Scala typing: "Expected _$1, got T" - scala

Question

I have a Lift project with mixed Java-Scala code. The project has a JPA backend written in Java with EclipseLink, which is accessed by the Scala side, which uses Lift and Lift-NG.
On the Java side, I have the following relevant interfaces:
interface IEntity
interface IDAO<T extends IEntity> {
void persist(T t);
}
On the Scala side, I have the following:
abstract class Binding[T <: IEntity] extends NgModel {
def unbind: T
}
class BasicService[B <: Binding[_ <: IEntity]](serviceName: String, dataAccessObject: IDAO[_ <: IEntity]) {
def persist(binding : B): Unit = {
val entity = binding.unbind
dataAccessObject.persist(entity)
}
}
The purpose of this hierarchy is to let me create Service instances which handle a given Binding for entity E, which can also receive and use an appropriate DAO created to handle type E. For example:
// Java side
class Todo implements IEntity
class TodoManager implements IDAO<Todo>
// Scala side
case class TodoBinding extends Binding[Todo] {
override def unbind: Todo = new Todo()
}
object HelloWorld extends BasicService[TodoBinding]("todoService", new TodoManager)
My problem is a compilation error that occurs inside the persist method of BasicService. On the last line, I get this:
Type mismatch: expected _$1, actual: IEntity
As I am a bit inexperienced with Scala, I might be missing something very obvious with the type system, but I genuinely cannot figure this out. How can I work around this error?

Answer 1

Your definition of BasicService does not require that the type argument of B:
B <: Binding[_ <: IEntity]
is compatible with the type argument of dataAccessObject:
dataAccessObject: IDAO[_ <: IEntity]
What if one is Binding[FooEntity] and the other is IDAO[BarEntity]?
You should take that type variable E that you say you are trying to use, and actually define it:
class BasicService[E <: IEntity, B <: Binding[E]]
(serviceName: String, dataAccessObject: IDAO[E])
Also note that in the example code you posted, the type variable B to BasicService might not be necessary, because you only use it to receive the argument binding to persist(). You could have simply used the type Binding[E] there:
class BasicService[E <: IEntity]
(serviceName: String, dataAccessObject: IDAO[E]) {
def persist(binding: Binding[E]): Unit = // ...
}