[swift-evolution] Custom annotation processors?

Fons Rademakers Fons.Rademakers at cern.ch
Tue Jan 19 09:10:17 CST 2016

Hi Chris,

  is there a document or notes on discussions on reflection metadata representations and an API to access them? We are very interested in this as it essential to have that make a powerful I/O system, like we have in ROOT, it is mandatory to be able to find out at run-time all possible object details. User defined attributes are in our case used to annotate e.g. transient data members that should not be streamed, or the precision with which a certain data member should be streamed (a double that could be streamed with a much lower precision saves a lot of bytes in the output, etc.).

Cheers, Fons.

> On 13 Jan 2016, at 22:58, Chris Lattner via swift-evolution <swift-evolution at swift.org> 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
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Dr. Fons Rademakers      CERN - European Organization for Nuclear Research
Chief Research Officer   1211 Geneve 23, Switzerland
CERN openlab             Tel: +41227679248            Mobile: +41754113742

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