[swift-evolution] Custom annotation processors?

Chris Lattner clattner at apple.com
Wed Jan 13 19:58:05 CST 2016

> 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/ <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.


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