Hi @Tortar, yes the correct way to do this would be to wrap the contents in a RefValue (or whatever other type of mutable struct you like).

You should definitely not be doing s.data.data[1][] = 3 though, that's bad.

Instead, you should do something like

s = A(Ref(0), Ref(0))
@cases s begin
    A(x, _) => x[] = 3
    B(x, _) => x[] = 3
end

thanks for the prompt responses @MasonProtter, what I'd like to ask you is if you think that creating an interface in another package which mimics the working of structs, mutable structs would make sense (and would be possible) in your opinion? Because for example I'm thinking to use SumTypes for some heterogeneous data containers (multi-agent systems) in Agents.jl but I need a syntax equivalent to that of mutable structs to expose to users

for example it seems maybe possible to wrap the cases macro in getproperty/setproperty

I guess it would be possible to make a Sum Type appear like a regular struct for the purposes of a user-facing interface, but how to do it would be mostly up to you and would depend on your use-case. I don't really have enough information to give advice on this