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Scala Def宏,从值传递参数

scala-macros scala

FrancMo · 技术社区 · 4 年前

我有一个工作宏,即:

object Main extends App {

  println("Testing assert macro...")
  val result = Asserts.assert(false, "abc")

}

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

object Asserts {
  val assertionsEnabled: Boolean = true

  def assert(cond: Boolean, msg: String): Unit = macro assertImpl

  def assertImpl(c: Context)(cond: c.Expr[Boolean], msg: c.Expr[String]) : c.Expr[Unit] = {
    import c.universe._
    cond.tree match {
      case Literal(Constant(cond: Boolean)) =>
        if (!cond) c.abort(c.enclosingPosition, "Fix the code, whatever.") else c.Expr(q"()")
    }
  }
}

但问题是:

  val cond: Boolean = false
  val result = Asserts.assert(cond, "abc")

现在我有一个错误:

错误:(8,30)宏扩展期间出现异常:scala.MatchError: Main.this.cond(属于scala.reflect.internal.Trees$Select类)位于断言

0 回复 | 直到 4 年前

Dmytro Mitin 4 年前

简短的回答 false 是的运行时值 cond 在里面

val cond: Boolean = false

你试着匹配 Literal 但是那棵树/ Expr 条件 不是字面意思。

所以呢 Asserts.assert(false, "abc") 会有用但是 Asserts.assert(cond, "abc")

长话短说 有几个小把戏。

你可以把你的项目分成三个子项目核心宏使用 c.eval Literal(Constant) (如果你把 条件 到而不是 常见的

object App {
  val cond: Boolean = false
}

核心 (取决于和 常见的

import App.cond

object Main {
  def main(args: Array[String]): Unit = {
    println("Testing assert macro...")
    val result = Asserts.assert(cond, "abc") 
    //scalac: macro expansion has failed: Fix the code, whatever.
  }
}

宏 (取决于 常见的 )

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

object Asserts {
  def assert(cond: Boolean, msg: String): Unit = macro assertImpl

  def assertImpl(c: blackbox.Context)(cond: c.Expr[Boolean], msg: c.Expr[String]) : c.Expr[Unit] = {
    import c.universe._
    val condEvaluated = c.eval(c.Expr[Boolean](c.untypecheck(cond.tree)))
    if (!condEvaluated) c.abort(c.enclosingPosition, "Fix the code, whatever.") else c.Expr(q"()")
  }
}

Scala: what can code in Context.eval reference?

第二招 如果你继续 条件 在里面然后你可以试着找到树的定义 条件 从右边开始:

(取决于宏 )

object Main {
  def main(args: Array[String]): Unit = {
    println("Testing assert macro...")
    val cond: Boolean = false
    val result = Asserts.assert(cond, "abc")
    //scalac: macro expansion has failed: Fix the code, whatever.
  }
}

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

object Asserts {    
  def assert(cond: Boolean, msg: String): Unit = macro assertImpl

  def assertImpl(c: blackbox.Context)(cond: c.Expr[Boolean], msg: c.Expr[String]) : c.Expr[Unit] = {
    import c.universe._

    var condValue: Option[Boolean] = None

    val traverser = new Traverser {
      override def traverse(tree: Tree): Unit = tree match {
        case q"$_ val cond: $_ = $expr" if tree.symbol == cond.tree.symbol => 
          expr match {
            case Literal(Constant(cond: Boolean)) =>
              condValue = Some(cond)
          }
        case _ => super.traverse(tree)
      }
    }

    c.enclosingRun.units.foreach(unit => traverser.traverse(unit.body))

    condValue.map(cond =>
      if (!cond) c.abort(c.enclosingPosition, "Fix the code, whatever.") else c.Expr(q"()")
    ).getOrElse(
      c.abort(c.enclosingPosition, "can't find cond")
    )
  }
}

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