Using this keyword to inherit? [duplicate]

问题:

Possible Duplicate:
What is the difference between scala self-types and trait subclasses?

From example in scalatest site. There is one particular thing I don't really understand

trait FunSuiteStackBehaviors { 
                   this: FunSuite => //This line
                         def a() {}
                         def b() {}
}

class StackFunSuite extends FunSuite with FunSuiteStackBehaviors {}

As far as I understand, it seems like they try to assign some defs into a trait. But what does this: FunSuite => part do ? I tried to use extends FunSuite instead like

trait FunSuiteStackBehaviors extends FunSuite { 
                         def a() {}
                         def b() {}
}

class StackFunSuite extends FunSuite with FunSuiteStackBehaviors {}

and I still end up with same result. Are they the same thing ?

回答1:

the this: => XXXX is called a self type annotation http://www.scala-lang.org/node/124 Basically, you're specifying the type of "this" (current object) to the type specified. Kind of a "cast from the inside"

回答2:

this: FunSuite => is called self type annotation. There is nothing special about "this" Both snippets are equivalent. Here is some discussion on its rationale. In short self type annotation helps to signify compositional nature rather than IS-A relation.