User defined attributes use

[simendsjo]

On Sunday, 15 September 2013 at 17:34:06 UTC, matovitch wrote:
> Hi everyone,
>
> I read the documentation about user defined attributes, but I 
> don't see their uses. Ok, it'a a template expression you can 
> link to a declaration, but what are they useful for ? (not sure 
> about the syntax ;-))
>
> Can you declare a template constraint as a user defined 
> attribute to do something like :
>
> void 
> template_function_which_go_back_and_forth(@("Bidirectional") 
> @("Range") BR)(BR br) {...}
>
> This would be awesome (even if not really occidental) to do 
> something like:
>
> @("SmallTypeSet") @("MediumTypeSet") @("LargeTypeSet") Type
>
> This could allow to build tree based category structure.

It enables declarative programming.
And because this is D, there is no runtime overhead.
A common use is to add semantics to types and instances that is 
difficult or very intrusive to do by creating structs/classes by 
hand.

A little validation example:

@nonNull // An instance shouldn't be allowed to be null
class C {
   @matches("[0-9]+")
   string someNumber;

   @interval!"(]"(0, 10) // (0, 10] range
   int someInt;
}

C c;
validate(c); // returns ["C is null", "someNumber doesn't match 
'[0-9]+'", "someInt is outside the interval '(0, 10]'"]

And ORMs usually use annotations:

@table("some_tablename")
class C {
   @id("id_field_name")
   int id;
}

Take a look at C# and Java libraries to see how many uses 
attributes/annotations - they are still quite new in D, so they 
are still underutilized.
A very big difference is of course that UDAs are available at 
compile time :)