On 2012-11-07 23:20, Walter Bright wrote:
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.
This is what I start to like less and less about the current
implementation of UDA. There is no actual attribute. There is the symbol
with a bunch of random attached values.
I want to be able to do something like this:
module bar;
@attribute struct foo
{
string name;
}
@foo int a;
alias Tuple!(__traits(getAttributes, foo)) TP;
enum bool yes = hasAttribute!(bar.foo);
static if (yes)
enum foo attr = getAttribute!(bar.foo);
The last two could actually be library functions.
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?
Have you ever heard of libraries. You can pass a symbol as an alias
parameter to a function of a completely different library.
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.
--
/Jacob Carlborg
