GoingNative 2012 to be livestreamed tomorrow - part 2

[Jacob Carlborg]

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".

-- 
/Jacob Carlborg