GoingNative 2012 to be livestreamed tomorrow - part 2

[Timon Gehr]

On 02/10/2012 08:17 AM, Jacob Carlborg wrote:
> On 2012-02-10 02:47, bearophile wrote:
>> Some more comments about the conference.
>>
>> --------------------------
>>
>> About "Variadic Templates are Funadic" by Andrei Alexandrescu fun talk:
>>
>> I have had to see it at only 1-1.1X speed, to understand the language.
>>
>> I can't see the laser spot in the video :-(
>>
>> Thank you to Walter for designing D varidic templates in a simpler
>> way. C++11 variadic templates look too much complex and
>> over-engineered (example: the lockstep expansion seems a bit crazy).
>>
>> Slide 21: I didn't know that default is OK as first switch case too :-)
>>
>> Even the questions&answers part of this talk was interesting enough.
>>
>> ------------------
>>
>> About Bjarne Stroustrup and Andrew Sutton "A Concept Design for C++":
>>
>> Regarding this code in Slide 12:
>>
>> template<Number Num> Num gsqrt(Num);
>> gsqrt(2); // fine
>> gsqrt("Silly!"); // error: char* is not a Number
>>
>>
>> In D template constraints have two (or more) different usages:
>> 1) To just remove a template from the pool of the usable ones;
>> 2) In other situations only one template is present, and its
>> constraints are a way to give it some static typing. In this case I'd
>> like better error messages.
>>
>>
>> Once this patch is applied:
>> https://github.com/D-Programming-Language/dmd/pull/692
>>
>> you are able to write something like this, that isn't exceptionally
>> nice looking, but it's useful (it's going to make Phobos code a bit
>> more hairy, but the user is going to see some better error messages):
>>
>>
>> template IsNumberWithError(T, string file, int line) {
>> enum bool IsNumberWithError = is( ...
>> static if (!IsNumberWithError)
>> __ctfeWriteln(file, "(", line, "): '", typeid(T), "' is not a number.");
>> }
>>
>> double gsqrt(T)(T x) if (IsNumberWithError!(T, __FILE__, __LINE__)) {
>> /*...*/ }
>
> Wouldn't this be possible:
>
>
> template IsNumberWithError(T, string file = __FILE__, int line =
> __LINE__) {
> enum bool IsNumberWithError = is( ...
> static if (!IsNumberWithError)
> __ctfeWriteln(file, "(", line, "): '", typeid(T), "' is not a number.");
> }
>
> double gsqrt(T)(T x) if (IsNumberWithError!(T)) { /*...*/ }
>
> __FILE__ and __LINE__ would be picked up from the "calling" point and
> not the declaration point of IsNumberWithError. We already have this in
> some cases.
>

Unfortunately, they would point to the template constraint. But I think 
it would be a quite useful enhancement to pick up __FILE__ and __LINE__ 
at template instantiation point for the template constraint.