scala: traits and abstract methods override

Question

I have a base abstract class (trait). It has an abstract method meth(). It is extended and implemented by several derived classes. I want to create a trait that can be mixed into the derived classes so that it implements meth() and then calls the derived class's meth()

Something like:

trait Foo {
  def foo()
}

trait M extends Foo {
  override def foo() {println("M"); super.foo()}
}

class FooImpl1 extends Foo {
  override def foo() {println("Impl")}
}

class FooImpl2 extends FooImpl1 with M 

I tried self types and structural types, but can't get it to work.

Answer 1

You were very close. Add the abstract modifier to M.foo, and you have the 'Stackable Trait' pattern: http://www.artima.com/scalazine/articles/stackable_trait_pattern.html

trait Foo {
  def foo()
}

trait M extends Foo {
  abstract override def foo() {println("M"); super.foo()}
}

class FooImpl1 extends Foo {
  override def foo() {println("Impl")}
}

class FooImpl2 extends FooImpl1 with M