How to convert context.universe.Annotation to MyAnnotation in a Scala Macro - scala

Question

I am on Scala 2.11.1, I have an annotation
case class MyAnnotation(id: String, message: String) extends StaticAnnotation
I would like to create a macro MessageFromMyAnnotation that transform the following code
class Foo {
#MyAnnotation("001", "James Bond 001")
def foox = {
... // my messy code
val x = MessageFromMyAnnotation("001")
... // my messy code
}
}
to
class Foo {
#MyAnnotation("001", "James Bond 001")
def foox = {
... // my messy code
val x = "Hello world, James Bond 001"
... // my messy code
}
}
In brief, the macro find, on its enclosing element, a message of #MyAnnotation whose id = "001" and return "Hello world, " + message
This is the macro
object MessageFromMyAnnotation {
def apply(id: String) = macro impl
def impl(c: Context)(id: c.Expr[String]): c.Expr[String] = {
c.internal.enclosingOwner.annotations.filter( anno =>
anno.tpe =:= c.universe.typeOf[MyAnnotation] &&
anno.asInstanceOf[MyAnnotation].id == id.value //this does not work
) match {
case anno :: Nil => c.universe.reify("Hello world, " + ...)
case x => c.abort(c.enclosingPosition, c.universe.showRaw(x))
}
}
}
I want to convert anno of type cuniverse.Annotation to MyAnnotation and compare its id with the argument id of type c.Expr[String], but the anno.asInstanceOf[MyAnnotation] yields a ClassCastException and id.value gives me an error message
cannot use value except for signatures of macro implementations
So, please help me with 2 questions:
How to convert anno of type cuniverse.Annotation to MyAnnotation
How to compare its id with the argument id of type c.Expr[String]

Answer 1

I have successfully made it thanks to #Imm's suggestion:
You don't have an instance of MyAnnotation - this is compile time, you only
have an AST that represents the call. You can get the Expr that's the
parameter given to the cuniverse.Annotation, and either splice it, or pattern
match it as a String literal and then take the value out of that.
And here is the code
object MessageFromMyAnnotation {
def apply(id: String) = macro impl
def impl(c: Context)(id: c.Expr[String]): c.Expr[String] = {
import c.universe._
id match { case Expr(Literal(Constant(idVal: String))) =>
(for { Annotation(tpe,
Literal(Constant(annoIdVal: String)) ::
Literal(Constant(annoMessageVal: String)) ::
Nil, _) <- c.internal.enclosingOwner.annotations
if tpe =:= typeOf[MyAnnotation] && idVal == annoIdVal
} yield (annoIdVal, annoMessageVal)
) match {
case (annoIdVal, annoMessageVal) :: Nil =>
reify(c.literal("Hello world, " + annoMessageVal).splice)
case matchedMore :: thanOne :: Nil => c.abort(c.enclosingPosition, "Found more than one #MyAnnotation with the same id")
case x => c.abort(c.enclosingPosition, "Not Found #MyAnnotation with the specified id")
}
}
}
}