How do I make a struct generic over two types for information that is sometimes hidden?-CodePudding

I am making a multiplayer game, where the server has full knowledge of the game state (Sure), but the client only sees a part of it (SometimesSure, parts of the world are hidden, e.g. cards in a deck). I want to use the same GameState struct though and make it generic over those two cases:

enum SometimesSure<T> {
    Hidden,
    Visibile(T),
}

struct Sure<T>(T);

struct GameState {
    pub map_state: MapState,
    pub player_states: PlayerStates,
}

where MapState and PlayerStates should be on the client:

struct MapState {
    pub tiles: Vec<SometimesSure<u8>>,
}
struct PlayerStates {
    pub points: SometimesSure<bool>,
    pub name: SometimesSure<String>,
}

and on the server:

struct MapState {
    pub tiles: Vec<Sure<u8>>,
}
struct PlayerStates {
    pub points: Sure<bool>,
    pub name: Sure<String>,
}

How can I solve this with Generics or Associated Types?

CodePudding user response：

Ideally you would use higher kinded types here, but Rust does not support the concept. However, since generic associated types (GATs) were recently stabilized, you can use a helper trait (StateType in the code below) with a generic associated type to fulfill the same purpose:

trait StateType {
    type Type<T>;
}

struct ClientStateType;

impl StateType for ClientStateType {
    type Type<T> = SometimesSure<T>;
}

struct ServerStateType;
impl StateType for ServerStateType {
    type Type<T> = Sure<T>;
}

struct MapState<S: StateType> {
    pub tiles: Vec<S::Type<u8>>,
}

struct GameState<S: StateType> {
    pub map_state: MapState<S>,
}