Scala 宏 : how to read an annotation object

Question

我想做一个和Scala Macros: Checking for a certain annotation类似的事情
我的注释看起来像:

class extract(val name: String) extends StaticAnnotation

我是这样使用它的:

case class MainClass(@extract("strings") foo: String, bar: Int)

我正在尝试获取 foo 参数 Symbol因为它有一个 @extract注解:

val extrList = params.map { param: Symbol =>
  param.annotations.collect {
    case extr if extr.tpe  <:< c.weakTypeOf[extract] =>
      val args = extr.scalaArgs
      if (args.size != 1)
        abort("@extract() should have exactly 1 parameter")
      getExtractValue(args.head) -> param
  }
}

getExtractValue方法如下所示:

def getExtractValue(tree: Tree): String = ???

我如何获得值(value) name的 @extract注解

更新

我从 scalaArgs 得到的树似乎也无法被 c.eval() 使用

param: Symbol =>
    param.annotations.collect {
      case ann if ann.tpe <:< c.weakTypeOf[extract] =>
        val args = ann.scalaArgs
        val arg0 = args.head
        val name: String = c.eval(c.Expr(arg0))
        echo(s"args @extract(name = $name)")
        name -> param
    }

给出错误

[error] exception during macro expansion:
[error] scala.tools.reflect.ToolBoxError: reflective toolbox has failed: cannot
operate on trees that are already typed
[error]         at scala.tools.reflect.ToolBoxFactory$ToolBoxImpl$ToolBoxGlobal.
verify(ToolBoxFactory.scala:74)

完整的堆栈跟踪指向 c.eval (我把 c.eval 和 c.Expr 分开了)

Answer 1

在这种情况下:

def getExtractValue(tree: Tree) = tree match {
  case Literal(Constant(str: String)) => str
}