MethodError calling constructor of parametric struct

Question

I'm trying to create a linked list in Julia. I have:

mutable struct LLNode{T}
    x::T
    next::Union{LLNode{T},Void}
    prev::Union{LLNode{T},Void}
end

mutable struct LinkedList{T}
   count::Int
   head::Union{LLNode{T},Void}
   tail::Union{LLNode{T},Void}
end

Now, the above code compiles fine. I can also run:x = LLNode(0,nothing,nothing) fine. But when I run y = LinkedList(0,nothing,nothing) I get a no method matching LinkedList(::Int64, ::Void, ::Void) error. What gives?

VERSION returns v"0.6.2"

Answer 1

When you write LLNode(0,nothing,nothing), Julia is able to figure out that it needs to construct an LLNode{Int} based upon the type of the first argument. But in LinkedList(0, nothing, nothing), there's quite literally nothing for it to go on to determine what the type parameter should be, so it doesn't know what to construct.

Instead, you either need to explicitly choose what you want T to be:

julia> LinkedList{Int}(0, nothing, nothing)
LinkedList{Int64}(0, nothing, nothing)

or it can get the T based upon a not-nothing argument:

julia> LinkedList(0, LLNode(0, nothing, nothing), nothing)
LinkedList{Int64}(0, LLNode{Int64}(0, nothing, nothing), nothing)