Instantiating immutable paired objects

Question

Is it possible to create a class with an immutable reference to a partner object, or does it have to be a var that I assign after creation?

e.g.

class PairedObject (p: PairedObject, id: String) {
  val partner: PairedObject = p  // but I need ref to this object to create p!
}

or similarly how could I instantiate the following pair?

class Chicken (e: Egg) { 
  val offspring = e
}

class Egg (c: Chicken) {
  val mother = c
}

Potherb

Answer 1

Here is a complete solution to the Chicken/Egg problem:

class Chicken (e: =>Egg) { 
  lazy val offspring = e 
}

class Egg (c: =>Chicken) {
  lazy val mother = c
}

lazy val chicken: Chicken = new Chicken(egg)
lazy val egg: Egg         = new Egg(chicken)

Note that you have to provide explicit types to the chicken and egg variables.

And for PairedObject:

class PairedObject (p: => PairedObject, val id: String) {
  lazy val partner: PairedObject = p
}

lazy val p1: PairedObject = new PairedObject(p2, "P1")
lazy val p2: PairedObject = new PairedObject(p1, "P2")

Waterline

I added solution for PairedObject as well. Also I found that vals for chicken and egg don't need to be lazy. Strictly you don't need to include types for both, as one can be inferred. – Potherb

This solution will work if entered at the top level of a class or object definition, but one gets an illegal forward reference error if chicken and egg are local variables (say, inside a function). A solution that works in any scope is: lazy val (egg: Egg, chicken: Chicken) = ..., which can be self-referential. – Khan

@KiptonBarros -- I think you should write up your comment as an answer, because, sorry Eric, as ingenious as the response was, it was woefully incomplete without KB's improvement. See here for more. – Petiolate

@Malvolio that was my fault: I tested this in 2.9 in an App object which explains why lazy was not required. More generally it will be within a function where it is, as @Kipton points out. I've editied it back to Eric's original. – Potherb

This is a good solution, but note that the lazy val is a little bit of "cheating", because the compiler generates a variable that can be internally mutated once (on first access). – Rhiamon

Answer 2

If your problem is circular references, you could use the solution posted in this SO question:

scala: circular reference while creating object?

This solves the chicken/egg problem.

Teheran

Works well if I just have a couple of case objects: just make the val lazy. – Potherb