Question

I have a set of decorator-traits (for simplicity here only trait T) mixing in classes (here subclasses of A).

trait T { def i: Int }
abstract class A
type AT = A with T
class B extends A
// class C extends A
// class D extends A
// ...

Now I have an instance a of e.g. class B which I handle as an instance of type AT (together with other A-subclass-AT-instances in a Seq[AT]).

val a: AT = new B with T { val i = 8 }

How can I generically decorate like this:

def toAT(a: A, i: Int): AT = {
 // how to ???
}

An obvious solution I thought about is:

trait T { def i: Int }
abstract class A {
  def asAT(i: Int): AT
}
class B extends A {
  def asAT(ii: Int): AT = new B with T { val i = ii }
}
type AT = A with T
def toAT(a: A, i: Int): AT = a.asAT(i)

But I don´t want to pollute my classes B, C, ..., Z with a new method and there are other decorations, so for every decoration-combo I need a new method for all subclasses!
Is there a more generic way?

EDIT:
An example why I need the toAT method, but still with the obvious solution approach from above in terms of a redecorated method:

trait T { def b: Boolean}
type AT = A with T
abstract class A(val i: Int) {
  def changedBy(x: Int): A
  def redecorated(oldAT: AT): AT
}
class B(x: Int) extends A(x) {
  def changedBy(x: Int): A = new B(i * x) 
  def redecorated(oldAT: AT): AT = new B(i) with T { val b = oldAT.b }
}
class C(x: Int) extends A(x) {
  def changedBy(x: Int): A = new C(i * x * x) 
  def redecorated(oldAT: AT): AT = new C(i) with T { val b = oldAT.b }
}
val b = new B(1) with T { val b = true  }
val c = new C(1) with T { val b = false }
val x = 8
val res: Seq[AT] = Seq(b,c) map { at => at.changedBy(x).redecorated(at) }

Answer 1

Basically, you can’t do it.

One alternative is to use the auto proxy plugin, by Kevin Wright. See also here and here for more information about it.