Sealed classes - would you want them in D?

[rikki cattermole]

On 11/05/2018 1:22 AM, Piotr Mitana wrote:
> Hello,
> 
> I've recently thought of sealed classes (sealed as in Scala, not as in 
> C#) and am thinking of writing a DIP for it. I decided to start this 
> thread first though to gather some opinions on the topic.
> 
> For those who never coded Scala and don't know sealed classes: a sealed 
> class is a class which can be only extended in the same source file.
> 
>      sealed class MyClass {}
> 
> Translating to D, a sealed class would could only be extended in the 
> same module. Additionally I would suggest "sealed(some.package) MyClass 
> {}" syntax for classes that could only be extended in the particular 
> package.
> 
> In Scala an important value of sealed classes is that compiler knows 
> that and can aid programmer in pattern matching by detecting the missed 
> cases. However, there is another value I see: limiting the extension of 
> classes.
> 
> Some time ago I was working on a simple CQRS library and created an 
> Entity class, where I wanted only a few specific subtypes for it. The 
> library is built over a few defined entity types and user is not able to 
> create their own types (should extend the particular subtype instead). 
> Sealing the Entity class could enable me to define the closed set of 
> subtypes and prevent new direct subtype creation outside the library.
> 
> Combining sealed with final library developer can create a completely 
> closed type hierarchy.
> 
> Any thoughts on that proposal?

Adding a keyword like sealed isn't desirable.

I'm trying to find fault of the concept, but it definitely is tough.

You basically want protected, but only for specific packages, otherwise 
final.

protected(foo, final)