Compilation error while implementing Trait

Question

Given this code: (no matter whether it doesn't make much sense)

    object Test {

          def main(args: Array[String]) {
           (new FooImpl2()).foo()
          }

          trait Foo {
            def foo()
          }

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

          abstract class FooImpl1 extends Foo {

          }

          class FooImpl2 extends FooImpl1 with M{
            override def foo() {println("Impl2")}
          }

    }

At compile time, this error occurs:

error: overriding method foo in trait M of type ()Unit;
method foo needs `abstract override' modifiers
override def foo() {println("Impl2")}

So at this place:

class FooImpl2 extends FooImpl1 with M{
    override def foo() {println("Impl2")}
}

Why doesn't override apply on FooImpl1 (in order to provide a concrete implementation for abstract trait method)? It seems like it matches the trait method's instead...and obviously there's a huge conflict with the pattern "abstract override"

som-snytt · Accepted Answer · 2012-12-12 17:31:58Z

1

M needs to be mixed-in after the concrete def (in linearization order).

answered Dec 12, 2012 at 17:31

som-snytt

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1 Comment

Mik378 Over a year ago

Thanks :) So that explains why this code works: class FooImpl1 extends Foo { def foo() {println("Impl")} } class FooImpl2 extends FooImpl1 with M{ override def foo() {println("Impl2")} } since concrete method implementation is defined before M is mixed-in.

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