Overriding the method of a generic trait in Scala

Question

I defined a generic Environment trait:

trait Environment[T]

For which I provide this implementation:

class MyEnvironment extends Environment[Integer] {
 val specific: Integer = 0
}

Furthermore, I defined a generic Event trait that has one method that accepts a generic Environment as parameter:

trait Event[T] {
  def exec(e: Environment[T])
}

For this Event trait, I provided the following implementation, where the exec() method accepts a parameter of the type MyEnvironment, to enable me to access the specific value of MyEnvironment.

class MyEvent extends Event[Integer] {
  override def exec(e: MyEnvironment): Unit = {
    println(e.specific)
  }
}

However, the Scala compilers outputs an error, from where it seems that MyEnvironment is not recognized as an Environment[Integer]:

Error: method exec overrides nothing.

Note: the super classes of class MyEvent contain the following, non final members named exec: def exec(t: main.vub.lidibm.test.Environment[Integer]): Unit

Is it possible to make this work, or are there patterns to circumvent this problem.

slouc slouc · Accepted Answer · 2016-03-15T23:17:30

You can't narrow down the signature of a method; it's not the same method any more. In your case, you can't override

def exec(e: Environment[T]): Unit

with

override def exec(e: MyEnvironment): Unit

Second method is more specific than the first one. It's conceptually the same as e.g. overriding def foo(a: Any) with def foo(s: String).

If you want it to work, you need to use the same type in both signatures (note that if you use an upper bound such as T <: Environment[_], that means that a method that accepts T actually accepts any subclass of Environment, so overriding using MyEnvironment will work OK in that case).

Overriding the method of a generic trait in Scala

2 Answers