template parameter list syntax

[Patrick Schluter]

On Sunday, 20 January 2019 at 16:22:09 UTC, Carl Sturtivant wrote:
> On Sunday, 20 January 2019 at 12:35:58 UTC, Patrick Schluter 
> wrote:
>> On Saturday, 19 January 2019 at 22:52:25 UTC, Carl Sturtivant
>>
>> Afaik it's a parser problem. Walter wanted to avoid the 
>> catastrophic state that exist in C++ of an ambiguous grammar. 
>> He insited that the the grammar of the D language be as 
>> straightforward as can be.
>> In the template declaration, both parenthesis pairs are 
>> required and the parser knows directly that first '(' == 
>> template parameter, second '(' rt parameter.
>> At the instanciation site, it was preferred to have parameter 
>> inference, i.e. that the template parameter [1] can be omitted 
>> if it can be deduced from the runtime parameter. This has the 
>> consequence that the starting '(' becomes ambiguous, it can 
>> mean template parameter start or runtime parameter start. 
>> Using another operator allows to lift the ambiguity. Forcing 
>> to have parenthesis would also lift the ambiguity, but it 
>> would be at the cost of parameter inference as it would be 
>> impossible distinguish between infered parameter or no 
>> parameter. ! was chosen because it is light and its only use 
>> is in prefix context, in template instantiation it's in a 
>> infix position and thus completely unambiguous with the ! 
>> operator.
>> [1]: https://tour.dlang.org/tour/en/basics/templates
>
> You've given a more detailed version of a response given 
> earlier.
> https://forum.dlang.org/post/xypridqjaqxosnxaqbjm@forum.dlang.org

The point is that Walter Bright insists on context free grammar. 
This is a key point that allows for fast parsing. C++ is in the 
mess it is because it cannot be parsed (without heroic efforts). 
Every syntax choice has to abide by that rule. Template parameter 
inference was deemed MORE important than a slight inconsistency 
in the syntax of definition and use of the template. If that had 
not been the case, you would have to explicitely give always all 
type parameter in the template instanciation, every single time. 
This would make the code much more tedious, would violate DRY 
principle and would not even work properly as there are types in 
D that cannot even be written out (Voldemort types).
D would be so much less expressive because of that.
So, this slight inconsistency is a small price to pay for the 
benefits it provides.

>
> So my reply to that is also pertinent here.
> https://forum.dlang.org/post/anlnnizkcohoznigynpj@forum.dlang.org