DCI MoneyTransfer in Scala

MoneyTransfer.scala

case class Account(var amount: Int)

import MoneyTransfer._

class MoneyTransfer private (src: Account with SourceRole,
                             dst: Account with DestinationRole,
                             amount: Int) {
    def transfer(): Unit = {
        src transfer amount
    }

    private implicit class Source(acc: Account with SourceRole) {
        def withdraw(amount: Int): Unit = {
            acc.amount -= amount
        }

        def transfer(amount: Int): Unit = {
            dst deposit amount
            src withdraw amount
        }
    }

    private implicit class Destination(acc: Account with DestinationRole) {
        def deposit(amount: Int): Unit = {
            acc.amount += amount
        }
    }
}

object MoneyTransfer {
    trait SourceRole
    trait DestinationRole

    def apply(src: Account, dst: Account, amount: Int) =
        new MoneyTransfer (src.asInstanceOf[Account with SourceRole],
                           dst.asInstanceOf[Account with DestinationRole],
                           amount)

}