User Defined Attributes

[Jacob Carlborg]

On 2012-11-06 15:23, Gor Gyolchanyan wrote:

> What if the data in the attribute needs to be specific to the symbol on
> which the attribute is set on? Can you query the symbol from inside the
> attribute?
> What if the attribute needs to change the symbol being defined?
> For instance (using commonly desired syntax):
>
> @flags enum A { ... }
>
> the "flags" attribute would replace the declaraction of A with another
> enum declaration with the same name and same members, but with replaced
> initialization and would static assert(false, "flags enum can't have
> initializers") if any initializers are given.
>
> The existing "[data] declaration" is one thing. It adds compile-time
> data to symbols, which is very very important, but it's not the only
> thing that is needed. What I described above is not quite an attribute,
> but an annotation. The semantics of attributes as Walter made them is
> perfect. The annotation should be  defined like so:
>
> template myAnnotation(alias symbol, annot_args...)
> {
>      // ...
>      ["myAnnoration adds this attribute"] alias symbol myAnnotation; //
> or alias anything else if a change in necessary.
> }
>
> @myAnnotation(arg1, arg2, arg3) class Declaration
> {
> }
>
> here the "symbol" is passed to the template as the first parameter and
> the parameters in the parentheses are passes after it. It's basically a
> thin syntactic sugar over some manual (and very ugly) manual template
> instantiations (and mixins), so it doesn't add anything new.
>
> Annotations then become a way to statically replace declarations.
> This would be perfect to replace
>
> Scoped!MyClass mc;
>
> with:
>
> @scoped MyClass mc;
>
> Which looks quite like a very beautiful built-in syntax, but is actually
> a library solution.
>
> IMHO, Attributes as they are should stay, but annotations should be added.
>
> P.S. Thank you, Walter very very much for making attributes!!! :-)

I like this proposal as well.

-- 
/Jacob Carlborg