C#'s greatest mistakes

[Andrei Alexandrescu]

On 11/27/10 7:55 PM, Adam Burton wrote:
> 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.

Walter and I discussed a number of times allowing structs to inherit 
interfaces. I even submitted an enhancement request once. Unfortunately, 
the feature wouldn't be a game-changer because interfaces are quite 
rigid - for example, you often want to enforce that a template defines a 
type. Before you know it, you need to add language to interfaces that 
makes them start looking like the stillborn C++ concepts. I think 
template constraints are a better solution.


Andrei