On 2012-11-08, 00:18, Walter Bright wrote:
Started a new thread on this.
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.
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.
I'm not sure what the correct answer is here, but I think the solutions
are either to allow any old type as a UDA, or have @attribute structs or
something along those lines. Just user defined types is a weird
restriction.
--
Simen
