User Defined Attributes

[deadalnix]

Le 07/11/2012 23:20, Walter Bright a écrit :
> On 11/7/2012 3:05 AM, Leandro Lucarella wrote:
>> OK, that's another thing. And maybe a reason for listening to people
>> having
>> more experience with UDAs than you.
>>
>> For me the analogy with Exceptions is pretty good. The issues an
>> conveniences
>> of throwing anything or annotating a symbol with anything instead of just
>> type are pretty much the same. I only see functions making sense to be
>> accepted
>> as annotations too (that's what Python do with annotations,
>> @annotation symbol
>> is the same as symbol = annotation(symbol), but is quite a different
>> language).
>
> There's another aspect to this.
>
> D's UDAs are a purely compile time system, attaching arbitrary metadata
> to specific symbols. The other UDA systems I'm aware of appear to be
> runtime systems.
>
> This implies the use cases will be different - how, I don't really know.
> But I don't know of any other compile time UDA system. Experience with
> runtime systems may not be as applicable.
>

Java is mostly compile time (and optionally runtime). See 
http://projectlombok.org/ for what can be done at compile time with 
attributes + compiler hooks.

> Another interesting data point is CTFE. C++11 has CTFE, but it was
> deliberately crippled and burdened with "constexpr". From what I read,
> this was out of fear that it would turn out to be an overused and
> overabused feature. Of course, this turned out to be a large error.
>
> One last thing. Sure, string attributes can (and surely would be) used
> for different purposes in different libraries. The presumption is that
> this would cause a conflict. But would it? There are two aspects to a
> UDA - the attribute itself, and the symbol it is attached to. In order
> to get the UDA for a symbol, one has to look up the symbol. There isn't
> a global repository of symbols in D. You'd have to say "I want to look
> in module X for symbols." Why would you look in module X for an
> attribute that you have no reason to believe applies to symbols from X?
> How would an attribute for module X's symbols leak out of X on their own?
>
> It's not quite analogous to exceptions, because arbitrary exceptions
> thrown from module X can flow through your code even though you have no
> idea module X even exists.
>