Scala - handling "multiple overloaded alternatives of method ... define default arguments"

Question

Let's say I have a setting such as this:

sealed trait Annotation {
    def notes : Seq[String]
}

trait Something extends Annotation{
    //do something funny
}

case class A(val i:Int)(val notes:Seq[String] = Nil) extends Something
object A{
    def apply(a:A)(notes:Seq[String] = Nil):A = A(a.i)(notes)
}

case class B(val b:Boolean)(val notes:Seq[String] = Nil) extends Something
object B{
    def apply(b:B)(notes:Seq[String] = Nil):B = B(b.b)(notes)
}

case class C(val s:String)(val notes:Seq[String] = Nil) extends Something
object C{
    def apply(c:C)(notes:Seq[String] = Nil) :C = C(c.s)(notes)
}

Trying to compile this will result in

Main.scala:10: error: in object A, multiple overloaded alternatives of method apply define
default arguments.
object A{
       ^

Main.scala:15: error: in object B, multiple overloaded alternatives of method apply define
default arguments.
object B{
       ^

Main.scala:20: error: in object C, multiple overloaded alternatives of method apply define
default arguments.
object C{
       ^
three errors found

I have read this, so I do at least have an idea as to why this is happening, what I don't know, however, is how I am supposed to resolve the issue.

One possibility would - of course - be to simply omit the default values and force the client to provide Nil when no notes are to be stored, but is there a better solution?

Yes, you are right about the linked issue. Alternatively you can create a different method than apply. e.g. def withNotes(notes:Seq[String] = Nil) — Johnny Everson
– Johnny Everson, Commented May 20, 2015 at 7:17

Kulu Limpa · Accepted Answer · 2015-05-20 12:47:57Z

My first guess was to simply make the default arguments explicit:

case class A(i: Int)(val notes: Seq[String]) extends Something
object A {
  def apply(i: Int): A = new A(i)(Nil)
  def apply(a: A)(notes: Seq[String]): A = new A(a.i)(notes)
  def apply(a: A): A = new A(a.i)(Nil)
}

However, now, because of currying, you just have a function Int => A and Int => Seq[String] => A (and analogous for A => A) with the same name in scope.

If you refrain from currying you can manually define the overloaded methods:

case class B(b: Boolean, notes: Seq[String]) extends Something

object B {
  def apply(b: Boolean): B = B(b, Nil)
  def apply(b: B, notes: Seq[String] = Nil): B = B(b.b, notes)
}

But, since notes is now part of the same parameter list as b, the behavior of the case-class methods such as toString is changed.

println(B(true))                  // B(true,List())
println(B(true, List("hello")))   // B(true,List(hello))
println(B(B(false)))              // B(false,List())

Finally, to mimic the original behavior more closely, you can implement your own equals, hashCode, toString, and unapply methods:

class C(val s:String, val notes:Seq[String] = Nil) extends Something {

  override def toString = s"C($s)"

  override def equals(o: Any) = o match {
    case C(`s`) => true
    case _ => false
  }

  override def hashCode = s.hashCode
}

object C{
  def apply(s: String, notes: Seq[String]) = new C(s, notes)
  def apply(s: String): C = C(s, Nil)
  def apply(c:C, notes:Seq[String] = Nil): C = C(c.s, notes)
  def unapply(c: C): Option[String] = Some(c.s)
}

Example:

val c1 = C("hello")
val c2 = C("hello", List("world"))
println(c1)                 // C(hello)
println(c2)                 // C(hello)
println(c1 == c2)           // true
c1 match {                  // hello
  case C(n) => println(n)
  case _ =>
}

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