Can I override a type parmeterized method with concrete type in scala?

Question

I want to override a type-parameterized method by assigning a concrete type to the type-parameter, something like the code below.

trait A {
  def amethod[T](x: T): T
}
trait B extends A {
  def amethod(x: String): String = x ++ x
}

However the compiler gives amethod overrides nothing . I cannot put [String] after amethod in trait B, as syntactically it means a type parameter named String not the type java.lang.String. I am wondering whether and how I can do something like that.

Thanks a lot.

Answer 1

amethod , as it's defined in trait A , is a generic method, meaning that each call can be applied to generic arguments.

What you are trying to say in trait B is to change the scope at which generality is expressed, moving it from the call site to the class definition itself. It's not possible to do it.

As it's been suggested in a comment, if you want to express generality in a class definition, the type parameter should be applied to the class, not to the method:

 trait A[T] {
   def amethod(x: T): T
 }

 trait B extends A[String] {
   override def amethod(x: String): String = x ++ x
 }

Answer 2

As an explanation of the comment saying " B breaks its contract with A ", consider this code:

def foo(): A = new B {} // legal because B extends A
def bar = foo().amethod[Int](0)

What should happen? If you are tempted to say that it should be rejected by looking at the body of foo , imagine it's an abstract method instead, and some class happens to implement it in this way.