[D.typesystem] Suggestion for improving OO inheritance models

[retard]

Wed, 01 Sep 2010 23:30:08 +0200, Jacob Carlborg wrote:

> On 2010-09-01 22:44, Philippe Sigaud wrote:
>> On Wed, Sep 1, 2010 at 18:13, retard <re at tard.com.invalid> wrote:
>>
>>
>>     Have you taken a loot at Scala & traits already? It would be a
>>     great starting point.
>>
>>
>> Scala's traits are great! Implicits in Scala are quite interesting too.
>> Also, Haskell typeclasses
>>
>> I wonder if D can have part of Scala traits functionality with mixins?
> 
> You can't use D template mixins to add methods that will overload
> existing methods.

I'm afraid D's template mixins are like a poor man's copy/paste macro 
system. Scala's approach takes subtyping into account. D 2 took a step 
towards traits actually since it allows some implementations & contracts 
in interfaces. However, Scala is more flexible. Scala's design had the 
focus on nice OOP properties the whole time, but D will probably only get 
features if implementing them efficiently is a low hanging fruit.

One interesting feature in Scala are the types of objects in this case:

class A {}
trait T {}

class B extends A with T {}

val foo = (new B) : (A with T)

--

I suppose D doesn't allow this:

class A {}
interface T {}

class B : A, T {}

(A & T) foo = new B


In Scala the inheritance graph looks like:

 Any
  |
AnyRef
  |
Object
  |
  A      ____ T
  |     /
A with T
  |
  B

In D it's just:

 Object
   |
   A  ____T
   | /
   B