Searching for implicit inside a reify call (scala macro)

Question

I need to search for an implicit value at a given position. I retained the position from a previous macro call in a class, like so :

class Delayed[+Kind[_[_]]](val sourceFilePath: String, val callSitePoint: Int) {
  def find[F[_]]: Kind[F] = macro Impl.find[Kind, F]
}

The previous macro is very simple :

def build[Kind[_[_]]](c: blackbox.Context): c.Expr[Delayed[Kind]] = {
    import c.universe._

    c.Expr(
      q"""
        new Delayed(${c.enclosingPosition.point}, ${c.enclosingPosition.source.path})
       """
    )
  }

With this I have the position, all I need to do is to launch the implicit search right ?

def find[Kind[_[_]], F[_]](c: blackbox.Context)(implicit kindTag: c.WeakTypeTag[Kind[F]], fTag: c.WeakTypeTag[F[_]]): c.Expr[Kind[F]] = {
    import c.universe._

    reify {
      val self = c.prefix.splice.asInstanceOf[Delayed[Kind]]
      val sourceFile = AbstractFile.getFile(self.sourceFilePath)
      val batchSourceFile = new BatchSourceFile(sourceFile, sourceFile.toCharArray)
      val implicitSearchPosition = new OffsetPosition(batchSourceFile, self.callSitePoint).asInstanceOf[c.Position]

      c.Expr[Kind[F]](c.inferImplicitValue(
        appliedType(kindTag.tpe.typeConstructor, fTag.tpe.typeConstructor),
        pos = implicitSearchPosition
      )).splice
    }
  }

I get the position using reify/splice calls and then apply inferImplicitValue . But the compiler complains about the last splice on the implicit value :

the splice cannot be resolved statically, 
which means there is a cross-stage evaluation involved

It asks me to add the compiler jar as dependencies, but by doing so I only get another error :

Macro expansion contains free term variable c defined by find in Delayed.scala

I understand that reify is, conceptually, in the world of values. What I don't understand is that the implicit search should be resolved before the macro-generated code is written to my source code. That the only way I can think of for the implicit search to work in macro contexts.

Where I am wrong ? I do understand the compiler messages, but to me, it makes no sense in this particular context. Maybe I don't get how inferImplicitValue works.

Answer 1

Try Context#eval(expr)

def find[Kind[_[_]], F[_]](c: blackbox.Context)(implicit kindTag: c.WeakTypeTag[Kind[F]], fTag: c.WeakTypeTag[F[_]]): c.Expr[Kind[F]] = {
  import c.universe._

  val self = c.eval(c.Expr[Delayed[Kind]](c.untypecheck(c.prefix.tree.duplicate)))
  val sourceFile = AbstractFile.getFile(self.sourceFilePath)
  val batchSourceFile = new BatchSourceFile(sourceFile, sourceFile.toCharArray)
  val implicitSearchPosition = new OffsetPosition(batchSourceFile, self.callSitePoint).asInstanceOf[c.Position]

  c.Expr[Kind[F]](c.inferImplicitValue(
    appliedType(kindTag.tpe.typeConstructor, fTag.tpe.typeConstructor),
    pos = implicitSearchPosition
  ))
}