Type erasure problem in method overloading

问题

I have two overloaded method having following signatures -

def fun(x: Seq[String]): Future[Seq[Int]] = ???
def fun(x: Seq[(String, String)]): Future[Seq[Int]] = ???

Due to type erasure, these methods can't be overloaded and hence showing compilation error. I tried using typetags as a workaround -

def fun[t: TypeTag](values: Seq[T]): Future[Seq[Int]] = {
    typeOf[T] match {
        case t if t =:= typeOf[String] => ???
        case t if t =:= typeOf[(String, String)] => ???
        case _ => ???    // Should not come here
     }
}

There are two problems with this approach that I am facing -

1. How can I use the values in the Seq ?
2. How can I avoid Should not come here case ?

Thanks in advance.

回答1:

Try DummyImplicit approach instead of TypeTag

def fun(x: Seq[String]): Future[Seq[Int]] = ???
def fun(x: Seq[(String, String)])(implicit ev: DummyImplicit): Future[Seq[Int]] = ???

How can I use the values in the Seq ?

Note even though TypeTag enables overcoming type erasure I do not think compiler inserts a cast automatically, so you would have manually call asInstanceOf or equivalent

case t if t =:= typeOf[String] => 
  val vs: Seq[String] = values.asInstanceOf[Seq[String]]
  ???

As a side note, TypeTag requires dependency on scala-reflect.jar (which might be a disadvantage, say, if we care about package size).

回答2:

Common practice for this situation is to use Magnet Pattern

回答3:

For method overloading you can use

1. DummyImplicit (see @MarioGalic's answer)

   def fun(x: Seq[String]): Future[Seq[Int]] = ???
   def fun(x: Seq[(String, String)])(implicit ev: DummyImplicit): Future[Seq[Int]] = ???
   
   fun(Seq("a", "b"))
   fun(Seq(("a", "b"), ("c", "d")))
   

2. type class

   trait FunTypeclass[A] {
     def fun(x: A): Future[Seq[Int]]
   }
   object FunTypeclass {
     implicit val string: FunTypeclass[Seq[String]] = x => ???
     implicit val stringTuple2: FunTypeclass[Seq[(String, String)]] = x => ???
   }
   def fun[A](x: A)(implicit ftc: FunTypeclass[A]): Future[Seq[Int]] = ftc.fun(x)
   
   fun(Seq("a", "b"))
   fun(Seq(("a", "b"), ("c", "d")))
   

or

3. magnet

   import scala.language.implicitConversions
   
   trait FunMagnet {
     def fun(): Future[Seq[Int]]
   }
   object FunMagnet {
     implicit def fromString(x: Seq[String]): FunMagnet = () => ???
     implicit def fromStringTuple2(x: Seq[(String, String)]): FunMagnet = () => ???
   }
   def fun(x: FunMagnet): Future[Seq[Int]] = x.fun()
   
   fun(Seq("a", "b"))
   fun(Seq(("a", "b"), ("c", "d")))
   

Overloading methods based on generics

来源：https://stackoverflow.com/questions/64313268/type-erasure-problem-in-method-overloading