Question

I got some difficulties designing my case classes. A simplified version looks like:

abstract class Base(s: Option[String]) {
   //code
}

case class CaseClass(s: Option[String] = None) extends Base(s) {
    //code
}

And I have a method where I want to do something like:

  def method(base : Base) = {
     //code
     base copy (s = Some("string"))
  }

Of course I get:

 value copy is not a member of Base

So what I want to do is create a new instance based on my base class (which is not a case class). Obviously one can not do this. But how would you solve this in a elegant way?

Thanks in advance!

Answer 1

The behavior you’re trying to achieve is not implementable. copy method of a case class is autogenerated by the compiler, and once you add a method called copy to your implementation, compiler will not generate any sugar.

You can reimplement copy with traits, but it will not be as flexible as the generated one (you will have to update the base trait, copy and method implementations every time the field-set of a case class changes):

sealed trait Base[T] {
   val s: Option[String]
   def copy(s: Option[String]) : T
}

case class CaseClass(override val s: Option[String] = None) extends Base[CaseClass] {
    override def copy(s: Option[String]) = CaseClass(s)
}

def method[T <: Base[T]](base : Base[T]) = base copy (s = Some("strng"))

---

Alternatively, you can implement method as follows:

case class CaseClass(s: Option[String] = None)

def method[X <: {def copy(s: Option[String]):X}](base : X) = 
    base copy(s = Some("string"))

scala> method(CaseClass())
res4: CaseClass = CaseClass(Some(string))

Thus you won’t need Base trait, and reduce the number of alterations, if your case classes change.