模式匹配语法可替代Scala宏中_ =：= typeOf [_]测试链

Question

Is there already a pattern match alternative for writing long winded tests like

if (aTpe =:= typeOf[Int]) 1
else if (aTpe =:= typeOf[Long]) 2
else if (aTpe =:= typeOf[Double]) 3
else ...

perhaps something that looks a little like

aTpe match {
  case tpe[Int] => 1
  case tpe[Long] => 2
  case tpe[Doble] => 3
  ...
}

It looks pretty easy to write an extractor for this, but I want to find out if there isn't already something like this.

Answer 1

What I've been doing in normal code so far is materialized types. I guess it could be used in macros as well:

val ClassOfInt = classOf[Int]
val ClassOfLong = classOf[Long]

aTpe match {
   case ClassOfInt => 1
   case ClassOfLong => 2
}

It works but I'd very much like to know if there's a 'terser' way of doing so.

Answer 2

Of course it is possible to write your if-else-chain nearly the same way with a pattern match and guards:

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> def f[A : TypeTag] = typeOf[A] match {
    case t if t =:= typeOf[Int] => 1
    case t if t =:= typeOf[Long] => 2
    case _ => 0
  }
f: [A](implicit evidence$1: reflect.runtime.universe.TypeTag[A])Int

scala> f[Int]
res5: Int = 1

scala> f[String]
res6: Int = 0

However a construct like case tpe[Int] => as shown in your question is not possible because of this ticket: SI-884 - it is just an unimplemented feature.