Scala-使用泛型方法定义特征

Question

How do I define a trait with generic arguments in its methods and then override with the specific typed arguments?

An example:

import shapeless._

case class UserA(name:String)
case class UserB(name:String)

trait User {
  def save(item:Option[T]): Option[T]
}
class UserAdmin extends User {
  def save(item:Option[UserA]): Option[UserA]
}
class UserSomethingElse extends User {
  def save(item:Option[UserB]): Option[UserB]
}

With this, I get the error not found: type T even though it is defined in shapeless

Answer 1

It works if you something along these lines:

trait User[T] {
  def save(item:Option[T]): Option[T]
}
class UserAdmin extends User[UserA] {
  def save(item: Option[UserA]): Option[UserA] = ???
}
...

Answer 2

One has to keep in mind that T at this moment does not denote a type or class but is used as a placeholder. So it may be that there is a T in shapeless but it has no connection to the placeholder T .

Here is a nice introduction to Type polymorphism : https://twitter.github.io/scala_school/type-basics.html .

So basically as @mfirry already suggested:

trait User[T] { // T is placeholder
  def save(item:Option[T]): Option[T]
}

new User[String] {
  // using override her is considered best practice
  override def save(item:Option[String]): Option[String] = item
}

Answer 3

Another option, besides @mfirry's is this, where we use a type member instead (versus a type parameter).

trait User {
  type T
  def save(item:Option[T]): Option[T]
}
class UserAdmin extends User {
  type T = UserA
  def save(item:Option[UserA]): Option[T]
}
class UserSomethingElse extends User {
  type T = UserB
  def save(item:Option[UserB]): Option[T]
}

In my real case this is actually better since I have several such types whose concrete implementations will vary in the subclasses (different types for the arguments and the return parameters). So something like this:

trait User {
  type T
  type R
  def save(item:Option[T]): Option[R]
}
class UserAdmin extends User {
  type T = UserA
  type R = UserAA
  def save(item:Option[T]): Option[R]
}
class UserSomethingElse extends User {
  type T = UserB
  type R = UserBB
  def save(item:Option[T]): Option[R]
} 

This article was helpful: http://www.artima.com/weblogs/viewpost.jsp?thread=270195