如何在"THEN"中打印"IF"条件的源代码

Question

我想打印IF条件的Scala源代码，而在THEN部分

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

现在让我们跳过THEN部分，问题是:如何在IF之后获得块的源代码?

我假设IF应该是一个宏…

注意:这个问题是在运行时访问函数源代码的宏的重新定义版本，我描述了{ val i = 5; List(1, 2, 3); true }.logValueImpl为我工作(根据其他问题宏在运行时访问源代码文本)。

Answer 1

即兴实现，因为我只有一分钟的时间:

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)

这是对来源的一种不加糖的表示，但我认为这是你能得到的最好的。

Answer 2

从2.13开始，您还可以通过包装表达式来实现这一点，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")
  }
}

与宏实现:

  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)