Approach to modify a struct using helpers within the struct?

help
3

I'm just not seeing how this disjointness can be done at a practical level. The apply_turn function, regardless of where it resides, needs to have access to a mutable State. Something has to determine which fields are to be modified, and then modify them. The only way that thing can access those fields is if it has access to a mutable state.

Consider the top-level game loop:

loop {
   read_state( &mut state );
   let turn = eval_turn( &state );
   state.apply_turn( turn );
}

apply_turn needs a mutable state object, otherwise it can't work. How can I create disjoint functions without just inlining the apply_turn function?