Scala - return a type

Question

I want to return a type from a function. For example:

class Super
case class One(a: Int) extends Super
case class Two(b: Float) extends Super
case class Unknown extends Super

def decide(criterion: String): ??? = {
  criterion match {
    case "one" => One
    case "two" => Two
    case _ => Unknown
  }
}

So I want to return the type itself, to store it in a Map so that I could apply it somewhere later:

val test = Buffer(
  ("ahaha" -> "one")
  ("ohoho" -> "two")
  ("lalala" -> "one")
)

var map = scala.collection.mutable.Map[String, Super]()

test.map {pair =>
  map(pair._1) = decide(pair._2)
}

So that later I could like:

def act(code: String) {
  map(code) match {
    case One => doSmth[One]()
    case Two => doSmth[Two]()
    case _ => doNothing()
  }
}

I know that some parts, like the unused parameters of the case classes may seem strange here, but this is how it is in the environment I am working in, and this example is that full because I am not sure if it will differ if I take something away...

So how can I make the decide function return a type and then use it in a manner similar to what I have shown?

Answer 1

I think you may want case object One, etc, rather than using Class or ClassTag. Then you get useful match support. For the act method, your case objects could return a ClassTag or similar, or just let act associate One with doSmth[OneClass] etc.

It seems you can make your case companions into case objects. Isn't that special.

package typeswitch
import reflect.runtime.universe._

sealed trait Selection

class Super
case class One(a: Int) extends Super
case object One extends Selection
case class Two(b: Float) extends Super
case object Two extends Selection
case class Unknown() extends Super
case object Unknown extends Selection

object Test extends App {
  type What = Selection

  def decide(criterion: String): What = criterion match {
    case "one" => One
    case "two" => Two
    case _ => Unknown
  }

  val test = List(
    "ahaha" -> "one",
    "ohoho" -> "two",
    "lalala" -> "one"
  )

  val m = scala.collection.mutable.Map[String, What]()

  test map (pair => m(pair._1) = decide(pair._2))

  def act(code: String) = m(code) match {
    case One => doSmth[One]()
    // non-exhaustive
    //case Two => doSmth[Two]()
    case Unknown => doNothing()
    // handle exhaustively
    case s: Selection => doSmthNew(s)
  }
  def doSmthElse[A <: Super]()(implicit t: TypeTag[A]): A = {
    Console println s"Do st with $t"
    val claas: Class[_] = t.mirror.runtimeClass(t.tpe)
    null.asInstanceOf[A]
  }
  def doSmth[A <: Super]()(implicit t: ClassTag[A]): A = {
    Console println s"Do st with $t"
    val claas: Class[_] = t.runtimeClass
    null.asInstanceOf[A]
  }
  def doSmthNew[A >: What : ClassTag, B <: Super](what: A): B = {
    Console println s"Do st new with $what"
    null.asInstanceOf[B]
  }
  def doNothing() { }

  val res = act("lalala")
  Console println s"Got $res?"
}

Answer 2

Sorry if this is too basic, but:

$ scala
Welcome to Scala version 2.10.0-RC2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_06).
Type in expressions to have them evaluated.
Type :help for more information.

scala> trait Bar
defined trait Bar

scala> case class Foo(i:Int) extends Bar
defined class Foo

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

scala> def f[A <: Bar : TypeTag]() = println(s" Do ${ implicitly[TypeTag[A]] }") 
f: [A <: Bar]()(implicit evidence$1: reflect.runtime.universe.TypeTag[A])Unit

scala> f[Foo]
 Do TypeTag[Foo]