具有关联类型的工厂方法

Question

这是服务:

trait QType {}

trait Service {
    type Query: QType;

    fn sanitize(&self, query: &str) -> Result<Self::Query, String>;

    fn run(&self, query: &Self::Query) -> Result<(), String>;
}

因此，其思想是sanitize函数返回query的一个实例，然后可以将该实例传递给run函数。

工厂看起来是这样的（不编译）：

fn factory<Q: QType>(name: &str) -> Box<dyn Service<Query = Q>> {
    match name {
        "amazon" => Box::new(amzn::Amazon {}),
        other => panic!("Invalid service {}", other),
    }
}

mod amzn {
    use super::*;

    pub struct Amazon {}

    pub struct Product {
        name: String,
    }

    impl QType for Product {}

    impl Service for Amazon {
        type Query = Product;
        fn sanitize(&self, query: &str) -> Result<Product, String> {}
        fn run(&self, query: &Product) -> Result<(), String> {}
    }
}

error[E0271]: type mismatch resolving `::Query == Q`
 --> src/main.rs:9:21
  |
9 |         "amazon" => Box::new(amzn::Amazon {}),
  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter, found struct `amzn::Product`
  |
  = note: expected type `Q`
             found type `amzn::Product`
  = help: type parameters must be constrained to match other types
  = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
  = note: required for the cast to the object type `dyn Service`

非常感谢任何帮助。

Answer 1

无法实现此签名：

fn factory<Q: QType>(name: &str) -> Box<dyn Service<Query = Q>>

关联类型始终由实现类型唯一确定。即。service的每个实现只选择一个关联类型query。

这与factory不一致，后者让调用方决定关联的类型应该是什么。应该清楚地看到，如果使用不是product的Q调用Factory，则match表达式中的代码不再进行类型检查。

fn factory(name: &str) -> Box<dyn Service<Query = Product>> {
    match name {
        "amazon" => Box::new(amzn::Amazon {}),
        other => panic!("Invalid service {}", other),
    }
}

trait QType {
    fn create_service(name: &str) -> Option<Box<dyn Service<Query = Self>>>;
}

fn factory<Q: QType>(name: &str) -> Box<dyn Service<Query = Q>> {
    Q::create_service(name).expect("Invalid service")
}

impl QType for Product {
    fn create_service(name: &str) -> Option<Box<dyn Service<Query = Self>>> {
        match name {
            "amazon" => Some(Box::new(amzn::Amazon {})),
            other => None,
        }
    }
}