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Nick Buckner
Nick Buckner

Reputation: 13

Implementing a list of Ints in Scala's case classes not working

Why do I get an error when trying to implement a linked list in Scala using case classes?

As I am quite familiar with Haskell I am essentially trying to convert this Haskell code into Scala:

Data List = Cons Int List | Nil

instance Show List where
    show x = helper x ""
        where
            helper Nil acc = acc
            helper (Cons n xs) acc = let y = acc ++ " " ++ show n in helper xs y

Here is my Scala code: 

abstract class List1
case class Cons(head : Int, tail : List1) extends List1
case object Nil extends List1

object thing {
    def printList (x : List1) {
        var acc = x
        while (acc != Nil) {
            println (acc.head)
            acc = acc.tail
            }
        }
    val a : List1 = Cons(1,Cons(2,Cons(3,Cons(4,Nil))))
    printList(a)
}

However, I get a strange error saying that when I call acc.tail, this is not part of List1, nor is Nil, even though they both extend List1. I don't really understand OOP (just procedural and functional programming) so I assume something funky is happening with the type system and the object system.

Upvotes: 1

Views: 108

Answers (1)

Didier Dupont
Didier Dupont

Reputation: 29538

You could do that very much like in Haskell

def printList(x: List1) = {
   x match {
     case Cons(hd, tl) => println(hd); printList(tl)
     case Nil => 
  }
}

Your problem is that acc != Nil is not enough for the compiler to understand that acc is a Cons. head and tail are available on Cons only. Pattern matching is the normal way to go. Otherwise (more object-like), make head and tail available in List1 : 

sealed trait List1 {
   def head: Int
   def tail: List1
   def isEmpty: Boolean
}

case class Cons(head: Int, tail: List1) extends List1 {
  def isEmpty = false
}

case object Nil extends List1 {
   def head = sys.error("Head on Nil")
   def tail = sys.error("Tail on Nil")
   def isEmpty = true
}

Upvotes: 5

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