Scala Factory Pattern return the concrete class depending on the generic type

Question

I use the factory pattern with generics. The idea is to create the right implementation (BlockType1Impl or BlockType2Impl) depending on the type of A which is a case class( BlockchainType1 or BlockchainType2). I don't put any Type Bounds constraints.

Code

After looking at this example on the apply method with generic types

trait BlockTypeFactory[A]{

  def findTransactions( blocks: Seq[A], address: String): Seq[TransactionResponse]

}


object BlockTypeFactory{
  // I want this method to return the correct implementations 
  def getBlockExplorer[A](blockType: A): BlockTypeFactory[A] = {
    blockType match {
      case type1: BlockchainType1 => new BlockTypeFactory[BlockchainType1](new BlockType1Impl)
    // error : Expression of type BlockTypeFactory[BlockType1Impl] doesn't conform with the expected type BlockTypeFactory[A]
      case type2: BlockchainType2 => new BlockType2Impl
    }
  }

def apply[A](implicit ev: BlockTypeFactory[A],blockType: A):BlockTypeFactory[A] = ev

}

But I get an error about expected type . What is exactly wrong ?

Other classes

class BlockType1Impl extends BlockTypeFactory[BlockchainType1]

class BlockType2Impl extends BlockTypeFactory[BlockchainType2]

case class BlockchainType1(...)
case class BlockchainType2(...)

Answer 1

Your code doesn't work because the compiler doesn't know where to get the implicit instances of BlockTypeFactory.

In order to achieve your goal you can use Type Classes.

This way is extensible, you can have more than one factory per class if you want (you need to play with implicits scope) and you can define standard factories for some types.

You can code implicit instances of your case classes inside BlockTypeFactory object, but this is the way it is usually done.

// your type class
trait BlockTypeFactory[A] {
  def create:A
} 

case class BlockchainType1()

object BlockchainType1 {
  // type 1 impl
  implicit val factory:BlockTypeFactory[BlockchainType1] = new BlockTypeFactory[BlockchainType1] {
    def create: BlockchainType1 = BlockchainType1()
  }
}

case class BlockchainType2()

object BlockchainType2 {
  // type 2 impl
  implicit val factory:BlockTypeFactory[BlockchainType2] = new BlockTypeFactory[BlockchainType2] {
    def create: BlockchainType2 = BlockchainType2()
  }
}

object BlockTypeFactory {

  // get factory
  def apply[A:BlockTypeFactory]:BlockTypeFactory[A] = implicitly[BlockTypeFactory[A]]

  // or create
  def create[A:BlockTypeFactory]:A = implicitly[BlockTypeFactory[A]].create

}

val instance1 = BlockTypeFactory[BlockchainType1].create

val instance2 = BlockTypeFactory.create[BlockchainType2]

This pattern is called Type Class, and it is used to get ad hoc polymorphism. In your example, you need a polymorphic method findTransactions for each class defined on BlockTypeFactory.