extends Trait versus with Trait

Hi,
Why do we have to extend a trait when we do _not_  extend a superclass.So a class that extends nothing, and you want to mix in a trait, you have to extend the trait. But when you extend some superclass you have to use a trait.
This is confusing for me. It would for me make more sense that extending a superclass uses 'extend'.And one ore more traits (with our without a superclass) use 'with' .
When you look at the definitions this might sound better.For example Frog extends Animal  (Animal as class) makes sense because it reads as "Frog is an Animal"
suppose a trait JumpingCapabilities,  would defineClass Frog extends Animal with JumpingCapabilities"A Frog is an Animal, has JumpingCapabilities"
But without the subclassing one must define: class Frog extends JumpingCapabilities"A Frog is a JumpingCapabilities" ... <-- doesn't make sense to me.
Maybe I do not understand it correctly, could someone shed a light on 'extends Trait'?
tia,
Alex

> Another advantage of the new convention is that subclasses are not affected
> when a class is changed to a trait or vice versa, something that happens
> quite often and naturally.

Ah, I didn't think of that one. Clever indeed, I understand.

Thanks

Alex

>
> Cheers
>
>  -- Martin
>
>
>
>
>
> On Wed, Jul 6, 2011 at 3:51 PM, Alex Wouda wrote:
>
> > Hi,
>
> > Why do we have to extend a trait when we do _not_  extend a superclass.
> > So a class that extends nothing, and you want to mix in a trait, you have
> > to extend the trait.
> > But when you extend some superclass you have to use a trait.
>
> > This is confusing for me. It would for me make more sense that extending a
> > superclass uses 'extend'.
> > And one ore more traits (with our without a superclass) use 'with' .
>
> > When you look at the definitions this might sound better.
> > For example Frog extends Animal  (Animal as class) makes sense because it
> > reads as
> > "Frog is an Animal"
>
> > suppose a trait JumpingCapabilities,  would define
> > Class Frog extends Animal with JumpingCapabilities
> > "A Frog is an Animal, has JumpingCapabilities"
>
> > But without the subclassing one must define:
> > class Frog extends JumpingCapabilities
> > "A Frog is a JumpingCapabilities" ... <-- doesn't make sense to me.
>
> > Maybe I do not understand it correctly, could someone shed a light on
> > 'extends Trait'?
>
> > tia,
>
> > Alex
>
> --
> Martin Odersky
> Prof., EPFL and Chairman, Typesafe
> PSED, 1015 Lausanne, Switzerland
> Tel. EPFL: +41 21 693 6863
> Tel. Typesafe: +41 21 691 4967