I wrote the following simple example and expected it to be compiled fine:
abstract class TestObject extends App{
type Type
def method[F[_]](ft: F[Type], t: Test[F]{
type Type = TestObject#Type
}) = t.doSomeAction(ft) //ERROR
}
trait Test[F[_]]{
type Type
def doSomeAction(t: F[Type]) = println(t)
}
But the compiler prints the following error message:
Error:(8, 23) type mismatch;
found : ft.type (with underlying type F[TestObject.this.Type])
required: F[t.Type]
(which expands to) F[TestObject#Type]
Note: TestObject.this.Type <: t.Type, but type F is invariant in type _.
You may wish to define _ as +_ instead. (SLS 4.5)
}) = t.doSomeAction(ft)
I do not really understand it since Test#Type = TestObject#Type.
So the problem is that TestObject#Type captures an existential type which is not what you want. you want to ensure that the Type of specific instances line up. you can do this like this:
https://scalafiddle.io/sf/wpI8iGg/0
or more commonly with the Aux-Pattern