[swift-evolution] Custom annotation processors?

Talin viridia at gmail.com
Thu Jan 14 11:18:51 CST 2016

BTW other use cases for this sort of thing are automated generation of
testing mocks and RPC stubs - things that require a lot of exacting,
repetitive drudgery and that no programmer should have to code manually.
It's possible to do these with reflection as well, however the problem with
reflection is that when you look at the code in the debugger you have to
peer inside a lot of obscure data structures to figure out what is going
on, whereas it's fairly intuitive to step through generated source code.

On Wed, Jan 13, 2016 at 9:55 PM, Chris Lattner <clattner at apple.com> wrote:

> On Jan 13, 2016, at 6:18 PM, Talin <viridia at gmail.com> wrote:
> 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
> Guice.
> 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.
> Ah I see, I don’t know anyone considering building those sorts of tools
> right now.
> -Chris
> 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

-- Talin
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