Rust:使用通用特征作为特征参数

Question

如何在 Rust 中使用相关的泛型类型？

这是我得到的(只有第一行给我带来了麻烦):

impl<G, GS> TreeNode<GS> where G: Game, GS: GameState<G>{
    pub fn expand(&mut self, game: &G){
        if !self.expanded{
            let child_states = self.data.generate_children(game);
            for state in child_states{
                self.add_child_with_value(state);
            }
        }
    }
}

GameState是对 Game 通用的特征, 和 self.data工具 GameState<Game>这种类型的。编译器告诉我

error[E0207]: the type parameter `G` is not constrained by the impl trait, self type, or predicates
  --> src/mcts.rs:42:6
   |
42 | impl<G, GS> TreeNode<GS> where G: Game, GS: GameState<G>{
   |      ^ unconstrained type parameter

error: aborting due to previous error

但在我看来，我在约束 G都在 expand功能，事实上， G需要属于 GS .我真的很感激任何帮助。

编辑:
到目前为止，这里还有一些定义

trait GameState<G: Game>: std::marker::Sized + Debug{
    fn generate_children(&self, game: &G) -> Vec<Self>;
    fn get_initial_state(game: &G) -> Self;
}

trait Game{}

struct TreeNode<S> where S: Sized{
    parent: *mut TreeNode<S>,
    expanded: bool,
    pub children: Vec<TreeNode<S>>,
    pub data: S,
    pub n: u32
}

impl<S> TreeNode<S>{
    pub fn new(data: S) -> Self{
        TreeNode {
            parent: null_mut(),
            expanded: false,
            children: vec![],
            data,
            n: 0
        }
    }

    pub fn add_child(&mut self, mut node: TreeNode<S>){
        node.parent = self;
        self.children.push(node);
    }

    pub fn add_child_with_value(&mut self, val: S){
        let new_node = TreeNode::new(val);
        self.add_child(new_node);
    }

    pub fn parent(&self) -> &Self{
        unsafe{
            &*self.parent
        }
    }

}

Answer 1

impl<G, GS> TreeNode<GS> where G: Game, GS: GameState<G>{
    // ...
}

问题是 G不受约束，因此此块中可能有多个(可能是冲突的)实现，因为 GS也许实现 GameState<G>为多个 G .参数 G是模棱两可的。

如果您想保留GameState<G>能够为多个 G 实现 ，您应该将约束从 impl 移开改为阻止该方法:

// note: G is now a type parameter of the method, not the impl block, which is fine
impl<GS> TreeNode<GS> {
    pub fn expand<G>(&mut self, game: &G) where G: Game, GS: GameState<G> {
        if !self.expanded{
            let child_states = self.data.generate_children(game);
            for state in child_states{
                self.add_child_with_value(state);
            }
        }
    }
}

如果您只想要 GameState为单个 G 实现 , 你应该做 G GameState 的关联类型而不是泛型类型参数:

trait GameState: std::marker::Sized + Debug {
    type G: Game;
    fn generate_children(&self, game: &Self::G) -> Vec<Self>;
    fn get_initial_state(game: &Self::G) -> Self;
}

// note: now G is given by the GameState implementation instead of
//       being a free type parameter
impl<GS> TreeNode<GS> where GS: GameState {
    pub fn expand(&mut self, game: &GS::G){
        if !self.expanded{
            let child_states = self.data.generate_children(game);
            for state in child_states{
                self.add_child_with_value(state);
            }
        }
    }
}