Is it possible to generate Apply from WeakTypeTag inside a scala macro?

问题

I have a WeakTypeTag of some type in my macro, and I want to generate code as follows:

macroCreate[SomeObject] // => SomeObject(1)

The definition of a macro will be something like this:

def macroCreate[A] = macro _macroCreate[A]
def _macroCreate[A](c: Context)(implicit wtt: c.WeakTypeTag[A]) = {
  c.Expr(Apply(Select(???, newTermName("apply")), List(c.literal(1).tree)))
}

The problem is, how do I get Select for the given type?

I can use a workaround of converting the type to string, splitting on "." and then creating a Select from list of strings, but that seems hacky.

Is it possible to create a Select directly from type tag?

回答1:

You can get the symbol of the companion object and then use the universe's Ident(sym: Symbol): Ident factory method:

def macroCreate[A] = macro _macroCreate[A]

def _macroCreate[A](c: Context)(implicit wtt: c.WeakTypeTag[A]) = {
  import c.universe._

  c.Expr(
    Apply(
      Select(Ident(wtt.tpe.typeSymbol.companionSymbol), newTermName("apply")),
      c.literal(1).tree :: Nil
    )
  )
}

And then:

scala> case class SomeObject(i: Int)
defined class SomeObject

scala> macroCreate[SomeObject]
res0: SomeObject = SomeObject(1)

scala> macroCreate[List[Int]]
res1: List[Int] = List(1)

If you really mean that SomeObject is the type of the object (i.e., not the type of its companion class), just remove the .companionSymbol above.

来源：https://stackoverflow.com/questions/17399998/is-it-possible-to-generate-apply-from-weaktypetag-inside-a-scala-macro