Traits and abstract methods override in Scala

Question

I have a base abstract class (trait). It has an abstract method foo(). 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 foo() and then calls the derived class's foo().

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 I 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