Templates - Numeric Types only

[Bill Baxter]

On Thu, Aug 6, 2009 at 11:19 PM, Andrei
Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
> Jarrett Billingsley wrote:
>>
>> On Fri, Aug 7, 2009 at 12:54 AM, Andrei
>> Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
>>>
>>> Jarrett Billingsley wrote:
>>>>
>>>> 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.
>>>
>>> I know, and I explained why that's arguably better.
>>
>> So what's the solution?  Integrate template conditions into the
>> specialization rules?  Use this as a justification for their behavior?
>
> Right now the compiler evaluates if clauses early in the matching process,
> eliminates those that are false, and "forgets" those that are true. It would
> be possible to make if-clauses an integral part of partial ordering by
> postulating that an instantiation originating from a matched if clause is
> more specialized than a template without an if-clause. That would interfere
> with the other partial ordering rules.
>
>>  Or, like so many other features in D, leave it half-specified,
>> half-useful, and half-implemented because "it got too hard"?
>
> Which features of D are you referring to?

Design by Contract
"Order of declaration doesn't matter"
Protection level enforcement of various kinds
Property syntax
.stringof
__traits
Fixing the linker
CTFE (in particular memory management or lack thereof)
immutable (lack of data initialization)

are the unfinished things that come to mind.
But I would call those most of those things  90% useful / 90%
implemented rather than half so.
Maybe DbC is closer to half than 90%.

--bb