Templates - Numeric Types only

[Jarrett Billingsley]

On Thu, Aug 6, 2009 at 9:53 PM, Andrei
Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
> Jarrett Billingsley wrote:
>>
>> On Thu, Aug 6, 2009 at 9:19 PM, Andrei
>> Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
>>>
>>> Kevin wrote:
>>>>
>>>> Hi,
>>>> simple Question:
>>>> Is it possible to allow just numeric types in templates?
>>>> For example to create a Vector Class like this:
>>>>
>>>> class Vector(T:"numeric" = float, int size = 4)
>>>>
>>>> Thanks,
>>>> Kevin
>>>
>>> import std.traits;
>>>
>>> class Vector(T, int size = 4) if (isNumeric!T)
>>> {
>>>   ...
>>> }
>>
>> Unfortunately,
>>
>> struct Vec(T) if(isNumeric!T) {}
>> struct Vec(T) {}
>> Vec!(int) x; // error
>>
>> foo.d(14): Error: template instance Vec!(int) matches more than one
>> template declaration, Vec(T) if (isNumeric!(T)) and Vec(T)
>>
>> *sigh*
>>
>> Wouldn't it be nice.
>
> struct Vec(T) if(isNumeric!T) {}
> struct Vec(T) if(!isNumeric!T) {}
>
> is actually superior because it's more modular; the semantics of the latter
> does not depend on the presence of the former.

But the condition of the latter now depends on the condition of the
former.  And on the condition of any other 'specializations':

struct Vec(T) if(isNumeric!T) {}
struct Vec(T) if(isSomeString!T) {}
struct Vec(T) if(!isNumeric!T) {} // uhoh
Vec!(string) x; // matches two

Now the fallback has to be declared as

struct Vec(T) if(!isNumeric!T && !isSomeString!T) {}

and it gets worse the more you add.