[mattnewport]

> The most obvious examples are ML and its derivatives.

I'm reasonably familiar with F# but I know that's strayed quite far from its ML roots and its generics are constrained to some degree by what the CLI supports. For the kinds of generic programming that C++ does well it's not clear to me where the ML style is superior. Can you give some specifics?

> If C++ had an actual notion of abstract types, then templates could be type-checked against ADT specifications (so-called “concepts”), rather than having to wait until instantiation time.

Yep, that's why everyone wants Concepts to be standardized.

> You can't encapsulate a single abstraction providing two or more ADTs without using `friend`. That's a fact.

I'm still not clear what the issue is here. A C++ class can implement multiple interfaces for dynamic polymorphism without using friend and can implement more than one 'concept' for static polymorphism without using friend. Can you explain what you mean in more detail?

> This is a reply to the wrong thing. Concepts are ADT specifications, not object specifications.

Are you talking about something like Go interfaces? For my use cases static polymorphism is generally preferable to dynamic polymorphism and in those cases where dynamic polymorphism is desirable C++ style interfaces are often sufficient. When the need arises for something like a Go interface in C++ people usually use type erasure but implementing that currently requires a bit more boilerplate than would be ideal (though there are libraries that help). That's something that could be solved in the future by something like Herb Sutter's metaclass proposal. Perhaps I'm still not understanding your point though.

[catnaroek]

> Can you give some specifics?

Concepts are what ML and Haskell have been calling “signatures” and “type classes”, respectively, for ages. Unlike concepts, which only exist in the collective minds of C++ programmers, signatures and type classes are actual features of existing type systems, so, for instance, the type checker can make sure that you aren't trying to use an inexistent (member) function - without ever attempting to instantiate your generic code.

It is very unfortunate that F# got rid of this important feature.

> Yep, that's why everyone wants Concepts to be standardized.

What I'm telling you is other languages have had this very same feature for well over two decades.

> I'm still not clear what the issue is here.

Here's a thought exercise: Design an API for manipulating graphs, nodes and edges. The concrete representation of these three types must be hidden from the user by language-enforced mechanisms. Using `friend` is not allowed. Using the pimpl pattern is not allowed. Bypassing type safety is not allowed.

The fundamental problem that you'll run into is that C++'s access levels work with one type at a time. ML modules don't have this problem, because a single signature can specify multiple abstract types. An implementation can have access to the representations of all three types (graph, node, edge), while at the same time hiding these representations from all clients.

> Are you talking about something like Go interfaces?

Yes.

> For my use cases static polymorphism is generally preferable to dynamic polymorphism and in those cases where dynamic polymorphism is desirable C++ style interfaces are often sufficient.

I agree: static polymorphism is preferable whenever possible. It is easier to reason about both for language users (who care about correctness) and compiler writers (who care about optimizations). However, sometimes you want dynamic polymorphism, and that's what objects (in the object-orientation sense, which you can think of as “thingies that have vtables”) are for.

[mattnewport]

I think the general consensus of the C++ community is that we need Concepts or something like it. I don't think anyone is really claiming no other language offers something similar. I'd love to learn more Haskell but for my primary domain (games and VR) it's not a terribly practical option. This gets back to my original point - yes Concepts will make C++ more complex (in the sense of adding features) but I think it will make the language better / simpler to use in practice and I look forward to it being standardized and widely available so I can use it.

> The concrete representation of these three types must be hidden from the user by language-enforced mechanisms.

This seems to be more an issue of encapsulation than support for ADTs. There's value in hiding concrete representations (ABI compatibility) but C++ works well for my use cases most of the time with concrete representations visible (and this helps with performance which is important in my domain).

As a language user I don't think it's just compiler writers who care about performance :)

I do see the value in something like Go interfaces although it's not a problem I encounter that frequently in my domain. Type erasure is a handy technique in those situations and I think better language support to eliminate some of the boiler plate is desirable. More complexity :)

[catnaroek]

> I'd love to learn more Haskell but for my primary domain (games and VR) it's not a terribly practical option.

Oh, sure, Haskell has lots of defects. (Chiefly among those, being lazy.) I only said that it has something that's essentially concepts, except it has been designed, implemented and used since ages ago.

> This seems to be more an issue of encapsulation than support for ADTs.

The whole point to ADTs is that clients don't get to manipulate the internal representation! What exactly is that, if not encapsulation?

> most of the time with concrete representations visible (and this helps with performance which is important in my domain).

ADTs are about hiding the representation from abstraction clients (other programmers), not from the compiler, of course! In fact, a compiler writer could use ADTs solely for type-checking purposes, and from then onwards proceed as if ADTs didn't exist. So I don't see how using (proper) ADTs must have any adverse effect on performance.

> I do see the value in something like Go interfaces although it's not a problem I encounter that frequently in my domain.

I have to agree, I don't use objects with dynamically dispatched methods much either. My original point was just that C++ classes are neither good ADTs nor good object builders. They're at an uncomfortable point at the middle, with the disadvantages of both, and the advantages of neither.

[mattnewport]

> I only said that it has something that's essentially concepts, except it has been designed, implemented and used since ages ago.

I'm not sure why this is relevant to the topic at hand though, other than historical interest. What's relevant is that something like concepts are a useful thing for a language to have and C++ will be a better / more usable language with them, even if it means adding 'complexity'.

> The whole point to ADTs is that clients don't get to manipulate the internal representation! What exactly is that, if not encapsulation?

You said "The concrete representation of these three types must be hidden from the user" and mentioned the pimpl pattern which led me to think you were talking about ABI issues. In C++ generally private members are not accessible but they are visible (in headers) and affect object size and layout. That can be a problem for build times and for versioning / binary compatibility but it also allows for private functions to be inlined and avoids pointer indirections and simplifies certain other optimizations (devirtualization for example).

C++ does not currently have a language level concept of modules (and I'm not sure the modules proposal working its way through standardization addresses your issue here) or anything like the C# internal access level. There are patterns to structure your code to enable implementation hiding for collaborating classes in a 'module' but they don't tend to be very widely used due to lack of first class language support. In my own experience I haven't found this to be a huge issue but maybe I just don't know what I'm missing.

[AnimalMuppet]

> Here's a thought exercise: Design an API for manipulating graphs, nodes and edges. The concrete representation of these three types must be hidden from the user by language-enforced mechanisms. Using `friend` is not allowed. Using the pimpl pattern is not allowed. Bypassing type safety is not allowed.

Perhaps I am not understanding your example, but...

I'd have a Node class, an Edge class, and a Graph class. None would be friends of any of the others. Many of the API functions would take a Graph (plus other parameters), but you might like them to be able to operate on an Edge or even a Node as well. But the way I think you handle this is by having a conversion operator (which is also a constructor). That is, if I have a Graph constructor that takes an Edge parameter, and one that takes a Node parameter, now I can use an Edge or a Node as a parameter to a function that takes a Graph.

Note that this does not require Graph to know about the internal details of Edge or Node. Also, it does not bypass type safety. It does sometimes return a different type than you passed in, but I'd argue that you want that: If you call addEdgeToGraph you expect to get a Graph back, and you will, even if you passed in a Node in place of the Graph.