GoingNative 2012 to be livestreamed tomorrow - part 2

[Timon Gehr]

On 02/10/2012 02:44 PM, Jacob Carlborg wrote:
> On 2012-02-10 13:58, Timon Gehr wrote:
>> 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.
>
> We already have this for regular templates so why wouldn't it work. It's
> documented here: http://www.dlang.org/template.html
>
> Search for "Template Value Parameters".
>

I know the spec by heart :P. But you are right, your example does the 
same thing as bearophile's. I did not read his post carefully enough, I 
assumed he wanted the error message point to instantiation side of the 
constrained template.