scala - StringContext和宏: a simple example

我正在尝试实现StringContext扩展，这将允许我编写以下代码:

val tz = zone"Europe/London" //tz is of type java.util.TimeZone

但要注意的是，如果提供的时区无效，它应该无法编译(假设可以在编译时确定)。

这是一个辅助函数:

def maybeTZ(s: String): Option[java.util.TimeZone] =
  java.util.TimeZone.getAvailableIDs collectFirst { case id if id == s =>
    java.util.TimeZone.getTimeZone(id)
  }

我可以很容易地创建一个非宏实现:

scala> implicit class TZContext(val sc: StringContext) extends AnyVal {
 |   def zone(args: Any *) = {
 |     val s = sc.raw(args.toSeq : _ *)
 |     maybeTZ(s) getOrElse sys.error(s"Invalid zone: $s")
 |   }
 | }

然后:

scala> zone"UTC"
res1: java.util.TimeZone = sun.util.calendar.ZoneInfo[id="UTC",offset=0,...

到现在为止还挺好。除非时区不合时宜(例如zone"foobar")，否则编译不会失败；代码在运行时崩溃。我想将其扩展到一个宏，但是，尽管阅读了docs，但我确实在细节方面苦苦挣扎(准确地说，是所有细节)。

谁能帮我从这里开始？全力以赴的解决方案应该查看StringContext是否定义了任何参数，并且(如果是)将计算推迟到运行时，否则尝试在编译时解析区域

我尝试了什么？

好吧，宏定义似乎必须位于静态可访问的对象中。所以:

package object oxbow {
  implicit class TZContext(val sc: StringContext) extends AnyVal {
    def zone(args: Any *) = macro zoneImpl //zoneImpl cannot be in TZContext
  }

  def zoneImpl(c: reflect.macros.Context)
    (args: c.Expr[Any] *): c.Expr[java.util.TimeZone] = {
      import c.universe._
      //1. How can I access sc from here?

      /// ... if I could, would this be right?
      if (args.isEmpty) {
        val s = sc.raw()
        reify(maybeTZ(s) getOrElse sys.error(s"Not valid $s"))
      }
      else {
        //Ok, now I'm stuck. What goes here?
      }
    }

}

根据以下som-snytt的建议，这是最新的尝试:

def zoneImpl(c: reflect.macros.Context)
           (args: c.Expr[Any] *): c.Expr[java.util.TimeZone] = {
  import c.universe._
  val z =
    c.prefix.tree match {
      case Apply(_, List(Apply(_, List(Literal(Constant(const: String)))))) => gsa.shared.datetime.XTimeZone.getTimeZone(const)
      case x => ??? //not sure what to put here
    }

  c.Expr[java.util.TimeZone](Literal(Constant(z))) //this compiles but doesn't work at the use-site
                             ^^^^^^^^^^^^^^^^^^^
                             this is wrong. What should it be?
}

在使用站点上，有效的zone"UTC"无法编译，并显示以下错误:

java.lang.Error: bad constant value: sun.util.calendar.ZoneInfo[id="UTC",offset=0,dstSavings=0,useDaylight=false,transitions=0,lastRule=null] of class class sun.util.calendar.ZoneInfo

大概我不应该使用Literal(Constant( .. ))来封装它。我应该用什么？

最后一个示例-基于下面的特拉维斯·布朗的答案

def zoneImpl(c: reflect.macros.Context)
         (args: c.Expr[Any] *): c.Expr[java.util.TimeZone] = {
  import c.universe._
  import java.util.TimeZone

  val tzExpr: c.Expr[String] = c.prefix.tree match {
    case Apply(_, Apply(_, List(tz @ Literal(Constant(s: String)))) :: Nil)
      if TimeZone.getAvailableIDs contains s => c.Expr(tz)
    case Apply(_, Apply(_, List(tz @ Literal(Constant(s: String)))) :: Nil) =>
      c.abort(c.enclosingPosition, s"Invalid time zone! $s")
    case _ => ???
//            ^^^ What do I do here? I do not want to abort, I merely wish to
//                "carry on as you were". I've tried ...
//                    c.prefix.tree.asInstanceOf[c.Expr[String]]
//                ...but that does not work
  }
  c.universe.reify(TimeZone.getTimeZone(tzExpr.splice))

}

最佳答案

这是处理时区插值的“歌曲和舞蹈”解决方案:

package object timezone {
  import scala.language.implicitConversions
  implicit def zoned(sc: StringContext) = new ZoneContext(sc)
}

package timezone {
  import scala.language.experimental.macros
  import scala.reflect.macros.Context
  import java.util.TimeZone

  class ZoneContext(sc: StringContext) {

    def tz(args: Any*): TimeZone = macro TimeZoned.tzImpl

    // invoked if runtime interpolation is required
    def tz0(args: Any*): TimeZone = {
      val s = sc.s(args: _*)
      val z = TimeZoned maybeTZ s getOrElse (throw new RuntimeException(s"Bad timezone $s"))
      TimeZone getTimeZone z
    }
  }
  object TimeZoned {
    def maybeTZ(s: String): Option[String] =
      if (TimeZone.getAvailableIDs contains s) Some(s) else None

    def tzImpl(c: Context)(args: c.Expr[Any]*): c.Expr[TimeZone] = {
      import c.universe._
      c.prefix.tree match {
        case Apply(_, List(Apply(_, List(tz @Literal(Constant(const: String)))))) =>
          maybeTZ(const) map (
            k => reify(TimeZone getTimeZone c.Expr[String](tz).splice)
          ) getOrElse c.abort(c.enclosingPosition, s"Bad timezone $const")
        case x =>
          val rts = x.tpe.declaration(newTermName("tz0"))
          val rt = treeBuild.mkAttributedSelect(x, rts)
          c.Expr[TimeZone](Apply(rt, args.map(_.tree).toList))
      }
    }
  }
}

用法:

package tztest

import timezone._

object Test extends App {

  val delta = 8
  //Console println tz"etc/GMT+$delta"  //java.lang.RuntimeException: Bad timezone etc/GMT+8
  Console println tz"Etc/GMT+$delta"
  Console println tz"US/Hawaii"
  //Console println tz"US/Nowayi"     //error: Bad timezone US/Nowayi
}

关于scala - StringContext和宏: a simple example，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/17028377/