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I am getting frustrated because I don't know how to handle the frontier between my typesafe code and the external API which uses polymorphism and inheritance.

My flow is the following. I receive an entry value of class Class1, which I use to retrieve from an external service an item of Class Class2. Then I need to subtype on both to obtain the runtime type of them and resolve the implicit. However, this is not possible because of type erasure. 

trait Typeclass1[A, B] {
  def hash(a: A, b: B): String
}

trait Typeclass2[A, B] {
  def hash(a: A, b: B): B
}

trait Entity

trait MyEntity1

trait MyEntity2

object db {
  def load(any:Any):Entity = new Entity{}
}


class MyClass[T](t: T, a: String) {

  def apply(timeout: Long): T = {
    val loadFromDB = db.load(t)
    loadFromDB match {
      case myEntity1: MyEntity1 => applyTypeSafe(myEntity1)
      case myEntity2: MyEntity2 => applyTypeSafe(myEntity2)
    }
  }


  def applyTypeSafe[C](c: C)(implicit typeClass1: Typeclass1[C, T], typeclass2: Typeclass2[C, T]): (String, T) = {
    typeClass1.hash(c, t) -> typeclass2.hash(c, t)
  }
}

I am wondering what is the right pattern to develop this frontier layer. I would probably need a type-constructor for my typeclass to provide at at the constructor of MyClass... or maybe to totally rethinkg my design?

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I do not see the problem. Which types get erased? As far as I can see you should be able to pattern match reliably on loadFromDB and then you have recovered all the type info the DB layer looses. – gzm0 17 hours ago

1 Answer

up vote 0 down vote

There is no compilation issue if you add implicit definitions in scope for MyEntity and MyEntity2. For example, the code below compiles ok:

trait Typeclass1[A, B] { def hash(a: A, b: B): String }
trait Typeclass2[A, B] { def hash(a: A, b: B): B }

trait Entity
trait MyEntity1 extends Entity
trait MyEntity2 extends Entity

object db { def load(any:Any):Entity = new Entity {} }

implicit def MyEntity1HasTypeclass1[T] = new Typeclass1[MyEntity1, T] {
  def hash(a: MyEntity1, t: T) = a.toString
}
implicit def MyEntity1HasTypeclass2[T] = new Typeclass2[MyEntity1, T] {
  def hash(a: MyEntity1, t: T) =t
}

class MyClass[T](t: T, a: String) {
  def apply(timeout: Long): (String, T) = {
    db.load(t) match {
      case myEntity1: MyEntity1 => applyTypeSafe(myEntity1)
    }
  }
  def applyTypeSafe[C](c: C)(implicit typeClass1: Typeclass1[C, T], 
                                      typeclass2: Typeclass2[C, T]): (String, T) = {
    typeClass1.hash(c, t) -> typeclass2.hash(c, t)
  }
}
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