Scala Macro获得术语名称的值

Question

我有以下代码:

usage.scala

object Test extends App {
   import Macros._

   val f = 1
   Macros.get(f) 
}

macros.scala

import language.experimental.macros
import scala.reflect.macros.Context

object Macros {
  def get(a: Int) = macro getImpl

  def getImpl(c: Context)(a: c.Expr[Int]) = {
    import c.universe._

     println(showRaw(a))
  }
}    

它返回:

Expr(Select(This(newTypeName("Test")), newTermName("f")))

如何从termName("f")一个1值中提取？有可能用宏吗？

Answer 1

一般来说-不。f可以是抽象的，也可以定义为外部呼叫或用户输入，也可以是随机的，也可以是许多其他情况之一。

但是在某些特定情况下，您可以得到它。您几乎知道所有编译器都知道！

看一下c.enclosingClass：

object Macros {
  def get(a: Int) = macro getImpl

  def getImpl(c: Context)(a: c.Expr[Int]) = {
    import c.universe._
    println(showRaw(c.enclosingClass))
    c.Expr[Unit](Literal(Constant(())))
  }
}    

object Test { val f = 1; Macros.get(f) }
// ModuleDef(Modifiers(), $line55.$read.$iw.$iw.$iw.$iw.Test, Template(List(Select(Ident(scala), newTypeName("AnyRef"))), emptyValDef, List(DefDef(Modifiers(), nme.CONSTRUCTOR, List(), List(List()), TypeTree(), Block(List(Apply(Select(Super(This(newTypeName("Test")), tpnme.EMPTY), nme.CONSTRUCTOR), List())), Literal(Constant(())))), ValDef(Modifiers(), newTermName("f "), TypeTree(), Literal(Constant(1))), Apply(Select(Ident(newTermName("Macros")), newTermName("get")), List(Ident(newTermName("f")))))))

这里有趣的是ValDef(Modifiers(), newTermName("f "), TypeTree(), Literal(Constant(1)))。

我们必须提取它：

object Macros {
  def get(a: Int) = macro getImpl

  def getImpl(c: Context)(a: c.Expr[Int]) = {
    import c.universe._

    val (enclosing, name) = a.tree match {
      case Select(This(enclosing), name) => enclosing -> name
      case _ => c.abort(c.enclosingPosition, "Not a `this` memver")
    }
    val impl = c.enclosingClass match {
      case impl: ImplDef if impl.name.toString == enclosing.toString => impl
      case impl: ImplDef => c.abort(c.enclosingPosition, "Should search in another parent")
      case _ => c.abort(c.enclosingPosition, "Not an `ImplDef`")
    }
    val body = impl.children.collect{
      case Template(_, _, body) => body
    } match {
      case Seq(body) => body
      case _ => c.abort(c.enclosingPosition, "Should be a single template.")
    }
    val rhss = body.collect{
      case ValDef(_, valName, _, rhs) if valName.toString == name.toString => rhs
    }
    val rhs = rhss match {
      case Seq(rhs) => rhs
      case Seq() => c.abort(c.enclosingPosition, "Not found. Maybe it's a DefDef or somethong else")
      case _ => c.abort(c.enclosingPosition, "Some other error.")
    }
    val res = rhs match {
      case Literal(Constant(i: Int)) => i
      case Literal(Constant(_)) => c.abort(c.enclosingPosition, "Literal, but not an Int.")
      case _ => c.abort(c.enclosingPosition, "Implemented not as literal.")
    }
    println(s"Int value in this particular case: $res")
    c.Expr[Any](Literal(Constant(res)))
  }
}   

结果：

object Test { val f = 1; Macros.get(f) }
// Int value in this particular case: 1

因此，我们f在编译时具有价值。

我很确定这不是您所期望的。