Source code annotations alla Java

[Ary Manzana]

On 1/20/11 5:48 PM, Jacob Carlborg wrote:
> On 2011-01-20 21:34, Steven Schveighoffer wrote:
>> On Thu, 20 Jan 2011 15:03:55 -0500, Jacob Carlborg <doob at me.com> wrote:
>>
>>> On 2011-01-20 19:18, Steven Schveighoffer wrote:
>>>> On Thu, 20 Jan 2011 13:07:58 -0500, Jacob Carlborg <doob at me.com> wrote:
>>>>
>>>>> On 2011-01-20 15:02, Steven Schveighoffer wrote:
>>>>>> On Thu, 20 Jan 2011 08:47:28 -0500, Justin Johansson <jj at nospam.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Not long ago the Java Language people introduced the idea of
>>>>>>> annotations together with an annotation processing tool (apt).
>>>>>>>
>>>>>>> Now perhaps the idea of source code annotations is not actually a
>>>>>>> Java
>>>>>>> invention per se, however for someone learning D is there any
>>>>>>> equivalent idiom [of Java annotations] in the D language?
>>>>>>
>>>>>> Haven't used Java since they added annotations, but I think they are
>>>>>> like C# attributes?
>>>>>>
>>>>>> In any case, D has an annotation syntax like:
>>>>>>
>>>>>> @property
>>>>>>
>>>>>> But I think at the moment, annotations have no custom ability. Only
>>>>>> compiler-defined annotations are allowed. This may change in the
>>>>>> future,
>>>>>> but probably not short-term. FWIW, I think we need a much richer
>>>>>> runtime-reflection capability before we can use custom annotations to
>>>>>> any great effect.
>>>>>>
>>>>>> -Steve
>>>>>
>>>>> I would rather formulate it like: currently D has a syntax for
>>>>> keywords that are similar to Java annotations.
>>>>
>>>> I don't think it's the same thing. Keywords are not allowed to be used
>>>> anywhere else, even for things that would parse properly were they not
>>>> keywords. They are anchors for the parser to determine where it is. In
>>>> contrast, a compiler-defined annotation is parsed just the same as a
>>>> custom one, it's just that the meaning is predefined.
>>>>
>>>> For example, you can legally do:
>>>>
>>>> int property;
>>>>
>>>> without error, but this won't even get past the parsing stage:
>>>>
>>>> int struct;
>>>>
>>>> -Steve
>>>
>>> I assume you meant "int @property;"?
>>
>> No. I meant int property;
>
> Of course that would work, isn't that like saying this won't work:
>
> int struct_; // ?
>
>> A keyword is specifically not allowed where the grammar would otherwise
>> allow it. A symbol isn't allowed to have @ in it, so this naturally
>> prevents a conflict. I realize the poor example, but it's definitely not
>> a keyword. Otherwise, it would be listed here:
>> http://www.digitalmars.com/d/2.0/lex.html#keyword (actually, are
>> annotations part of the lexical grammar there?).
>>
>> It's more like Object, which is not a keyword, but you aren't allowed to
>> use it to mean anything besides what it means.
>>
>> The end result is, it fails if you use it in the wrong place, but the
>> keyword status makes it fail at the parsing stage. I am not a compiler
>> writer, so I'm talking a bit from my ass here.
>>
>> -Steve
>
> Ok, maybe you're right. I'm pretty sure, as you say, that a keyword in
> the wrong place would fail during parsing. But I don't know where a
> misplaced annotation/attribute would fail.
>

Jacob is right here.

This, as you say, fails:

int struct;

And also this fails:

int @property;

So yes, currently @property is just a keyword with a prepended @.

Annotations in Java anc C# can have arguments. That is not the case in 
D. For example, as it is said in DIP6, you could have:

@extern('C')
void someFunc();

instead of having extern a keyword and thus being unable to use it for 
identifiers.