Implementing leaf and non-leaf node by using parametric type

General Usage
1

Hello everyone,
I would like to implement a binary tree by using the parametric type instead of inheritance.
The structure is something like

# leaf node
struct Node{T}
  vale::T
  first_weights::Nothing
  second_weights::Nothing
  lchild::Nothing
  rchild::Nothing
end

#non-leaf node
struct Node{T}
  vale::T
  first_weights::Vector{T}
  second_weights::Vector{T}
  lchild::Node{T}
  rchild::Node{T}
end

For a leaf node, the types of first_weights, second_weights, lchild and rchild are Nothing.

The idea I have so far is

struct Node{T,W<:Union{Vector{T},Nothing},ND<:Union{Node,Nothing}}
  val::T
  first_weights::W
  second_weights::W
  lchild::ND
  rchild::ND
end

However, this design allows the possibility, for example

# an unreasonable Node
struct Node{T}
  vale::T
  first_weights::Vector{T}
  second_weights::Vector{T}
  lchild::Nothing
  rchild::Nothing
end

How can I avoid this problem? Is there any better ways to design this structure? Thanks.

Solution provided by @Henrique_Becker: defining a internal structure Branch.

struct Node{T}
  value::T
  branch::Union{Nothing,Branch{T}}
end

struct Branch{T}
  first_weights::Vector{T}
  second_weights::Vector{T}
  lchild::Node{T}
  rchild::Node{T}
end
2

Instead of the four last fields you could use two fields (one for each side) that are either nothing or a composite internal struct (lets call it Branch{T}) that has the two associated fields (child and weights). Note the child and weights do not need to support Nothing values as the Branch{T} will only exist if it exists.

3

Thanks for your reply. I just updated your solution to the post.

4

Is the tree complete? (This is, it always has the two branches or none?) What I proposed was slightly different:

struct Branch{T, W}
  weights::Vector{W}
  child::Node{T, W}
end

struct Node{T, W}
  value::T
  left::Union{Nothing,Branch{W}}
  right::Union{Nothing,Branch{W}}
end