Scala copy case class with generic type

Question

I have two classes `PixelObject`, `ImageRefObject` and some more, but here are just these two classes to simplify things. They all are subclasses of a `trait Object` that contains an uid. I need universal method which will copy a case class instance with a given new `uid`. The reason I need it because my task is to create a class ObjectRepository which will save instance of any subclass of `Object` and return it with new `uid`. My attempt: ``` trait Object { val uid: Option[String] } trait UidBuilder[A <: Object] { def withUid(uid: String): A = { this match { case x: PixelObject => x.copy(uid = Some(uid)) case x: ImageRefObject => x.copy(uid = Some(uid)) } } } case class PixelObject(uid: Option[String], targetUrl: String) extends Object with UidBuilder[PixelObject] case class ImageRefObject(uid: Option[String], targetUrl: String, imageUrl: String) extends Object with UidBuilder[ImageRefObject] val pix = PixelObject(Some("oldUid"), "http://example.com") val newPix = pix.withUid("newUid…

Accepted Answer

I would stick with the solution proposed by Sean. I have done the same a couple of months ago. For example:

```
trait Entity[E <: Entity[E]] {
  // self-typing to E to force withId to return this type
  self: E => def id: Option[Long]
  def withId(id: Long): E
}
case class Foo extends Entity[Foo] {
  def withId(id:Long) = this.copy(id = Some(id))
}
```

So, instead of defining an UuiBuilder with a match for all implementations of your trait, you define the method in your implementation itself. You probably don't want to modify UuiBuilder every time you add a new implementation.

In addition, I would also recommend you to use a self typing to enforce the return type of your withId() method.

Scala copy case class with generic type

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