Sealed classes - would you want them in D? (v2)

[Mike Parker]

On Friday, 18 May 2018 at 12:16:55 UTC, aliak wrote:

> You may not need a new word at all. You can also enhance 
> private to take arguments. Package already does this. You can 
> give private a symbol list that says which symbols this is 
> private for. So:
>
> class A {
>   private int x;
>   private(A) int y;
> }
> void main() {
>   A a = new A();
>   a.x = 7; // ok, it's private to module
>   a.y = 3; // error, it's sealed to class
> }
>

That's actually a decent way to go about it. It matches the 
package syntax and avoids new keywords. It's also not as onerous 
as repurposing an existing keyword (like `module`), nor does it 
cause any breakage. Though, I would argue that rather than 
allowing any old symbol, it be restricted to `this`:

private(this) int y;

This keeps the implementation simple and the scope focused. If a 
DIP were put forward, I think this would be the approach to take. 
Though a fairly strong case will still need to be made as to why 
this is beneficial (use cases, example code, etc). To bolster the 
case, I would look at the reasoning behind the recommendation in 
Java that classes use the public API internally rather than 
manipulating member variables directly to improve maintainability.