Scala: Return type of copy method when implicit evidence parameter has a representation type


Let's say I have a class with a type parameter and a method that should return a copy of the instance only if the class is parameterized with a particular trait that has a representation type. I can get that to happen pretty easily. What I can't do is put a sensible return type that method:

case class Foo[+A](a: A) { // Compiles def gotFooBar(implicit evidence: A <:< Bar[_]) = copy(a = a.Copy()) // Does not compile def gotFooBar(implicit evidence: A <:< Bar[_]): Foo[A] = copy(a = a.Copy()) } trait Bar[+B <: Bar[B]] { def Copy(): B // Return underlying type } case class Grill() extends Bar[Grill] { def Copy() = Grill() }

What is the return type of that function, or perhaps more importantly, how would I set up the types so that that was the return type? Could someone also point out how the real return type could be a supertype of Foo[A]?


Well, since you only require Bar[_] you get Any as result of calling a.Copy. You need a type parameter for the gotFooBar method:

case class Foo[+A](a: A) { def gotFooBar[B <: Bar[B]](implicit evidence: A <:< B): Foo[B] = { val bar = a: B copy(a = bar.Copy()) } }

The second question is, how to enforce that Foo[B] is a supertype of Foo[A]. You just need to add A as a lower bound for B:

def gotFooBar[B >: A <: Bar[B]](implicit evidence: A <:< B): Foo[B]


This is a follow-up answer to the answer by @0__. The situation gets a little more complicated if A has a Manifest associated with it:

case class Foo[+A : Manifest](a: A)

The method type parameter B needs a Manifest too (even if you don't use it). The usual way of adding a Manifest (: Manifest) won't work either because that isn't allowed when the method also has implicit parameters (the implicit evidence in this case). The Manifest can be put in the implicit parameter list though, like this:

def gotFooBar[B >: A <: Bar[B]] (implicit m: Manifest[B], evidence: A <:< B): Foo[B] = copy(a = a.Copy())


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