.

While reading Scala in Depth , I put this code into REPL, but I'm getting a compile-time error.

scala> class Outer {
     |    trait Inner
     |     def y = new Inner {}
     |    def foo(x: this.Inner) = null
     |    def bar(x: X#Inner) = null
     | }
<console>:11: error: not found: type X
          def bar(x: X#Inner) = null
                     ^

Also, what is the compile-time error here?

Answer 1

The X in X#Inner refers to the outer type. You do not have a type X anywhere, that's what the compiler error says exactly.

In your example you will want to refer to type Outer instead:

class Outer {
  trait Inner
  def foo(x: this.Inner) = ???
  def bar(x: Outer#Inner) = ???
}

Answer 2

X#Inner应该是Outer#Inner ，我认为Scala in depth的作者在书中打错了字眼。