Question

我想在THEN部分打印IF条件的Scala源代码。

示例：IF{ 2 + 2 < 5 } THEN { println("I am in THEN because: " + sourceCodeOfCondition) }

我们现在跳过THEN部分，问题是：如何在IF之后获取块的源代码？

我认为IF应该是一个宏......

注意：此问题是Macro to access source code of function at runtime的重新定义版本，其中我描述{ val i = 5; List(1, 2, 3); true }.logValueImpl适用于我（根据其他问题Macro to access source code text at runtime）。

Answer 1

袖手旁观的实施，因为我只有一分钟：

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

case class Conditional(conditionCode: String, value: Boolean) {
  def THEN(doIt: Unit) = macro Conditional.THEN_impl
}

object Conditional {
  def sourceCodeOfCondition: String = ???

  def IF(condition: Boolean) = macro IF_impl

  def IF_impl(c: Context)(condition: c.Expr[Boolean]): c.Expr[Conditional] = {
    import c.universe._

    c.Expr(q"Conditional(${ show(condition.tree) }, $condition)")
  }

  def THEN_impl(c: Context)(doIt: c.Expr[Unit]): c.Expr[Unit] = {
    import c.universe._

    val rewriter = new Transformer {
      override def transform(tree: Tree) = tree match {
        case Select(_, TermName("sourceCodeOfCondition")) =>
          c.typeCheck(q"${ c.prefix.tree }.conditionCode")
        case other => super.transform(other)
      }
    }

    c.Expr(q"if (${ c.prefix.tree }.value) ${ rewriter.transform(doIt.tree) }")
  }
}

然后：

object Demo {
  import Conditional._

  val x = 1

  def demo = IF { x + 5 < 10 } THEN { println(sourceCodeOfCondition) }
}

最后：

scala> Demo.demo
Demo.this.x.+(5).<(10)

这是对来源的一种贬低的表现，但是我认为这是你最好的。

有关该技术的一些讨论，请参阅我的博文here。

Answer 2

从 2.13 开始，您还可以通过包装表达式来执行此操作，这意味着您不必定义自定义 if 函数：

implicit def debugIf[A]: DebugIf => Unit = { cond: DebugIf =>
  logger.info(s"condition = {}, result = ${cond.result}", cond.code)
}

decorateIfs {
  if (System.currentTimeMillis() % 2 == 0) {
    println("decorateIfs: if block")
  } else {
    println("decorateIfs: else block")
  }
}

使用宏实现：

  def decorateIfs[A: c.WeakTypeTag](a: c.Expr[A])(output: c.Expr[DebugIf => Unit]): c.Expr[A] = {
    def isEmpty(tree: Trees#Tree): Boolean = {
      tree match {
        case Literal(Constant(())) =>
          true
        case other =>
          false
      }
    }

    c.Expr[A] {
      a.tree match {
        // https://docs.scala-lang.org/overviews/quasiquotes/expression-details.html#if
        case q"if ($cond) $thenp else $elsep" =>
          val condSource = extractRange(cond) getOrElse ""
          val printThen = q"$output(DebugIf($condSource, true))"
          val elseThen = q"$output(DebugIf($condSource, false))"

          val thenTree = q"""{ $printThen; $thenp }"""
          val elseTree = if (isEmpty(elsep)) elsep else q"""{ $elseThen; $elsep }"""
          q"if ($cond) $thenTree else $elseTree"
        case other =>
          other
      }
    }
  }

  private def extractRange(t: Trees#Tree): Option[String] = {
    val pos = t.pos
    val source = pos.source.content
    if (pos.isRange) Option(new String(source.drop(pos.start).take(pos.end - pos.start))) else None
  }

  case class DebugIf(code: String, result: Boolean)

如何打印＆＃34; IF＆＃34;的源代码？条件在＆＃34; THEN＆＃34;

2 个答案: