Non-scala source positions with scala macros - scala

Question

I have a scala macro which depends on an arbitrary xml file that is specified through a static string containing it's location.
def myMacro(path: String) = macro myMacroImpl
def myMacroImpl(c: Context)(path: c.Expr[String]): c.Expr[Any] = {
// load file specified by path and generate some code
...
}
This means, that if the xml file is malformed the macro will not be able to expand. In the moment I am providing an error message that contains a textual representation of the location of the error in the xml file. This however is obviously not the nicest solution.
Is it possible to provide source locations in different (possible non-scala) files for my generated code so that errors will point to the xml file instead of the scala file where the xml file is included? I don't see how I can create locations myself instead of altering existing.

Answer 1

This use case is definitely very interesting, and it looks like something that should be supported in the reflection API. Unfortunately, at the moment, there's no public API to achieve this, even though the internal, albeit quite low-level, machinery is in place.
import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context
import scala.reflect.io.AbstractFile
import scala.reflect.internal.util.BatchSourceFile
import scala.reflect.internal.util.OffsetPosition
class Impl(val c: Context) {
def impl: c.Tree = {
val filePath = "foo.txt"
val af = AbstractFile.getFile(filePath)
val content = scala.io.Source.fromFile(filePath).mkString
val sf = new BatchSourceFile(af, content)
val pos = new OffsetPosition(sf, 3).asInstanceOf[c.universe.Position]
c.abort(pos, "it works")
}
}
object Macros {
def foo: Any = macro Impl.impl
}
object Test extends App {
Macros.foo
}
Running this code on a simple text file produces the following result:
20:56 ~/Projects/Master/sandbox (master)$ cat foo.txt
hello
world
20:56 ~/Projects/Master/sandbox (master)$ scalac Test.scala
foo.txt:1: error: it works
hello
^
one error found
Please note that this solution involves scala.reflect.internal and a cast (both of which invalidate all compatibility guarantees that we provide for scala-reflect.jar), so it's not something that I would recommend for production code.