Passing by reference an passing by value in julialang

function_parameter_pasing.jl

#= assignment of a function to a new name
    - is shallow copy of a function
    - in programming parlance: done by reference
=#

f(x) = 2x
g = f
g #f (generic function with 1 method)
g(2) #4
f(x) = 4x
g(2) #8


# alter variable by reference

# by use of Ref keyword
f(x::Ref) = x[] = 4
a = Ref{Int}(0)
f(a)
(a[], typeof(a[])) #(4, Int64)

# use of struct (somehow more idiomatic?? @ https://discourse.julialang.org/t/how-to-modify-value-of-scalar-argument-passed-to-function-so-that-it-persists-in-calling-function/27151/5)

f(x) = x.value = 1000
mutable struct s
    value::Int64
    s() = new(1)
end
s_instance = s()
s_instance.value |> typeof
f(s_instance)
s_instance.value # 1000

# wrapping the value into an array
f(x::Vector) = x[1] = 4
x = [0]
x
f(x)
x
# 1-element Vector{Int64}:
# 4

# use of global variable
# Note! Variable 'a' has to be initilised before the function definition

a = 0
function f(a::Int)
     global a = 1000
    return nothing
end
f(a)
a #1000