concepts and interfaces

[janderson]

Martin Hinsch wrote:
> Inspired by this video (linked by an earlier post, and very interesting; I really hope that D will get something similar)
> 
> http://video.google.com/videoplay?docid=-1790714981047186825&hl=en
> 
> I started to think about the relation of concepts and interfaces.
> In a way both serve very similar purposes, namely supporting polymorphism. The major difference is of course that one is used at compile time and the other one at run-time. Still, in the end both are just a way to describe the constraints which have to be fulfilled by a type in order to be usable in a specific generic way.
> Interestingly the syntactic differences are nevertheless considerable. 
> My main point in this post is that this is mainly for historical reasons and that it might be beneficial to think about making interfaces and concepts more similar to each other.
> 
> Some more specific thoughts:
> 
> - why does conformance to an interface have to be specified explicitly (as in class C implements interface I)? Why not do post-hoc checking as with concepts/templates?
> Using an object via an interface implies two things - the compiler has to check whether the objects class actually implements all methods required by the interface and the object reference has to be manipulated in such a way that when using it when assigned to an interface reference the right method calls are done. If we assume that all casts from an object to an interface reference are visible at compile time both of these could be done *at the point of the cast* (equivalent to the way type checking is done for template instantiation).
> 
> - why not make concepts obligatory?
> One of the beneficial aspects of using interfaces is in my opinion that it forces you to be specific about which properties you expect from the plugged in types. This is something which is missing from generic programming and which is supposed to be improved by introducing concepts. C++ of course has to stay backwards-compatible therefore concepts will be completely optional.
> In D OTOH concepts could actually be made obligatory forcing the programmer to be explicit about the properties of the types which can be plugged into a template/generic class.
> 
> - interface_maps
> With implicit implementation of interfaces the concept_map idea might come in handy for interfaces as well, allowing to do some adjustments to make a class conforming to an interface.
> 
> Just some food for thought.
> 
> cheers
> Martin Hinsch

I'm not sure D necessarily needs to support this the same way as C++. 
However the C++ developers of concepts are very smart people and have 
obviously put a lot of thought into this.  We should consider all options.

It would be nice when C++0x comes out we could say, D's had all these 
features for years.  If your going to switch, why not switch to D.

-Joel