关联特征类型的约束 [英] Constraints on associated trait types

问题描述

这是一个(有些人为设计的)例子，用来说明我想做的事

Here's a (somewhat contrived) example to illustrate what I would like to do

pub trait Node: Eq + Hash {
    type Edge: Edge;
    fn get_in_edges(&self)  -> Vec<&Self::Edge>;
    fn get_out_edges(&self) -> Vec<&Self::Edge>;
}

pub trait Edge {
    type Node: Node;
    fn get_src(&self) -> &Self::Node;
    fn get_dst(&self) -> &Self::Node;
}

pub trait Graph {
    type Node: Node;
    type Edge: Edge;
    fn get_nodes(&self) -> Vec<Self::Node>;
}

pub fn dfs<G: Graph>(root: &G::Node) {
    let mut stack = VecDeque::new();
    let mut visited = HashSet::new();

    stack.push_front(root);
    while let Some(n) = stack.pop_front() {
        if visited.contains(n) {
            continue
        }
        visited.insert(n);
        for e in n.get_out_edges() {
            stack.push_front(e.get_dst());
        }
    }
}

是否可以用Graph特征表达Graph::Node必须与Graph::Edge::Node相同的类型，并且Graph::Edge必须与Graph::Node::Edge相同的类型?

Is there a way to express in the Graph trait that Graph::Node must be the same type as Graph::Edge::Node and that Graph::Edge must be the same type as Graph::Node::Edge?

我记得阅读过有关某个功能的信息(当时尚未实现)，该功能将允许对此类功能进行更丰富的约束，但是我不记得它的名称，也找不到它.

I remember reading something about a feature (not implemented at the time) that would allow richer constraints for this sort of thing, but I don't remember its name and cannot find it.

推荐答案

在Graph的定义中，可以将每个关联类型的关联类型(！)约束为等于Graph中的相应关联类型./p>

In Graph's definition, you can constrain each associated type's associated type (!) to be equal to the corresponding associated type in Graph.

pub trait Graph {
    type Node: Node<Edge = Self::Edge>;
    type Edge: Edge<Node = Self::Node>;
    fn get_nodes(&self) -> Vec<Self::Node>;
}

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