Access modifier for extensions

[Boyd]

Hi there,

I have an idea for an access modifier that I wanted to throw out 
there, in the hopes that it will resonate with someone.

Basically I've found that in practice, when building a module, I 
always need to keep two kind of users in mind. Those who want to 
use it, and those who want to extend it.

For extending, there is the 'protected' attribute, but it's 
specific for class overriding only. Very often, extensions are 
not merely limited to derived classes. What I find myself wanting 
is more like the 'package' attribute, except it needs to work 
outside of the package as well.

So right now, the only viable choice is making all functionality 
for users and extenders 'public'.

So let's imagine the 'extendable' attribute for a moment. Here's 
what I would like it to do:

Let's say someone built a GUI library, and without access to the 
library, I want to add support for another platform.

------
module buttoncontrol

public class ButtonControl
{
     public Event pressEvent;
     extendable void click(Point position);
}
------

Now, you don't want a user to be able to generate a click. But 
someone whose purpose it is to extend the library, should be able 
to do it.

extender
------
import extendable buttoncontrol;

void click(Point position)
{
     auto button = findButtonUnderCursor(position);
     button.click(position);
}
------

user
------
import buttoncontrol;

void createUi()
{
     auto button = new ButtonControl();
     button.click(Point(10,10)); // ERROR, no access to click 
function
}
------

So, that's the general idea. I haven't thought very hard about 
the syntax part, so ignore that. Let me know what you think. Is 
it useful? feasible? worth the effort?

Cheers,
Boyd