[swift-evolution] Custom annotation processors?

Talin viridia at gmail.com
Wed Jan 13 20:18:44 CST 2016

Understood. I might add that one of the advantages of Dagger2 over the
older Guice framework is that it does all of the reflection work at compile
time, rather than at runtime, so a lot of the expense of runtime reflection
is avoided. This also means that the generated code for the injection logic
is debuggable, which was historically one of the biggest complaints about

So the bottom line for me is, I don't care as much about the ability to
access the attributes at runtime - I'm more interested as to whether the
module artifacts output by the compiler can be introspected via some simple
API, or are stored in some relatively transparent encoding.

On Wed, Jan 13, 2016 at 5:58 PM, Chris Lattner <clattner at apple.com> wrote:

> On Jan 13, 2016, at 5:24 PM, Talin via swift-evolution <
> swift-evolution at swift.org> wrote:
> As a former Googler, I've spent a lot of years writing Java code that uses
> dependency injection, and this relies heavily on the ability to have custom
> annotations/attributes in the language - particularly, user-defined
> attributes on function parameters - and to generate additional code at
> compile time via annotation processors. Although dependency injection does
> have it's detractors, it's getting better (current best of breed is
> http://google.github.io/dagger/), and it solves an amazing array of
> problems, including the ability for asynchronous programming to disappear
> into the underlying framework - you just write synchronous code and the
> framework handles the rest (no more futures!).
> Now, you can of course do dependency injection without custom attribute
> support in the language, but it's much more cumbersome. The user-defined
> attributes allow you to specify, in a simple declarative way, the runtime
> dependencies between various classes. Without it you have to build up those
> dependencies in code, using some sort of fluent interface or builder
> pattern.
> So my question is, is there any plan for Swift to support user-created
> annotations, and annotation processing compilation stages?
> Hi Talin,
> We have no concrete plans for user defined attributes, but it is a natural
> extension.  One of our goals for Swift 3 is to nail down the reflection
> metadata representation.  We should design this to be extensible to support
> user defined attributes so that we don’t close this off in the future.
> -Chris

-- Talin
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