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Overriding Parameterized Methods in Scala

Note. Apologies for changing this question, I was previously unable to express the exact question I ran into.

Let's say we have an abstract parameterized class abs

,

abstract class abs[T] {
    def getA(): T
}

class ABS[Int] extends abs[T] {
    override def getA(): Int = 4
}

gives the following weird error:

<console>:28: error: type mismatch;
 found   : scala.Int(4)
 required: Int
       override def getA(): Int = 4
                              ^

I want to override a method getA

.

It would also be very helpful to get an explanation or a helpful link.

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3 answers


Maybe you want something like this


 abstract class Abs[T] {
    def getA: T
    def getA(i: T): T
  }

class Absz extends Abs[Int]{ override def getA = 4 override def getA(i: Int) = i }





(new Absz()).getA //> res0: Int = 4 (new Absz()).getA(3) //> res1: Int = 3









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With the current base method signature, this is not possible as the generics are being erased.

Take the type tag (I also renamed the method T

to U

to prevent confusion due to shading):



abstract class abs[T] {
    def getA[U: TypeTag](): U
}

class ABS[T] extends abs[T] {
    override def getA[U]()(implicit tag: TypeTag[U]): U = {
        if (tag == typeTag[Int]) new Integer(1)
        else if (tag == typeTag[Char]) new Character('1')
        else throw new Exception("bad type")
    }.asInstanceOf[U]
}
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You can do this with Shapeless polymorphic functions: https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0

import shapeless._

object Abs extends Poly0{
  implicit def caseInt = at[Int](4)
  implicit def caseChar = at[Char]('4')
}

println(Abs.apply[Int])
println(Abs.apply[Char])
0


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