Passing class as a parameter to StaticAnnotation macro in scala

Question

I would like to pass a class as a parameter to a StaticAnnotation macro in Scala, let's say I need a macro that checks on DateTime instance:

class CheckDate(date: DateTime) extends StaticAnnotation {
    def macroTransform(annottees: Any*): Any = macro CheckDate.impl
}

object CheckDate {
    def impl(c: Context)(annottees: c.Expr[Any]*): c.Expr[Any] = {
        import c.universe._

        ???
}

And implementation:

@CheckDate(new DateTime(...))
class Sth(...) {...}

How can I retrieve the DateTime instance from an AST?

Answer 1

After a bit of tweaking I finally found a way to retrieve more complex parameters for StaticAnnotation macro in scala.

First of all I extracted all parameters through pattern matching on c.prefix.tree , which actually refers to the annotation class, like so:

val params = c.prefix.tree match {
  case q"""new CheckDate(...$paramss)""" => paramss
  case _ => c.warning(NoPosition, "Oops, something went terribly wrong!")
}

I tried to inspect the tree for params by printing the tree with showRaw(params) , and there are two possible patterns (with or without parameter name):

List(List(AssignOrNamedArg(
Ident(TermName("date")),
Apply(Select(New(Ident(TypeName("DateTime"))), termNames.CONSTRUCTOR),
List(Literal(Constant(2016)), Literal(Constant(5)), Literal(Constant(10)))))))

or

List(List(Apply(
Select(New(Ident(TypeName("DateTime"))), termNames.CONSTRUCTOR), 
List(Literal(Constant(2016)), Literal(Constant(5)), Literal(Constant(11))))))

In order to use the DateTime object we need to extract the appropriate fields and create new DateTime instances:

val d: DateTime = params match {
   case List(List(AssignOrNamedArg(
   _, Apply(_, List(Literal(Constant(y)), Literal(Constant(m)), Literal(Constant(d))))))) =>
       new DateTime(y.asInstanceOf[Int], m.asInstanceOf[Int], d.asInstanceOf[Int], 0, 0)
   case List(List(Apply(_, List(Literal(Constant(y)),
   Literal(Constant(m)), Literal(Constant(d)))))) =>
       new DateTime(y.asInstanceOf[Int], m.asInstanceOf[Int], d.asInstanceOf[Int], 0, 0)

Lot's of boilerplate actually, but the way turns out quite efficient. The key is to inspect the AST of the structure to be parsed and create appropriate cases for it.