Is there any good reason why C++ namespaces are "closed" in D?

[Steven Schveighoffer]

On 7/30/18 12:45 PM, Manu wrote:
> On Mon, 30 Jul 2018 at 01:45, Walter Bright via Digitalmars-d
> <digitalmars-d at puremagic.com> wrote:
>>
>> On 7/29/2018 8:05 PM, Manu wrote:
>>> ab.d
>>> ------
>>> module ab;
>>> extern(C++, "ab") void foo();
>>>
>>>
>>> cd.d
>>> ------
>>> module cd;
>>> extern(C++, "cd") void foo();
>>>
>>> Perfect!
>>
>>     extern(C++, ab) void foo();
>>     extern(C++, cd) void foo();
>>
>> Perfect!
>>
>>     struct S1 { extern(C++, ns) void foo(); } alias foo = S1.ns.foo;
>>     struct S2 { extern(C++, ns) void bar(); } alias bar = S2.ns.bar;
>>
>> Perfect!

What is this? I don't get what you are doing here.

>>> We have what already exists, that can't go away now, but please just give us
>>> string namespaces as a mangle-only option. Not only will I shut up about it
>>> forever, but we will also be able to specify namespaces that conflict with D
>>> keywords, and also organise our modules just like all other D code ever.
>>
>> I'm the one that gets stuck with the fallout from bad designs. The solution I
>> proposed above will work for you as best as I can tell. And it works right now,
>> you don't have to wait for another version.
> 
> Sure it 'works', but it's an annoying point of friction, and it
> doesn't need to be that way.
> It requires over-specification of the namespace at calls, and it
> creates friction for scanning meta.
> It offers nothing, and only makes things harder and unintuitive.
> 
> Why do you have such a strong opinion about a thing that you don't use?
> None of the customers ever suggested it should be like it is.
> Just let us mangle our symbols! We'll design our modules how we like;
> we're not morons, we want to present API's to users in logical and
> sensible ways.
> 
> 
>> You haven't explained why you can't just move the namespace ns declarations in
>> one file together.
> 
> Are you serious?
> 
> https://github.com/dlang/druntime/pull/2259 <- There, I fixed it for
> you. I can't wait to get all the container classes in there too!
> If you really believe that, and not just trolling me, then click merge.

hehe, you need to put them all into one extern(C++, std), right? Not to 
mention having std as an identifier is somewhat problematic in D.

> Why would I want to have one gigantic module with everything dumped in
> it? No end-user wants that.
> We just want to use D as it is. What is so bad about D's module system
> that you plainly refuse to let us use it as it was intended?
> 
> Just allow a string version. It's going to be okay. Everybody will be
> happier. The sky won't fall.
> When torrents of people complain, you can send every single one of
> them to me, tell me "I told you so", and I'll eat my hat.
> 

While I agree with Manu that extern(C++, ns) would be better as just 
mangling, please don't just make the difference that it's a string 
literal vs. symbol. That difference is going to be confusing. Not to 
mention the abuse from calling CTFE functions. e.g.:

string ns() { return "theNamespace"; }

extern(C++, ns) or extern(C++, ns())?

I'd suggest something like (shudder) extern(mangleC++, ns).

None of this is attractive...

Note that namespaces in D have been doable via templates or classes or 
structs for a long time. There is really no need to deal with C++ 
namespaces in D.

-Steve