main.scala

import user
import message

object Main {
  def main(args: Array[String]) {
    import MessageSender._
    
    val natsuki = User("Natsuki Ando")
    val sakura = User("Sakura Domyoji")
    
    val message = natsuki.sendMessage(sakura, "A message")
    
    println(message)
  }
}

message.scala

package message

import user

case class Message(sender: User, recipient: User, body: String)

object Message {
  def send(sender: User, recipient: User, body: String): Message =
    Message(sender, recipient, body)
}

class MessageSender(user: User) {
  def sendMessage(recipient: User, body: String): Message =
    Message(user, recipient, body)
}

object MessageSender {
  implicit def userToMessageSender(user: User): MessageSender = new MessageSender(user)
}

user.scala

package user

case class User(name: String)