递归地使用TypeTag调用函数

Question

I'm playing with Scala TypeTag. I want to recursively call a function with a TypeTag parameter. Here is a simplified example of what I'm trying to do:

import scala.reflect.runtime.universe._

object TypeTagTest extends App {

  def intValue[T](value: T)(implicit tag: TypeTag[T]): Int = {
    tag.tpe match {
      // integer
      case intType if intType <:< typeOf[Int] =>
        value.asInstanceOf[Int]
      // string
      case stringType if stringType <:< typeOf[String] =>
        value.asInstanceOf[String].toInt
      // option of either string or integer
      case optionType @ TypeRef(_, _, typeArg::Nil) if optionType <:< typeOf[Option[_]] =>
        println(s"Unwrapped type is $typeArg")
        val option = value.asInstanceOf[Option[_]]
        option.map { optionValue =>
          // how to pass the typeArg here?
          intValue(optionValue)
        }.getOrElse(0)
    }
  }

  println(intValue(1))
  println(intValue("1"))
  println(intValue(Some("1")))

}

This code compiles and runs:

1
1
Exception in thread "main" scala.MatchError: Any (of class scala.reflect.internal.Types$TypeRef$$anon$6)
    at TypeTagTest$.intValue(TypeTagTest.scala:7)
    at TypeTagTest$$anonfun$intValue$2.apply(TypeTagTest.scala:19)
    at TypeTagTest$$anonfun$intValue$2.apply(TypeTagTest.scala:18)
    at scala.Option.map(Option.scala:145)

Couple of questions:

1. How to pass the type information when the recursive call is made?
2. Is there a way to make this pattern matching a little less ugly?

Answer 1

It's not a recursion problem - your "Unwrapped type..." line never prints. I suspect the problem is that optionType , in this case Some[String] , is not a strict subtype of Option[_] which expands to Option[A] forSome {type A} .

In this specific instance perhaps you want to test whether optionType <:< Option[Any] , since Option is covariant? Or perhaps you could check that the first argument of the TypeRef is Option .

Once you've fixed that, I don't think it's possible to obtain the TypeTag for a parameter since they're always generated by the compiler, but since you're only using the Type you can make the method work with those instead:

def intValueInner(value: Any, tpe: Type) = tpe match {
  ...
  case optionType @ TypeRef(_, _, typeArg::Nil) if optionType <:< typeOf[Option[Any]] =>
    value.asInstanceOf[Option[_]].map(v => intValueInner(v, typeArg)).getOrElse(0)
}
def intValue[T: TypeTag](t: T) = intValueInner(t, typeOf[T])