Scala: Why using self-type instead of mixin produces stackoverflow? - scala

Question

trait UserRepository {
def findByFirstName(firstName: String): Seq[User]
}
trait UserBusinessDelegate extends UserRepository {
abstract override def findByFirstName(firstName: String) = {
super.findByFirstName(firstName)
}
}
class MockUserRepository extends UserRepository {
override def findByFirstName(firstName: String) = {
// whatever
}
}
val userRepository = new MockUserRepository with UserBusinessDelegate
userRepository.findByFirstName("John") // OK
However, if I change UserBusinessDelegate as follows:
trait UserBusinessDelegate {
self: UserRepository =>
override def findByFirstName(firstName: String): Seq[User] = {
self.findByFirstName(firstName) // requires explicit return type, thinks this is a recursive call
}
}
val userRepository = new MockUserRepository with UserBusinessDelegate
userRepository.findByFirstName("John") // StackOverflow!!!
I understand stackable pattern and hence how the first case works. My question is why the 2nd doesn't.

Answer 1

In the second snippet you have a recursive call without an exit condition:
override def findByFirstName(firstName: String): Seq[User] = {
self.findByFirstName(firstName)
}
This will always call findByFirstName from UserBusinessDelegate (because you're using self, which basically says that this object will have this kind of behaviour at runtime, not that it's parent will have it and therefore we should call parent's method) creating a new stack frame with each call -> stack overflow.
In the second snippet UserBusinessDelegate's findByFirstName will be called and then you call MockUserRepository's method from it using super -> no recursion -> no stack overflow. You can check Scala's stackable trait pattern for more info.
#Edit: to make it more clear, in the snippet that throws a SO exception the findByFirstName method from MockUserRepository won't be called because you are overriding it in UserBusinessDelegate therefore the anonymous class created with new MockUserRepository with UserBusinessDelegate will only contain the overriden method and that's why the SO, is that clear?
Why would you assume that the method from MockUserRepository would get invoked?
#Edit2: the code doesn't compile without override because self: UserRepository => tells the compiler that a method with such a signature will already be there at runtime and you cannot have 2 methods with the same signature. The first example works only because it's a stackable trait, such traits are dynamically bound and can modify the behaviour but have to call super at some point (which normally isn't allowed without the abstract override modifier, I really recommend going through the link I posted about stackable pattern).
Maybe someone else knows a way, from what I know there's no way to call the mock method unless you change the method name in UserBusinessDelegate and drop the override, then you can call self.findByFirstName and it will call the method from MockUserRepository.