ScalaZ：什么是“ type Tagged [T] = {type Tag = T}”？

Question

I started to read scalaz's source code .

package object scalaz {
  import Id._

  implicit val idInstance: Traverse1[Id] with Each[Id] with Monad[Id] with Comonad[Id] with Distributive[Id] with Zip[Id] with Unzip[Id] with Align[Id] with Cozip[Id] = Id.id

  type Tagged[T] = {type Tag = T}
  .
  .
  .

How should I interpret type Tagged[T] = {type Tag = T} ?

What does this mean ?

Where is this Scala syntax described ?

I am totally confused with this.

What does it do ?

Why ?

Could someone give a super simple example that would explain this syntax for "dummies" ?

... and what it is good for ?

Answer 1

It's scala structural typing, basically it means that type that satisfying Tagged[T] should have declared 'type Tag' inside

  type Tagged[T] = {type Tag = T}

  def acceptOnlyTagged[T: ClassTag](tagged: Tagged[T]): Unit = {
    println(implicitly[ClassTag[T]])
  }

  class A {
    type Tag = Int
  }

  class B {
    type Tag = String
  }

  acceptOnlyTagged(new A)
  acceptOnlyTagged(new B)

Class A & B doesn't know anything about Tagged but can be viewed as Tagged because they share common structure