如如何将参数传递给宏注释下的 ScalaMeta Paradise 描述中所述 ?部分
您匹配this
为 Scalameta 树
package scalaworld.macros
import scala.meta._
class Argument(arg: Int) extends scala.annotation.StaticAnnotation {
inline def apply(defn: Any): Any = meta {
// `this` is a scala.meta tree.
println(this.structure)
val arg = this match {
// The argument needs to be a literal like `1` or a string like `"foobar"`.
// You can't pass in a variable name.
case q"new $_(${Lit.Int(arg)})" => arg
// Example if you have more than one argument.
case q"new $_(${Lit.Int(arg)}, ${Lit.String(foo)})" => arg
case _ => ??? // default value
}
println(s"Arg is $arg")
defn.asInstanceOf[Stat]
}
}
请注意,此代码有点幼稚,并且不处理命名参数。如果你有很多参数,编写所有可能的常用参数和命名参数的组合是很无聊的。所以你可以尝试这样的事情:
package examples
import scala.collection.immutable
import scala.meta._
class MyMacro(p1: String, p2: Int) extends scala.annotation.StaticAnnotation {
inline def apply(defn: Any): Any = meta {
val params = Params.extractParams(this)
//some implementation
...
}
}
case class Params(p1: String, p2: Int) {
def update(name: String, value: Any): Params = name match {
case "p1" => copy(p1 = value.asInstanceOf[String])
case "p2" => copy(p2 = value.asInstanceOf[Int])
case _ => ???
}
}
object Params {
private val paramsNames = List("p1", "p2")
def extractParams(tree: Stat): Params = {
val args: immutable.Seq[Term.Arg] = tree.asInstanceOf[Term.New].templ.parents.head.asInstanceOf[Term.Apply].args
args.zipWithIndex.foldLeft(Params(null, 0))((acc, argAndIndex) => argAndIndex._1 match {
case q"${Lit(value)}" => acc.update(paramsNames(argAndIndex._2), value)
case q"${Term.Arg.Named(name, Lit(value))}" => acc.update(name.value, value)
})
}
}