Trying to get a parametrized function that would return the specified coproduct of a trait.
sealed trait Tr { def x: Int }
case class Cl1(x: Int) extends Tr
case class Cl2(x: Int) extends Tr
def getTr[A <: Tr](i: Int): A = {
???
}
How can this be achieved in Scala 2.11?
You could use a TypeTag for A and compare it to Cl1 and Cl2:
import scala.reflect.runtime.universe._
def getTr[A <: Tr : TypeTag](i: Int): Tr =
if (typeOf[A] <:< typeOf[Cl1]) Cl1(i)
else if (typeOf[A] <:< typeOf[Cl2]) Cl2(i)
else ??? // etc
getTr[Cl1](42) // Cl1(42)
getTr[Cl2](42) // Cl2(42)
Note that I had to change the return type to Tr. It can't work with A because the compiler wouldn't allow having branches that resolve into different types for A (we don't know the actual A, but it surely can't be both Cl1 and Cl2 at the same time). Side note: GADTs actually have a bit of special treatment by the compiler which allows such cases, e.g.:
object Test {
def eval[T](e: Expr[T]): T = e match {
case IntExpr(i) => i // is T Int?
case BoolExpr(b) => b // ...or is it Boolean?
}
}
By the way, is your question a contrived example or a real scenario? If it's the latter, I feel it should be possible to implement it in a more elegant way.
