尝试使用类型参数编写DRY Scala代码

你是对的, this.type 需要使用,但有一些限制:

trait EnforceIdType {
  def id: Id[_ >: this.type <: EnforceIdType]
}

case class A(id: Id[A]) extends EnforceIdType
case class B(id: Id[B]) extends EnforceIdType
//case class C(id: Id[B]) extends EnforceIdType // Won't compile, as expected

更新时间:

关于第二个限制(由Alexey Romanov展示)。它可以消除,但需要很长的路:

//Embeded polymorphism used
class Id(val value: Long) {
  type M
}

// Factory method for shift type variable to type parameter field
object Id {
  def apply[T](value : Long) = new Id(value) { type M = T }
}

trait EnforceIdType {
  type This = this.type // store this.type for use inside Id { type M = .. }
  val id: Id { type M >: This <: EnforceIdType } // need to be val

  def show(x : id.M) { println (x) } // dependent method
}

case class A(id: Id { type M = A }) extends EnforceIdType
case class B(id: Id { type M = B }) extends EnforceIdType
//  case class C(id: Id { type M = B }) extends EnforceIdType // Won't compile, as expected

val a = A( Id[A](10))
val b = B( Id[B](10))

a.show(a)
// a.show(b) // Won't compile, as expected