[swift-evolution] Adding custom attributes

[James Campbell]

Maybe we could implement a template system like C++

On Sat, Dec 26, 2015 at 3:20 PM, Dmitri Gribenko via swift-evolution <
swift-evolution at swift.org> wrote:

> On Sat, Dec 26, 2015 at 3:59 PM, Radosław Smogura <rsmogura at icloud.com>
> wrote:
> > So,
> >
> > What do you think about syntax like this:
> >
> > // Declaration
> > @AttributeUsage(RUNTIME) //Can be SOURCE - attribute not emitted into
> binary
> > @AttributeTarget(PROPERTY) //Other CLASS, ENUM, METHOD, INIT
> > @attribute JSONProperty {
> > var name:String!;
> > var serializable:Bool? = true;
> > var deserializable:Bool? = true;
> > }
>
> Is this a new declaration kind?  Do you think we could make it work
> with existing language constructs instead, like structs?  Any
> downsides to that?
>
> > // Usage
> > @JSONProperty(name=“id”)
> > var id:Int;
> >
> >
> > Attributes should be module aware (actual attributes should be redefined
> as Swift attribute, for beginning still can be hardcoded).
>
> Sorry, I'm not sure what this means.
>
> > The attribute’s name have to be unique in module. It’s compile time
> error if attribute, class, enum, struct, etc has same name.
> >
> > Attribute’s properties are read only and can’t be assigned - so during
> reflection no-one would change ‘shared’ values.
>
> I think using structs and requiring that attributes are value types
> would solve both issues.
>
> > Attribute’s properties can only support basic types: primitives,
> strings, types, and other attributes (optional).
>
> Well, there is no such thing as "basic types" in Swift.  Strings,
> integers etc. are defined in the standard library and the compiler
> does not know anything special about them.
>
> > When declaring attributes, properties can be set to constant values
> (static let) or enum values, however the final value is stored in binary,
> not a reference to it.
>
> Again, given that strings are defined in the standard library, and
> that the language does not have a notion of a constant expression, I'm
> not sure how this would work.  I'm not saying it can't, I'm just
> saying you need to introduce a lot of new language concepts and
> compiler machinery.
>
> > The compiler has build in support for core attributes, which can affect
> code generation. Compiler may perform additional checks on core attributes.
>
> OK.
>
> Another question is, how would you inspect attributes at runtime?
>
> Dmitri
>
> --
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
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