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November 27, 2012
The future of UDAs.
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. Moreover, only global
declarations can have UDAs, which removes some very useful possible uses of
UDAs.

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.

-- 
Bye,
Gor Gyolchanyan.
November 27, 2012
Re: The future of UDAs.
On 11/27/2012 6:42 PM, Gor Gyolchanyan wrote:
> AFAIK, currently UDAs are set at declaration site, are immutable and the
> declaration cannot get additional UDAs externally. Moreover, only global
> declarations can have UDAs, which removes some very useful possible uses
> of UDAs.

What very useful thing could be done with UDAs on local variables?


> 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.

I don't understand this. It is perfectly possible to create compile time 
tuple of (int, long, float, int*, S).


> 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.

I don't understand the difficulty here. UDAs are additive, so you could 
mixin your template in one space at the top, and it will apply to all 
the declarations nested under it, exactly like other attributes do now.


> 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.

That can be done now. Loop over the declarations, checking to see if 
each has a particular attribute.

> 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.

That could cause ugly forward reference issues, and also recall that a 
UDA can be applied to many declarations at once.
November 27, 2012
Re: The future of UDAs.
I think I really don't know what exactly do the UDAs do at the moment,
judging from your reply.The only documentation was the original D.anounce
post of yours. Is there a way I can get a reliable documentation on this?

Use cases:

1. I have a declaration with a UDA struct. I introspect the types I need
and add their names to an array in that struct. read that struct in a
static constructor and do necessary stuff with all those types. Is this
possible currently?

2. I have some code, which must be run on any type, marked with a special
UDA. Can I have that code executed without explicitly having to mix stuff
in every module?

I guess the way UDAs were originally designed, they have absolutely nothing
to do with what I envisioned.




On Tue, Nov 27, 2012 at 3:11 PM, Walter Bright
<newshound2@digitalmars.com>wrote:

> On 11/27/2012 6:42 PM, Gor Gyolchanyan wrote:
>
>> AFAIK, currently UDAs are set at declaration site, are immutable and the
>> declaration cannot get additional UDAs externally. Moreover, only global
>> declarations can have UDAs, which removes some very useful possible uses
>> of UDAs.
>>
>
> What very useful thing could be done with UDAs on local variables?
>
>
>
>  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.
>>
>
> I don't understand this. It is perfectly possible to create compile time
> tuple of (int, long, float, int*, S).
>
>
>
>  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.
>>
>
> I don't understand the difficulty here. UDAs are additive, so you could
> mixin your template in one space at the top, and it will apply to all the
> declarations nested under it, exactly like other attributes do now.
>
>
>
>  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.
>>
>
> That can be done now. Loop over the declarations, checking to see if each
> has a particular attribute.
>
>
>  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.
>>
>
> That could cause ugly forward reference issues, and also recall that a UDA
> can be applied to many declarations at once.
>
>


-- 
Bye,
Gor Gyolchanyan.
November 27, 2012
Re: The future of UDAs.
On 11/27/2012 10:27 PM, Gor Gyolchanyan wrote:
> I think I really don't know what exactly do the UDAs do at the moment,
> judging from your reply.The only documentation was the original
> D.anounce post of yours. Is there a way I can get a reliable
> documentation on this?
>
> Use cases:
>
> 1. I have a declaration with a UDA struct. I introspect the types I need
> and add their names to an array in that struct. read that struct in a
> static constructor and do necessary stuff with all those types. Is this
> possible currently?

Yes.


> 2. I have some code, which must be run on any type, marked with a
> special UDA. Can I have that code executed without explicitly having to
> mix stuff in every module?

It'll either have to be explicitly mixed in with every module that uses 
the UDA or some other code you write will explicitly need to apply it to 
each module that uses it. I suggest the former would be more convenient. 
Note that if a module is to use a UDA, you'll have to edit it anyway, so 
it shouldn't be a big issue to add the mixin.
November 28, 2012
Re: The future of UDAs.
So you're saying that UDAs are mutable after all? You can change a UDA but
you can't add a new one, right? If that's the case, it will solve ALL my
problems. I'll have all types marked with my struct and BAM! I have mutable
compile-time variables, which I solely needed.


On Wed, Nov 28, 2012 at 1:10 AM, Walter Bright
<newshound2@digitalmars.com>wrote:

> On 11/27/2012 10:27 PM, Gor Gyolchanyan wrote:
>
>> I think I really don't know what exactly do the UDAs do at the moment,
>> judging from your reply.The only documentation was the original
>> D.anounce post of yours. Is there a way I can get a reliable
>> documentation on this?
>>
>> Use cases:
>>
>> 1. I have a declaration with a UDA struct. I introspect the types I need
>> and add their names to an array in that struct. read that struct in a
>> static constructor and do necessary stuff with all those types. Is this
>> possible currently?
>>
>
> Yes.
>
>
>
>  2. I have some code, which must be run on any type, marked with a
>> special UDA. Can I have that code executed without explicitly having to
>> mix stuff in every module?
>>
>
> It'll either have to be explicitly mixed in with every module that uses
> the UDA or some other code you write will explicitly need to apply it to
> each module that uses it. I suggest the former would be more convenient.
> Note that if a module is to use a UDA, you'll have to edit it anyway, so it
> shouldn't be a big issue to add the mixin.
>
>


-- 
Bye,
Gor Gyolchanyan.
November 28, 2012
Re: The future of UDAs.
On 11/28/2012 07:38 AM, Gor Gyolchanyan wrote:
> So you're saying that UDAs are mutable after all? You can change a UDA
> but you can't add a new one, right? If that's the case, it will solve
> ALL my problems. I'll have all types marked with my struct and BAM! I
> have mutable compile-time variables, which I solely needed.
> ...

There is no such thing as a 'mutable compile-time variable'. How would 
such a thing behave? The term is meaningless until this is explained.
November 28, 2012
Re: The future of UDAs.
On 11/28/2012 5:38 PM, Gor Gyolchanyan wrote:
> So you're saying that UDAs are mutable after all? You can change a UDA
> but you can't add a new one, right? If that's the case, it will solve
> ALL my problems. I'll have all types marked with my struct and BAM! I
> have mutable compile-time variables, which I solely needed.

I don't know what you mean.

The static constructor thing happens at runtime.
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