Why mutable struct cannot be defined inside a function in Julia

Question

Why mutable struct cannot be defined inside a function in Julia?

function test()
    mutable struct ABC
        x
        y
        z
        a
    end 
end

throws error:

ERROR: LoadError: syntax: "struct" expression not at top level

But if the struct is global that is outside the function and accessed inside function, the code works fine.

Wordplay

The question is: what is it that you want to do in the end? If you know the field names x, y, z, a beforehand, there's no need to define the struct within the function in the first place. And then, what should the function return? A type? An instance? – Conciliator

@Conciliator I want to create multiple instances of struct. Function uses struct. Function does not return struct. – Wordplay

@Wordplay I think you are confusing a type with instances of a type. A struct block does not create instances of a type, it defines the type itself. It is similar to the difference between a class and an object of a class in OOP languages. – Kilmarx

Answer 1

Struct types must be defined at top-level (i.e. in the "module scope"), as the error suggest, and you can not define function-local structs like in your example.

If you really don't want to define the struct type in a module, then you can use a NamedTuple, which can sometimes take the place of an "anonymous struct type". Example:

function test()
    nt = (x = 1, y = 2, z = 3, a = "hello")
    # ...
end

Holothurian

And Dict can work like kind-of mutable struct. – Lizettelizotte

Actually I want to make many instances of struct. Can it be done with NamedTuple? – Wordplay

The standard way is to just define the struct outside the function, what is wrong with doing that? Yes, you can create as many tuples/dicts as you want. – Holothurian

Answer 2

You could use meta-programming. Strongly not recommended unless you very exactly know that you want and need to use meta-programming.

function test()
    Main.eval(
        quote
           mutable struct ABC
               x
               y
               z
               a
           end
        end
    )
end

Testing:

julia> test()

julia> fieldnames(ABC)
(:x, :y, :z, :a)

Stheno