Rewriting val and var constructor arguments using macros - scala

Question

I am evaluating the possibility and effort to create a macro annotation to turn this:
#txn class Foo(var bar: Int)
into this:
import concurrent.stm.{Ref, InTxn}
class Foo(bar0: Int) {
private val _bar = Ref(bar0)
def bar(implicit tx: InTxn): Int = _bar()
def bar_=(value: Int)(implicit tx: InTxn): Unit = _bar() = value
}
(or perhaps creating a Foo companion object with apply method, no sure yet)
Now what I'm seeing from the ClassDef body is something like this
List(<paramaccessor> var bar: Int = _,
def <init>(bar: Int) = {
super.<init>();
()
})
So bar appears as param-accessor with no init (_), and also as argument to the <init> method.
How would I go about rewriting that body?
import scala.reflect.macros.Context
import scala.language.experimental.macros
import scala.annotation.StaticAnnotation
class txn extends StaticAnnotation {
def macroTransform(annottees: Any*) = macro txnMacro.impl
}
object txnMacro {
def impl(c: Context)(annottees: c.Expr[Any]*): c.Expr[Any] = {
import c.universe._
val inputs = annottees.map(_.tree).toList
def reportInvalidAnnotationTarget(): Unit =
c.error(c.enclosingPosition,
"This annotation can only be used on a class definition")
val expandees: List[Tree] = inputs match {
case cd # ClassDef(mods, nme, tp, tmp # Template(parents, self, body)) :: Nil =>
println(s"Body: $body")
???
case _ =>
reportInvalidAnnotationTarget()
inputs
}
val outputs = expandees
c.Expr[Any](Block(outputs, Literal(Constant(()))))
}
}

Answer 1

As Eugene suggests, using quasiquotes can make this much easier. I haven't put together a full solution, but I tried some pieces so I think something along these lines will work:
case q"class $name extends $parent with ..$traits { ..$body }"=>
val newBody = body.flatMap {
case q"var $varName = $varBody" =>
List(
//put the three defs you need here. some variant of this ...
q"""private val ${stringToTermName("_" + varName.toString)} = Ref(${stringToTermName(varName.name + "0")})""",
//etc ...
)
case other => other
}
q"class $name extends $parent with ..$ttraits { ..${(newBody).toList} }"
You might find my blog post relevant: http://imranrashid.com/posts/learning-scala-macros/
The most relevant code snippet is here:
https://github.com/squito/learn_macros/blob/master/macros/src/main/scala/com/imranrashid/oleander/macros/FillTraitDefs.scala#L78
You may also find this useful for defining the getters & setters:
https://groups.google.com/forum/#!searchin/scala-user/macro|sort:date/scala-user/Ug75FW73mJI/F1ijU0uxZAUJ