Scala Macros: Convert/parse a Tree to a Name - scala

Question

This is a simplified example but the problem remains the same.
I want to achieve this using macros (scala based pseudocode):
(a: Int) => {
val z = "toShort"
a.z
}
If I reify it, I would obtain something similar to this:
Function(
List(
ValDef(
Modifiers(Flag.PARAM),
newTermName("a"),
Ident(scala.Int),
EmptyTree
)
),
Block(
List(
ValDef(
Modifiers(),
newTermName("z"),
TypeTree(),
Literal(Constant("toShort"))
)
),
Apply(
Select(
Ident(newTermName("a")),
newTermName("toShort")
),
List()
)
)
)
I dont know how to access to a value and then use it as a TermName.
I tried replacing newTermName("toShort") with newTermName(c.Expr[String](Select(Ident(newTermName("z")))).splice) but the compiler doesn't seem to like:
exception during macro expansion:
java.lang.UnsupportedOperationException: the function you're calling has not been spliced by > the compiler.
this means there is a cross-stage evaluation involved, and it needs to be invoked explicitly.
if you're sure this is not an oversight, add scala-compiler.jar to the classpath,
import scala.tools.reflect.Eval and call <your expr>.eval instead.
I've also tried 'eval' as suggested by compiler: newTermName(c.eval(c.Expr[String](...)) but neither worked.
How could I convert a tree like Select(Ident(newTermName("z"))) (which is a access to a value of a local val) to a Name a string which can be used as a parameter for newTermName? Is it possible?
UPDATE:
Here the real problem brought to you as a gist!
Thanks in advance,

Answer 1

I have a hard time understanding what you're trying to achieve, and why you are using Trees everywhere. Trees are really low level, hard to use, tricky, and it is very difficult to understand what the code does. Quasiquotes (http://docs.scala-lang.org/overviews/macros/quasiquotes.html) are the way to go indeed and you can use them on scala 2.10.x production release thanks to the macro paradise plugin (http://docs.scala-lang.org/overviews/macros/paradise.html). The you can simply write q"(a: Int) => {val z = "toShort"; a.z}" and you directly get the tree expression you just typed.
To answer your question, the first point is to remember that macros are evaluated at compile time. They therefore can not generate code which depends on a runtime value. This is why the compiler is complaining about your splice. But if you pass a value which can be computed at compile time, typically a literal, then you can use eval to get its value within your macro code. Eval does suffer a bug though, as indicated in scaladoc. It should only be called on untyped trees. So the way to call eval on an s: c.Expr[String] expression would be val s2 = c.eval(c.Expr[String](c.resetAllAttrs(c.tree.duplicate))) which gives you a String you can then use normally in your code, for instance q"(a: Int) => a.${newTermName(s2)}".
To put it all together, let's imagine you to create a macro that'll output a string value from an object and one of its String field. It'll give something like
def attr[A](a: A, field: String): String = macro attrImpl[A]
def attrImpl[A: c.WeakTypeTag](c: Context)(a: c.Expr[A], field: c.Expr[String]) = {
import c.universe._
val s = c.eval(c.Expr[String](c.resetAllAttrs(field.tree.duplicate)))
c.Expr[String](q"a.${newTermName(s)}")
}
REPL session test:
scala> object a { val field1 = "field1"; val field2 = "field2" }
defined module a
scala> attr(a, "field1")
res0: String = field1
scala> attr(a, "field2")
res1: String = field2
To understand the difference between compile time and runtime, you can meditate about the following result in REPL ;-)
scala> val s = "field1"; attr(a, s)
error: exception during macro expansion:
scala.tools.reflect.ToolBoxError: reflective compilation has failed:
$iw is not an enclosing class
at scala.tools.reflect.ToolBoxFactory$ToolBoxImpl$ToolBoxGlobal.throwIfErrors(ToolBoxFactory.scala:311)
at scala.tools.reflect.ToolBoxFactory$ToolBoxImpl$ToolBoxGlobal.compile(ToolBoxFactory.scala:244)
at scala.tools.reflect.ToolBoxFactory$ToolBoxImpl.compile(ToolBoxFactory.scala:408)
at scala.tools.reflect.ToolBoxFactory$ToolBoxImpl.eval(ToolBoxFactory.scala:411)
at scala.reflect.macros.runtime.Evals$class.eval(Evals.scala:16)
at scala.reflect.macros.runtime.Context.eval(Context.scala:6)
at .attrImpl(<console>:14)
scala> val s = "field1"
s: String = field1
scala> attr(a, s)
res3: String = field1
Hope it helps ;))