class extensions

[Robert Fraser]

Michael Deardeuff Wrote:

> I am very excited about the future of D. Walter's and Andrei's talk at the conference was awesome.
> 
> However, there are still a few quirks to get out:
> This is regarding slide 9 of the presentation.
> 
> I noticed that most people at the conference liked the idea of extending library classes, but were not to pleased with the interchangability of 
> ---
> foo(a, args)
> ---
> and
> ---
> a.foo(args)
> ---
> like the arrays have.

Really? I got the feeling it was very well-received. I like the idea quite a bit, not necessarily just for class extensions (I think it might encourage a lower degree of encapsulation, although if the fields are package-private this might be mitigated by adding class extensions in other modules in the same package), but for primitive types.

> 
> I have a suggestion: class extensions kindof like C#'s partial classes
> ----
> class A {...}
> ...
> extension class A {
>    void foo(args) {...}
> }
> ...
> a.foo(args);
> ----
> (maybe the syntax could be "class extension A" or "extension A"...)
> 
> In this way class extensions are clearly marked and the syntax is consistent.
> 
> It would be easy to add interfaces to the library class:
> 
> extension A : Visitable {
>    void accept(Visitor v) {...}
> }
> 
> Just an idea for you guys, Walter, Andrei, and the community.
> 
> --Michael Deardeuff
> 

I proposed this a while ago, and I agree this would be a good idea. However, the more I think about it, the more I realize that in a native-compiled language we're talking vtable problems - how do we know when the vtable is finished? If partials are cross-module, it would require recompilation of every involved module, and just be generally problematic.

The suggestion that cam eup in the other thread was to use templates:
---
interface IFoo
{
    int bar();
}

template IFoo_Impl()
{
    int bar()
    {
        return 3;
    }
}

interface IBaz
{
    string quux();
}

template IBaz_Impl()
{
    string quux() { return "abc"; }
}

class Baz : IFoo, IBaz
{
    mixin IFoo_Impl!();
    mixin IBaz_Impl();
}
---