Selftype: an idea from Scala for D2.0

[Reiner Pope]

eao197 wrote:
> Disclaimer. Yes, I know that Sensor could mixin Subject, not inherit it. 
> But I think there is probability that in some situation inheritance must 
> be used by some very important reasons.
I think mixins actually are the best way to do this. Here's how they 
would look with your example (note that there is very little 
modification of the code):

> template Subject(O, S)
> {
>     private O[] observers_ = [];
>     
>     public void subscribe(O observer)
>     {
>         observers_ ~= observer;
>     }
> 
>     public void publish()
>     {
>         foreach ( o; observers_ )
>             o.notify( this );
>     }
> }
> 
> interface Observer(S)
> {
>     void notify(S subj);
> }
> 
> class Sensor
>   {
>     mixin Subject!(Display, Sensor);
> 
>     private char[] label_;
>     private double value_;
> 
>     this( char[] label, double value )
>       {
>         label_ = label.dup;
>         value_ = value;
>       }
> 
>     char[] label() { return label_; }
>     double value() { return value_; }
>     void value( double v ) { value_ = v; publish; }
>   }
> 
> class Display : Observer!(Sensor)
>   {
>     void notify( Sensor subj )
>       {
>         Stdout( subj.label )( " has value " )( subj.value ).newline;
>       }
>   }

Using the syntax Subject is S automatically requires that any 
implementors of Subject!(Display, Sensor) inherit from Sensor. So why 
not simply make Sensor the inheritance root, as the mixin does? I don't 
see that any information is lost this way.

-- Reiner