Scala - Overriding trait method using a sub context bound method

1
votes

For 

sealed trait User {...}
sealed trait Trader extends User {...}

trait AuthObject

trait AuthUserObject {
  def authorize[U <: User](u: U): Boolean
}

trait AuthTraderObject extends AuthUserObject {
  def authorize[T <: Trader](t: T): Boolean
}

object HasPaidTax extends AuthTraderObject {
  def authorize[T <: Trader](t: T): Boolean = t.hasPaidTax
}

This doesnt build. Error:

Error:(15, 7) overriding method authorize in trait AuthUserObject of type [U <: users.User](u: U)Boolean; method authorize has incompatible type def authorize[T <: Trader](t: T): Boolean ^

I need to restrict AuthTraderObject to Trader user since only trader pays tax. Please, how is this override possible?

2
scalatraitscontext-bound

2 Answers

5
votes

First, you should understand the reason for the error: since HasPaidTax extends AuthUserObject, you can ascribe this type to it:

val auth: AuthUserObject = HasPaidTax

But for any auth: AuthUserObject and user: User, you can call auth.authorize(user). So when overriding methods, you can't narrow the argument types or type bounds.

For the solution, you can go with pamu's answer or just move the type parameter to the type (should be equivalent for this case):

trait AuthUserObject[U <: User] {
    def authorize(u: U): Boolean
}

trait AuthTraderObject[T <: Trader] extends AuthUserObject[T] {
    def authorize(t: T): Boolean
}

object HasPaidTax extends AuthTraderObject[Trader] {
    def authorize(t: Trader): Boolean = t.hasPaidTax
}
4
votes

User type and then override type when necessory

trait User

trait Trader extends User


trait AuthUserObject {
  type U <: User
  def authorize(u: U): Boolean
}

trait AuthTraderObject extends AuthUserObject {
  override type U <: Trader
  override def authorize(u: U): Boolean
}