How to create a circular reference in Scala?

Question

Given the following code example

case class Thing(
    value: String,
    parents: Seq[Thing],
    children: Seq[Thing],
  )

val x = Thing(
    value = "foo",
    parents = Seq(y),
    children = Seq(),
)

val y = Thing(
    value = "bar",
    parents = Seq(x),
    children = Seq(),
)

errors out because y is not initialized when creating x, is there any way of performing this "circular referencing" without using a secondary data structure such as a hash map to manually create pointers to each object?

https://stackoverflow.com/questions/8374010/scala-circular-references-in-immutable-data-types — volia17, Mar 29 '22 at 15:30
Does this answer your question? [Scala: circular references in immutable data types?](https://stackoverflow.com/questions/8374010/scala-circular-references-in-immutable-data-types) — volia17, Mar 29 '22 at 15:30
Easiest way is to define parents/children as var, then update them — SwiftMango, Mar 29 '22 at 15:47

Goosefand · Accepted Answer · 2022-03-29T19:55:11.693

2

One way to fix the compile code and the runtime exception is:

class Thing(
  _value: => String,
  _parents: => Seq[Thing],
  _children: => Seq[Thing]
) {
  lazy val value = _value
  lazy val parents = _parents
  lazy val children = _children
}

object Thing {
  def apply(
      value: => String,
      parents: => Seq[Thing],
      children: => Seq[Thing]
  ): Thing = {
    new Thing(value, parents, children)
  }
}

val x: Thing = Thing(
  value = "foo",
  parents = Seq(y),
  children = Seq()
)

val y: Thing = Thing(
  value = "bar",
  parents = Seq(x),
  children = Seq()
)

Fields used in Thing class must be evaluated lazily but since Scala prohibits using call-by-name for public class parameters I stored them in private class parameters and made them publicly accessible through public lazy vals.

edited Mar 29 '22 at 19:55

answered Mar 29 '22 at 14:18

Goosefand

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Oh nice - how does it manage that? – Kieran Mar 29 '22 at 14:30
Also using a lazy val causes a stack overflow as I guess it's trying to endless instantiate the parent/child – Kieran Mar 29 '22 at 14:35
@Kieran You're right making x and y lazy just fixes the compile error but throws an exception during runtime. I've edited my answer. – Goosefand Mar 29 '22 at 15:47
You should explain how this help and why ;) – Gaël J Mar 29 '22 at 19:09
@GaëlJ Thanks for mentioning it. I added a little more explanation. – Goosefand Mar 29 '22 at 19:58

How to create a circular reference in Scala?

1 Answers1