传递给Scala宏的内省参数

我想编写一个Scala宏，它以case类的一个实例作为参数。所有可以传递给宏的对象都必须实现特定的标记特征

以下代码段显示了marker trait和实现它的两个示例案例类：

trait Domain
case class Country( id: String, name: String ) extends Domain
case class Town( id: String, longitude: Double, latitude: Double ) extends Domain

现在，我想使用宏编写以下代码，以避免运行时反射的严重性及其线程的不安全性：

object Test extends App {

  // instantiate example domain object
  val myCountry = Country( "CH", "Switzerland" )

  // this is a macro call
  logDomain( myCountry )
} 

宏

logDomain

在不同的项目中实现，看起来类似于：

object Macros {
  def logDomain( domain: Domain ): Unit = macro logDomainMacroImpl

  def logDomainMacroImpl( c: Context )( domain: c.Expr[Domain] ): c.Expr[Unit] = {
    // Here I would like to introspect the argument object but do not know how?
    // I would like to generate code that prints out all val's with their values
  }
}

宏的目的应该是生成代码，该代码在运行时输出给定对象的所有值（

id

和

name

），并打印它们，如下所示：

id (String) : CH
name (String) : Switzerland

要实现这一点，我必须动态检查传递的类型参数并确定其成员（VAL）。然后我必须生成一个表示创建日志输出的代码的AST。无论将实现标记特征“域”的特定对象传递给宏，宏都应该工作

---

在这一点上我迷路了。如果有人能给我一个起点或给我一些文档，我将不胜感激？我对Scala比较陌生，在Scala API文档或宏指南中还没有找到解决方案。

列出case类的访问器是使用宏时的常见操作，因此我倾向于保留如下方法：

def accessors[A: u.WeakTypeTag](u: scala.reflect.api.Universe) = {
  import u._

  u.weakTypeOf[A].declarations.collect {
    case acc: MethodSymbol if acc.isCaseAccessor => acc
  }.toList
}

这将为我们提供

A

的所有case类访问器方法符号（如果有）。请注意，我在这里使用的是通用反射API，现在还没有必要使这个宏特定

我们可以用其他一些方便的东西来概括这个方法：

trait ReflectionUtils {
  import scala.reflect.api.Universe

  def accessors[A: u.WeakTypeTag](u: Universe) = {
    import u._

    u.weakTypeOf[A].declarations.collect {
      case acc: MethodSymbol if acc.isCaseAccessor => acc
    }.toList
  }

  def printfTree(u: Universe)(format: String, trees: u.Tree*) = {
    import u._

    Apply(
      Select(reify(Predef).tree, "printf"),
      Literal(Constant(format)) :: trees.toList
    )
  }
}

现在我们可以非常简洁地编写实际的宏代码：

trait Domain

object Macros extends ReflectionUtils {
  import scala.language.experimental.macros
  import scala.reflect.macros.Context

  def log[D <: Domain](domain: D): Unit = macro log_impl[D]
  def log_impl[D <: Domain: c.WeakTypeTag](c: Context)(domain: c.Expr[D]) = {
    import c.universe._

    if (!weakTypeOf[D].typeSymbol.asClass.isCaseClass) c.abort(
      c.enclosingPosition,
      "Need something typed as a case class!"
    ) else c.Expr(
      Block(
        accessors[D](c.universe).map(acc =>
          printfTree(c.universe)(
            "%s (%s) : %%s\n".format(
              acc.name.decoded,
              acc.typeSignature.typeSymbol.name.decoded
            ),
            Select(domain.tree.duplicate, acc.name)
          )
        ),
        c.literalUnit.tree
      )
    )
  }
}

或：

---

根据需要。

在我看来，您需要解决两个问题：1）从宏参数获取必要的信息，2）生成表示所需代码的树

在Scala2.10中，这些工作是通过反射API完成的。请按照下面的说明查看有哪些文档可供使用

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

trait Domain
case class Country(id: String, name: String) extends Domain
case class Town(id: String, longitude: Double, latitude: Double) extends Domain

object Macros {
  def logDomain(domain: Domain): Unit = macro logDomainMacroImpl

  def logDomainMacroImpl(c: Context)(domain: c.Expr[Domain]): c.Expr[Unit] = {
    import c.universe._

    // problem 1: getting the list of all declared vals and their types
    //   * declarations return declared, but not inherited members
    //   * collect filters out non-methods
    //   * isCaseAccessor only leaves accessors of case class vals
    //   * typeSignature is how you get types of members
    //     (for generic members you might need to use typeSignatureIn)
    val vals = typeOf[Country].declarations.toList.collect{ case sym if sym.isMethod => sym.asMethod }.filter(_.isCaseAccessor)
    val types = vals map (_.typeSignature)

    // problem 2: generating the code which would print:
    // id (String) : CH
    // name (String) : Switzerland
    //
    // usually reify is of limited usefulness
    // (see https://stackoverflow.com/questions/13795490/how-to-use-type-calculated-in-scala-macro-in-a-reify-clause)
    // but here it's perfectly suitable
    // a subtle detail: `domain` will be possibly used multiple times
    // therefore we need to duplicate it
    val stmts = vals.map(v => c.universe.reify(println(
      c.literal(v.name.toString).splice +
      "(" + c.literal(v.returnType.toString).splice + ")" +
      " : " + c.Expr[Any](Select(domain.tree.duplicate, v)).splice)).tree)

    c.Expr[Unit](Block(stmts, Literal(Constant(()))))
  }
}

---

你赢了我5分钟！：）非常感谢您的详细回答！你的例子正是我想要的。今晚我要试试。解决方案的优点在于使用了type参数和WeakTypeTag，这使代码完全通用。它应该适用于任何实现“域”的case类。非常感谢您的详细回答。我越来越喜欢Scala，新的反射和宏支持非常好。我将更多地使用它，并尝试生成一些代码，使用case-vals的提取值实例化不同的对象。

scala> Macros.log(Country("CH", "Switzerland"))
id (String) : CH
name (String) : Switzerland

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

trait Domain
case class Country(id: String, name: String) extends Domain
case class Town(id: String, longitude: Double, latitude: Double) extends Domain

object Macros {
  def logDomain(domain: Domain): Unit = macro logDomainMacroImpl

  def logDomainMacroImpl(c: Context)(domain: c.Expr[Domain]): c.Expr[Unit] = {
    import c.universe._

    // problem 1: getting the list of all declared vals and their types
    //   * declarations return declared, but not inherited members
    //   * collect filters out non-methods
    //   * isCaseAccessor only leaves accessors of case class vals
    //   * typeSignature is how you get types of members
    //     (for generic members you might need to use typeSignatureIn)
    val vals = typeOf[Country].declarations.toList.collect{ case sym if sym.isMethod => sym.asMethod }.filter(_.isCaseAccessor)
    val types = vals map (_.typeSignature)

    // problem 2: generating the code which would print:
    // id (String) : CH
    // name (String) : Switzerland
    //
    // usually reify is of limited usefulness
    // (see https://stackoverflow.com/questions/13795490/how-to-use-type-calculated-in-scala-macro-in-a-reify-clause)
    // but here it's perfectly suitable
    // a subtle detail: `domain` will be possibly used multiple times
    // therefore we need to duplicate it
    val stmts = vals.map(v => c.universe.reify(println(
      c.literal(v.name.toString).splice +
      "(" + c.literal(v.returnType.toString).splice + ")" +
      " : " + c.Expr[Any](Select(domain.tree.duplicate, v)).splice)).tree)

    c.Expr[Unit](Block(stmts, Literal(Constant(()))))
  }
}