```data class Node( val author: String, val mess...

# announcements

[LastExceed]

data class Node(
	val author: String,
	val message: String,
	val children: List<Node>,
	val parent: Node = this
)

why is this not defined in this context ?

context: Im making a chat client with tree structure instead of linear conversation. Every message is a comment on the message it replies to, except the root node which is a comment on itself (or should i make it nullable? i like to avoid nullability whenever possible)

[Kashif]

this is not yet created

[LastExceed]

is it even possible for an object to contain an immutable reference to itself?

[Kashif]

i don’t think so

[Kashif]

but you can make it Nullable

[Kashif]

data class Node(
	val author: String,
	val message: String,
	val children: List<Node>,
	val parent: Node?
)

[LastExceed]

yeah thats what i was considering as an alternative, but i'd prefer to avoid nullability if possible

[Kashif]

data class Node(
    val author: String,
    val message: String,
    val children: List<Node>,
    var parent: Node
) {
    init {
        parent = this
    }
}

[Kashif]

you can try this as well to avoid null ability

[LastExceed]

how would i specify a different node as parent in this case?

[LastExceed]

oh

[LastExceed]

its not immutable anymore

[Kashif]

yup indeed but an alternate for avoiding nullability

[LastExceed]

is it possible with neither nullability nor mutability?

[Kashif]

not in my knowledge

[Marc Knaup]

You should not create recursive data class instances. Calling .toString(), .equals() or .hashCode() on them will cause a StackOverflowError.

[LastExceed]

fair point

[LastExceed]

will go with nullability then for my case

[Marc Knaup]

Wil work for your case, but not fix the SOE 🙂

[LastExceed]

if i use null for the root node's parent i wont have selfreferences and therefore no SOE

[Marc Knaup]

That will do 👍

[LastExceed]

but i'd still be interested (just for the heck of it): can an immutable self-reference exist ?

[Marc Knaup]

Sure, but not in a data class afaik.

[Marc Knaup]

hmm, let me test something

[Kashif]

yes thats my understanding too, immutable self-reference not exists for data class

[LastExceed]

how is the fact that its a data class relevant here ?

[LastExceed]

if i remove the data keyword i still get the same error

[Marc Knaup]

Yeah but if its not a data class then you can do that:

class Node {   
    val parent = this
}

[Marc Knaup]

But a data class constructor’s parameters must all be val, so they cannot use this since you’re in the constructor.

[Marc Knaup]

Or actually, before the constructor

[Marc Knaup]

Technically it should work because this is already known when the default value is evaluated.

[Marc Knaup]

It’s just not allowed

[LastExceed]

so that would be topic for language design then

[Marc Knaup]

yeah I think so

[LastExceed]

alright that concludes my questions, thanks guys

[Kashif]

you are welcome

[Marc Knaup]

For sake of completion. Without `data`:

class Node private constructor(parent: Node?, foo: Unit) {

    val parent = parent ?: this


    constructor(parent: Node = useThis) :
            this(parent = parent.takeIf { it !== useThis }, foo = Unit)


    companion object {

        private val useThis = Node(parent = null, foo = Unit)
    }
}

[LastExceed]

class Node(
	val author: String,
	val message: String,
	val children: List<Node>,
	parent: Node? = null
) {
	val parent = parent ?: this
}

wouldn't this suffice ?

[Marc Knaup]

You wanted it to be non-nullable 😄

[LastExceed]

well in the end parent is non-nullable which is was i was aiming for. to be fair, the parameter specifying the value of a non-nullable variable being nullable is a mess, but frankly so is your solution 😄

[LastExceed]

anyway we're getting offtopic, i've decided to just use null as the roots parent