Type Parameters on Scala Macro Annotations

Question

I\'m trying to use macro annotations in scala, where my macro annotation would take an argument of another type. It would then use scala reflection to look at the passed in

Answer 1

As Eugene points out in his answer it is possible to match on the tree of the whole macro application. Like every Scala method, annotation macro applications can take multiple type argument lists as well as multiple value argument lists.

Consider the macro application of an annotation macro called test:

    @test[A, B][C, D](a, b)(c, d) trait Foo

In the implementation of test we can inspect the macro application by

    println(show(c.macroApplication))

which will result in:

    new test[A, B][C, D](a, b)(c, d).macroTransform(abstract trait Foo extends scala.AnyRef)

To extract the (type/value) parameters from the tree you have to pattern match on the tree. A parser for an arbitrary amount of parameter lists can be found in this project

Using this parser retrieving the first value argument of the macro application is as easy as

    val List(List(arg)) = MacroApp(c.macroApplication).termArgs

Answer 2

In macro paradise 2.0.0-SNAPSHOT we have quite a tricky way of accessing type parameters for macro annotations (the situation will improve later on when we have dedicated APIs for that, but right now it's very difficult to introduce new functionality to scala-reflect.jar in macro paradise, so the current API is a bit rough).

For now it's necessary to specify the type parameter on the annotation class and not to declare any type parameters on the macroTransform method. Then, in macro expansion, access c.macroApplication and extract the untyped tree corresponding to the passed type parameter. Afterwards, do c.typeCheck as described in Can't access Parent's Members while dealing with Macro Annotations.