Why don't purely functional languages use reference counting?

Question

In purely functional languages, data is immutable. With reference counting, creating a reference cycle requires changing already created data. It seems like purely functional la

Answer 1

Your question is based on a faulty assumption. It's perfectly possible to have circular references and immutable data. Consider the following C# example which uses immutable data to create a circular reference.

class Node { 
  public readonly Node other;
  public Node() { 
    other = new Node(this);
  }
  public Node(Node node) {
    other = node;
  }
}

This type of trick can be done in many functional languages and hence any collection mechanism must deal with the possibility of circular references. I'm not saying a ref counting mechanism is impossible with a circular reference, just that it must be dealt with.

Edit by ephemient

In response to the comment... this is trivial in Haskell

data Node a = Node { other :: Node a }
recursiveNode = Node { other = recursiveNode }

and barely any more effort in SML.

datatype 'a node = NODE of unit -> 'a node
val recursiveNode : unit node =
    let fun mkRecursiveNode () = NODE mkRecursiveNode
    in mkRecursiveNode () end

No mutation required.