I couldn't find an answer in both stackoverflow and the Julia docs to the following "design problem":

Let's say I want to define the following object

struct Person
birthplace::String
age::Int
end

I sometimes need to change the value in a single field of an instance of an immutable type. This is especially useful when an instance of this type is spread around different places and I need to update one of its values. This way the update occurs everywhere this instance sits, because it’s the same instance.  The way I accomplish this is by wrapping that specific field in a Vector:  julia> immutable A            x::Int            y::Vector{Int} # wrap it        end julia> a = A(1,[1]) # a live...

According to the docs, operating on immutable types can result in increased performance. On first thought, I assumed this was because the data contained inside immutable objects is known to be unchanging, but upon tinkering a bit, I realized immutable data can be changed when the data lies in an array:  immutable Test     x1::Float64     x2::Array{Float64,1} end  test = Test(1.0,[1.0]) # This is an invalid operation test.x1 = 2.0 # This is a valid operation test.x2[1] = 2.0  So my questions are: ...