Question

Is there anything that I should watch out for when I use classes that keeps themselves as members?

This works (it’s from a Scala Worksheet in Scala-IDE), but will this bite me at some point, ie Is this normal practice or bad practice and why?

object Play {

 println("Playing a bit")                         //> Playing a bit

 case class X(a: Int = 1, x: List[X]){
 }

 val y = X(3, List())                             //> y  : Play.X = X(3,List())
 val z = X(5, List(X(6, List())))                 //> z  : Play.X = X(5,List(X(6,List())))
 println(z)                                       //> X(5,List(X(6,List())))
 println(z.x.head.a)                              //> 6
}

Answer 1

This is a perfectly fine use of a case class. Case classes are great for defining Recursive structures like this.

For instance, if I wanted to define my own linked-list class, I could use a case class to facilitate easy pattern-matching functionality:

trait MyList[T]
case class Link[T](head: T, tail: MyList[T]) extends MyList[T]
case class End[T]() extends MyList[T]

@tailrec
def last[T](list: MyList[T]): Option[T] =
  list match {
    case End() => None
    case Link(head, End()) => Some(head)
    case Link(head, tail) => last(tail)
  }

println(last(Link(1, Link(2, Link(3, End())))))  // Some(3)
println(last(Link(1, End())))                    // Some(1)
println(last(End()))                             // None