在 Rust 中，如何创建一个接受 "Marker Component" 作为类型参数的函数？

Question

我目前正在研究 Rust Sokoban tutorial, playing with the code as I type it in, to see how I can "improve" it without breaking it. In the chapter on Pushing boxes 他们引入了两个“标记组件”来“告诉我们哪些实体是可移动的，哪些不是”：

#[derive(Component, Default)]
#[storage(NullStorage)]
pub struct Movable;

#[derive(Component, Default)]
#[storage(NullStorage)]
pub struct Immovable;

后来，我们有如下代码：

                let mut mov: HashMap<(u8, u8), Index> = (&entities, &movables, &positions)
                    .join()
                    .map(|t| ((t.2.x, t.2.y), t.0.id()))
                    .collect::<HashMap<_, _>>();
                let mut immov: HashMap<(u8, u8), Index> = (&entities, &immovables, &positions)
                    .join()
                    .map(|t| ((t.2.x, t.2.y), t.0.id()))
                    .collect::<HashMap<_, _>>();

其中&entities是Entities的实例，&positions是WriteStorage<Position>的实例，&movables和&immovables分别是ReadStorage<'a, Movable> 和 ReadStorage<'a, Immovable>.

的实例

作为一个对 DRY code 很神经质的人，上面的两个函数真的让我很烦躁，我很想重构它，但是我一直没能弄清楚如何编写一个可以处理的函数&movables 和 &immovables.

的不同类型

例如，如果我尝试这个函数：

fn collect<T>(entities: &Entities, storable: &ReadStorage<T>, positions: &WriteStorage<Position>)
    -> HashMap<(u8, u8), Index> {
    (&entities, &storable, &positions)
            .join()
            .map(|t| ((t.2.x, t.2.y), t.0.id()))
            .collect::<HashMap<_, _>>()
}

并像这样调用它：

let mov: HashMap<(u8, u8), Index> = collect(&entities, &movables, &positions);

或喜欢：

let mov: HashMap<(u8, u8), Index> = collect::<Movable>(&entities, &movables, &positions);

...编译失败：

error[E0277]: the trait bound T: specs::Component is not satisfied
--> src\resources\input_system.rs:95:46 | 95 | fn collect(entities: &Entities, storable: &ReadStorage, positions: &WriteStorage) | ^^^^^^^^^^^^^^^ the trait specs::Component is not implemented for T | ::: C:\Users\BrianKessler.cargo\registry\src\github.com-1ecc6299db9ec823\specs-0.17.0\src\storage\mod.rs:143:29 | 143 | pub struct MaskedStorage<T: Component> { | --------- required by this bound in MaskedStorage | help: consider restricting type parameter T | 95 | fn collect<T: specs::Component>(entities: &Entities, storable: &ReadStorage, positions: &WriteStorage) | ^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try rustc --explain E0277. error: could not compile rust-sokoban

To learn more, run the command again with --verbose.

例如，如果我尝试这个函数（根据@AlexeyLarionov 的建议）：

fn collect<T: specs::Component>(entities: &Entities, storable: &ReadStorage<T>, positions: &WriteStorage<Position>)
    -> HashMap<(u8, u8), Index> {
    (&entities, &storable, &positions)
            .join()
            .map(|t| ((t.2.x, t.2.y), t.0.id()))
            .collect::<HashMap<_, _>>()
}

编译失败：

error[E0599]: the method join exists for tuple (&&specs::Read<'_, EntitiesRes>, &&specs::Storage<'_, T, Fetch<'_, MaskedStorage<T>>>, && specs::Storage<'_, Position, FetchMut<'_, MaskedStorage<Position>>>), but its trait bounds were not satisfied --> src\resources\input_system.rs:94:14 | 94 | .join() | ^^^^ method cannot be called on (&&specs::Read<'_, EntitiesRes>, &&specs::Storage<'_, T, Fetch<'_, MaskedStorage<T>>>, & &specs::Storage<'_, Position, FetchMut<'_, MaskedStorage<Position>>>) due to unsatisfied trait bounds | = note: the following trait bounds were not satisfied: &&specs::Read<'_, EntitiesRes>: specs::Join which is required by (&&specs::Read<'_, EntitiesRes>, &&specs::Storage<'_, T, Fetch<'_, MaskedStorage<T>>>, &&specs::Storage<'_ , Position, FetchMut<'_, MaskedStorage<Position>>>): specs::Join &&specs::Storage<'_, T, Fetch<'_, MaskedStorage<T>>>: specs::Join which is required by (&&specs::Read<'_, EntitiesRes>, &&specs::Storage<'_, T, Fetch<'_, MaskedStorage<T>>>, &&specs::Storage<'_ , Position, FetchMut<'_, MaskedStorage<Position>>>): specs::Join &&specs::Storage<'_, Position, FetchMut<'_, MaskedStorage<Position>>>: specs::Join which is required by (&&specs::Read<'_, EntitiesRes>, &&specs::Storage<'_, T, Fetch<'_, MaskedStorage<T>>>, &&specs::Storage<'_ , Position, FetchMut<'_, MaskedStorage<Position>>>): specs::Join

error: aborting due to previous error

For more information about this error, try rustc --explain E0599. error: could not compile rust-sokoban

To learn more, run the command again with --verbose.

我真的需要引入特征才能使这项工作有效吗？如果是这样，该特征应该是什么样的？我还需要做出哪些其他改变？我是否需要增加如此多的复杂性以致于治疗变得比疾病本身更糟？

Answer 1

正如我们在评论中所讨论的，有两个错误来源：

在 ReadStorage<T> 或 WriteStorage<T> 上运行的方法要求 T 是 Component，幸运的是 Movable 和 Immovable 已经是，所以要修复它，我们可以简单地将 T 限制在这个特征上。函数的声明看起来像这样 fn collect<T: specs::Component> (...)
由于复制粘贴，在对象 (&entities, &storable, &positions) 上调用了 .join() 方法，其中 entities、storable、positions 已作为引用在函数声明中指定，因此 .join() 在类型 (&&A, &&B, &&C) 上被调用（简单地说），而它是为 (&A, &B, &C) 定义的。要解决这个问题我们需要在 collect 函数

(entities, storable, positions).join()

代码的最终版本如下所示：

fn collect<T: specs::Component>(entities: &Entities, storable: &ReadStorage<T>, positions: &WriteStorage<Position>)
    -> HashMap<(u8, u8), Index> {
    (entities, storable, positions)
            .join()
            .map(|t| ((t.2.x, t.2.y), t.0.id()))
            .collect::<HashMap<_, _>>()
}

在 Rust 中，如何创建一个接受 "Marker Component" 作为类型参数的函数？

In Rust, how can I create a function which will accept a "Marker Component" as a type parameter?

generics

rust

functional-programming

traits

dry