In Scala, is there a way to have two overloaded methods that only differ in whether an implicit is available?

Question

I'm writing a Scala application that accesses a database. Most of the time, there will be a connection available, but sometimes there won't be. What I'd like to do is something like the following:

object User {
  def authenticate(username: String, password: String)
      (implicit conn: Connection): Option[User] = {
    // use conn to grab user from db and check that password matches
    // return Some(user) if so, None if not
  }
  def authenticate(username: String, password: String): Option[User] = {
    implicit val conn = DB.getConnection()
    authenticate(username, password)
  }
}

What I hoped would happen is that, if there's an implicit value of type Connection available, the compiler would use the first method. If not, it would use the second. Unfortunately, I've discovered that the compiler isn't quite that smart or, if it is, I'm not telling it what to do in the right way.

So, my basic question is, is there a way to write a method that expects an implicit argument and then provide an overloaded version of the same method that creates an acceptable value of the implicit parameter's type if there isn't one available.

You might say, "Why would you want to do such a thing? If you can create an acceptable value of the appropriate type, why not just always do it?" And that's mostly true, except that if I have an open database connection, I'd prefer to go ahead and use it rather than creating a new one. However, if I don't have an open database connection, I know where to get one.

I mean, the simple answer is to just give the two methods different names, but I shouldn't have to, gosh-darn-it. But maybe I do...

Thanks! Todd

Answer 1

You don't need overloaded methods. Just give your implicit parameter a default value, i.e.

object User {
  def authenticate(username:String, password:String)(implicit conn:Connection = null): Option[User] = {
    val real_conn = Option(conn).getOrElse(DB.getConnection())
    // do the rest with the real_conn
  }
}