How do I generate new source code in text form in a Scala compiler plugin?

Question

I have just finished the first version of a Java 6 compiler plugin, that automatically generates wrappers (proxy, adapter, delegate, call it what you like) based on an annotation.

Since I am doing mixed Java/Scala projects, I would like to be able to use the same annotation inside my Scala code, and get the same generated code (except of course in Scala). That basically means starting from scratch.

What I would like to do, and for which I haven't found an example yet, is how do I generate the code inside a Scala compiler plugin in the same way as in the Java compiler plugin. That is, I match/find where my annotation is used, get the AST for the annotated interface, and then ask the API to give me a Stream/Writer in which I output the generated Scala source code, using String manipulation.

That last part is what I could not find. So how do I tell the API to create a new Scala source file, and give me a Stream/Writer/File/Handle, so I can just write in it, and when I'm done, the Scala compiler compiles it, within the same run in which the plugin was invoked?

Why would I want to do that? Firstly, because than both plugins have the same structure, so maintenance is easy. Secondly, I want to open source it, and there is just no way to support every option that anyone would want, so I expect potential users to want to extend the generation with their own code. This will be a lot easier for them if they just have to do some printf(), instead of learning the AST API (this also applies to me).

Answer 1

Short answer:

It can't be done

Long answer:

You could conceivably generate your source file and push that through a parser instance within your plugin. But not in any way that's likely to be of any use to you, because you'd now have a bigger problem to contend with:

In order to grab all the type/name information for generating the delagate/proxy, you'll have to pick up the annotated type's AST after it has run through both the namer and typer phases (which are inseperable). The catch is that any attempts to call your generated code will already have failed typechecking, the compiler will have thrown an error, and any further bets are off.

Method synthesis is possible in limited cases, so long as you can somehow fool the typechecker for just long enough to get your code generated, which is the trick I pulled with my Autoproxy 'lite' plugin. Even then, you're far better off working with TreeDSL to generate code instead of pumping out raw source.