Source code annotations alla Java

[Jonathan M Davis]

On Friday, January 21, 2011 12:36:23 Ary Manzana wrote:
> 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.

No. No attributes are keywords. The reasoning is simple. A keyword is a word 
that would be a valid identifier but isn't, because the language treats it as 
special. @ is not a valid part of an identifier. So, @property can't be a 
keyword. And property certainly isn't a keyword, since you _can_ use it as an 
identifier, so it isn't a keyword with @ prepended on either.

True, @property is treated as a special symbol by the compiler, and I don't 
expect that it really treats it like an attribute with the name property (which 
it will pretty much have to do if we ever get user-defined attributes), but 
definitely isn't a keyword, because it was never possible for it to be a valid 
identifier in the first place.

- Jonathan M Davis