C#'s greatest mistakes

[Adam Burton]

Jonathan M Davis wrote:

> On Saturday 27 November 2010 14:59:09 BLS wrote:
>> On 27/11/2010 16:59, Torarin wrote:
>> > 2010/11/27 Andrei Alexandrescu<SeeWebsiteForEmail at erdani.org>:
>> >> We use template constraints for that kind of stuff.
>> >> 
>> >> Andrei
>> > 
>> > Yes, and that's great, but is there a way to check whether a template
>> > argument matches a defined interface?
>> 
>> I could not resist..
>> We should have Implements!
>> 
>> Luca has done some work on it.. but it does not compile anymore. However
>> I think the intension is clear.
>> 
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&a
>> rticle_id=101673
> 
> If you're checking whether it implements an interface, then : should work
> just like it does with classes. If you're checking whether it has the same
> functions that an interface requires, then you're not really using
> interfaces correctly. structs don't implement interfaces. Classes do that.
> So, either make it a class, or don't use interfaces. If you want to verify
> that a struct has particular functions needed for the template function in
> question, then just check whether calling them compiles ( e.g.
> __traits(compiles, foo.bar()) ). If you have a set of functions that are
> expected to be there (for instance, to verify that the given type is a
> certain type of range), then just create template which checks for each of
> the functions that are supposed to be there and use the template ( e.g.
> isForwardRange!T ). Interfaces and structs don't have anything to do with
> each other.
> 
> - Jonathan M Davis
The more I see these explanations the more I think interface is the wrong 
word for what it is. Structs and base types have a public interface. 
Contraints like isForwardRange are checking the public interface of the type 
passed too them. Seems to me Ds interfaces are more like types than 
interfaces (consider you can cast to an interface).

Personally I think having an "implements" makes sense (that loops through 
the methods of an interface and checks the type has matching methods, not 
the same as having a class "implement" an interface which is more like 
inheritance). However as you say, that is not how interfaces in their 
current form are designed to be used so I understand why people disagree.