# How to self reference a struct
**URL:** https://discourse.julialang.org/t/how-to-self-reference-a-struct/91795
**Category:** New to Julia
**Created:** [December 17, 2022, 9:44pm UTC](https://discourse.julialang.org/t/how-to-self-reference-a-struct/91795 "2022-12-17T21:44:11Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Ray\_Walker](https://discourse.julialang.org/u/Ray_Walker)
#### Post date: [December 17, 2022, 9:44pm UTC](https://discourse.julialang.org/t/how-to-self-reference-a-struct/91795/1 "2022-12-17T21:44:11Z")
How do you define an array in a struct of itself?
julia\> mutable struct node
level::Int
candidate::Int
children::Array{node, 3}
end
ERROR: invalid redefinition of constant node
Stacktrace:
[1] top-level scope
@ REPL[5]:1
---
### Author:  [@jmair](https://discourse.julialang.org/u/jmair)
#### Post date: [December 17, 2022, 9:57pm UTC](https://discourse.julialang.org/t/how-to-self-reference-a-struct/91795/2 "2022-12-17T21:57:46Z")
Interesting, I guess the following would work:
```julia
abstract type AbstractNode end
struct Node{T<:AbstractNode}<:AbstractNode
level::Int
candidate::Int
children::Array{T,3}
end
```
But I think you’d never be able to instantiate this since you can’t use leaf nodes, or they can’t be null. You probably want to use a union or something to make it practical.
EDIT: As stated below, an empty array is perfectly fine, so there’s not an issue with instantiation.
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [December 17, 2022, 11:46pm UTC](https://discourse.julialang.org/t/how-to-self-reference-a-struct/91795/3 "2022-12-17T23:46:48Z")
> [@Ray\_Walker](#):
>
> ERROR: invalid redefinition of constant node
That should work, I’ve done such constructs a number of times - do you perhaps already have something called `node` in that session? That’s what the error suggests, at least. What do you get in a new session?
Also, please quote your code, to make it easier to help you.
> [@Please read: make it easier to help you](https://discourse.julialang.org/t/please-read-make-it-easier-to-help-you/14757):
>
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> [@jmair](#):
>
> But I think you’d never be able to instantiate this since you can’t use leaf nodes, or they can’t be null
You can use that - leaves just don’t have children, the array is empty. You can even have a `parent` field too - if the parent is equal to the object itself, you’re at the root. It’s also a bit more efficient to do it this way, since there’s no type instability involved anymore, unlike with the union approach.
---
### Author:  [@Ray\_Walker](https://discourse.julialang.org/u/Ray_Walker)
#### Post date: [December 19, 2022, 8:11pm UTC](https://discourse.julialang.org/t/how-to-self-reference-a-struct/91795/4 "2022-12-19T20:11:24Z")
I think that may have been the case. Thank you so much for your help. Closing.