StringContext and macros: a simple example

Question

I'm trying to achieve a StringContext extension which will allow me to write this:

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

But with the added caveat that it should fail to compile if the supplied time-zone is invalid (assuming that can be determined at compile-time).

Here's a helper function:

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

I can create a non-macro implementation very easily:

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")
 |   }
 | }

Then:

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

So far, so good. Except that this doesn't fail compilation if the timezone is nonsensical (e.g. zone"foobar"); the code falls over at runtime. I'd like to extend it to a macro but, despite reading the docs, I'm really struggling with the details (All of the details, to be precise.)

Can anyone help to get me started here? The all-singing, all-dancing solution should look to see if the StringContext defines any arguments and (if so), defer calculation until runtime, otherwise attempting to parse the zone at compile-time

What have I tried?

Well, macro defs appear to have to be in statically accessible objects. So:

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?
      }
    }

}

Based on som-snytt's suggestion below, here's the latest attempt:

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?
}

At the use-site, a valid zone"UTC" fails to compile with the error:

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

Presumably I should not have used a Literal(Constant( .. )) to enclose it. What should I have used?

Last example - based on Travis Brown's answer below

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))

}

som-snytt · Accepted Answer

This is the "song-and-dance" solution that handles interpolation of the timezone:

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))
      }
    }
  }
}

Usage:

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
}

StringContext and macros: a simple example

Answers (2)

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