A module comprehensive template-specialization

[Rory McGuire]

On Mon, 28 Jun 2010 11:09:13 +0200, Matthias Walter  
<xammy at xammy.homelinux.net> wrote:

> On 06/28/2010 09:49 AM, Justin Spahr-Summers wrote:
>> On Sun, 27 Jun 2010 18:51:35 +0200, Matthias Walter
>> <xammy at xammy.homelinux.net> wrote:
>>>
>>> Hi list,
>>>
>>> I tried to write a traits class comparable to iterator_traits in C++  
>>> STL
>>> or graph_traits in Boost Graph Library in D 2.0, but failed to do so  
>>> via
>>> template specialization which is put into different modules. Putting
>>> everything into one module interferes with extensibility. I tried the
>>> following:
>>>
>>> == Module a ==
>>> | module a;
>>> |
>>> | template Base (T)
>>> | {
>>> |   alias T Base;
>>> | }
>>>
>>> == Module b ==
>>> | module b;
>>> |
>>> | import a;
>>> |
>>> | template Base(T: T*)
>>> | {
>>> |   alias Base !(T) Base;
>>> | }
>>>
>>> == Main module ==
>>> |
>>> |  import a, b;
>>> |
>>> | int main(char[][] args)
>>> | {
>>> |   alias Base !(int*) foo;
>>> |
>>> |   return 0;
>>> | }
>>>
>>> The error message is:
>>> "bug.d(8): Error: template instance ambiguous template declaration
>>> b.Base(T : T*) and a.Base(T)"
>>>
>>> Can I handle this in another way (like making the template a  
>>> conditional
>>> one)?
>>>
>>> best regards
>>> Matthias Walter
>>
>> I believe this is intended behavior, as it prevents template hijacking
>> and the like. Using alias to import the two templates into the same
>> scope might help, though I'm not sure exactly how it should be done.
>
> I tried to do so in some variants but did not succeed unfortunately. If
> you have a precise idea, please let me know!
>
>>
>> On another note, though, have you looked at __traits() and std.traits?
>
> I looked at them but didn't find them helpful for this precise problem.
> The whole reason for doing this is to make it possible to make another
> existing class model the concept (i.e. have some aliases / typedefs
> done) of my library class without editing any of them. As I mentioned in
> my other response, a prominent example for Boost Graph
> Library is the LEDA graph class, which can be enabled to be used by BGL
> by more or less just specializing the graph_traits template. I'd like to
> have this kind of technique available, too.
>
> Any further suggestions?

I haven't looked at the boost stuff you mention but is it possible that  
using alias this, solves a similar or the same problem?
TDPL addresses the use of aliasing to bring multiple declarations into the  
same scope/module but it only uses actual functions not templates.