How to correctly define and use global variables in the module in Julia?

General Usage
2

Hi CRquantum! And welcome to the Julia discourse!

Having global variables is not considered the best practice because it makes code difficult to track. The other disadvantage is that unless it has a constant type it will affect your performance. But this is what I think you are trying to solve. If you still want to do this, you can use Ref:

global const var = Ref{Float64}(0.0) # Store it into a `Ref` which it's mutable but has a constant type

function f(x::Float64)
    global var[] = 2.0*x
    return nothing
end

f(2.0)

var[] #prints 4.0

You only need to write global the first time that the variable appears in a given scope and its child scopes.

And it doesn’t accept values of other types:

julia> var[] = "hola"
ERROR: MethodError: Cannot `convert` an object of type String to an object of type Float64
Closest candidates are:
  convert(::Type{T}, ::Base.TwicePrecision) where T<:Number at twiceprecision.jl:250
  convert(::Type{T}, ::AbstractChar) where T<:Number at char.jl:180
  convert(::Type{T}, ::CartesianIndex{1}) where T<:Number at multidimensional.jl:136
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