The future of UDAs.

[Manu]

On 27 November 2012 09:42, Gor Gyolchanyan <gor.f.gyolchanyan at gmail.com>wrote:

> Hi, fellow D programmers.
>
> I'd like to know (and I think I'm not alone with this) the future plans
> about UDAs, the features they're planned to have and the features they're
> planned not to have.
> I have a curious project, which would be vastly easier and cleaner with
> proper UDAs, so naturally I'd like to know what to expect, because if what
> I need is planned to be available, I'll postpone my project, instead of
> rushing into an ugly solution.
>
> AFAIK, currently UDAs are set at declaration site, are immutable and the
> declaration cannot get additional UDAs externally.
>

This is true, and variable UDA's would be nice. Attributed variable
declarations it should be trivial, but I guess the problem is if you
attribute members of a struct, or any type its self for that matter, then
each instance of that type would have to have respective UDA instances,
that's not so simple. Where do they allocate? Are they part of the struct
or not?

Moreover, only global declarations can have UDAs, which removes some very
> useful possible uses of UDAs.
>

This is just not true. You can attribute basically anything. I attribute
shit loads of stuff in my project.

Lack of mutable compile-time variables sometimes cripples the
> metaprogramming in D. For instance all classes, derived from a certain type
> must be dealt with in a special way, which requires a tuple of those types.
> Gathering a tuple of unrelated types is currently impossible, because that
> would require mutable compile-time variables.
>
> Mutable compile-time variables would also be extremely useful for
> implementing a very powerful compile-time reflection library without the
> need for compiler magic. All you'd have to do is mix in a template in your
> class or your module and voila. The mixin would then add the introspected
> declarations to the central compile-time declaration repository.
>
> There are also many cases when some actions need to happen in case a
> declaration gets a UDA of a specific type. For instance, a powerful RTTI
> library, which (when a type gets a dedicated RTTI-typed UDA) adds the
> run-time information about the class to the central registry at load-time.
> This kind of stuff could be easily achieved using the constructors and
> destructors of the structures, being added to the UDAs of a declaration (in
> this case a class). The only missing thing for this to work is the ability
> for the constructor to see the declaration it's being put on. I'd
> personally expect there to be some sort of a
> __traits(getAttributeDeclaration), which would evaluate to the symbol on
> which the enclosing type (a structure, a union or whatever) is being placed
> on as an attribute.
>
> The point is, that with a tiny little boost, the UDAs could make D's
> meta-programming a tool so powerful, it would be very difficult to predict
> the limit of possibilities..
>
> Note, that this thread isn't about the syntax, but about the expected and
> planned functionality of UDAs and their use cases.
>
> Please share your thoughts about this.
>

I guess I'm curious too to hear peoples theoretical limits on the subject.
But what's there now handles about 80% of my use cases, and is a huge bonus!
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