Justin Johansson:
> I was wondering if any of the concepts regarding C++0x's "concept"
> keyword would be of interest to D people,
Concepts look nice on paper, but they add a good amount of complexity to a
language that's already dying for complexity overload. So in the end Concepts
will not be present in the first versions of C++x0.
D2 answer to Concepts are the Template Constraints, that are way simpler, they
don't require new keywords, and you need just seconds to learn what they are.
Template Constraints don't allow you to do everything you can do with Concepts,
but I like them, because they give you back more than the small amount of
complexity they add to the language.
One of the main purposes of Concepts is shared by Template Constraints, if you
instantiate the following Add templated function on a type that doesn't support
the plus operator, you get a compilation error at the calling point instead
inside the template.
T Add(T)(T x, T y) if (IsAddable!T) {
return x + y;
}
In practice the recent stacked error messages added by Don to D2 help find the
calling point of a template that lacks a Constraints too :-) (And G++ has a
similar feature that improves error messages).
"Groups" of Concepts are a bit less handy to manage in D compared to that C++x0
proposal, but I think I will not miss this too much.
In the proposal for Concepts there are Axioms too, and they are not present in
D2. One of the nice thing about Axioms is that they in theory will allow the
user to give more semantics to the compiler, I think this is something that
will be present in future compilers. But in practice I think all such
flexibility is not that important. A language with macros is much more flexible
than Java, but in practice you can write a large number of programs in Java. So
I think it can be enough to take just the most common usages of Axioms and to
"hard-code" them in D as few D2 attributes. This can mean to add to D just a
@associative and @symmetric annotations, that can be applied to
functions/methods to denote some of their qualities, that later can be used by
the optimizer stages of the compiler.
Bye,
bearophile
