一次检查多个嵌套枚举中的 Some, None

Question

我正在寻找一种在我的项目中使用功能性事件类型机制的方法。尚未投入生产，因此甚至可以从头开始重写：

我想找两件事：

存在价值 (Option<T>)
不同类型的函数处理，确定性

我已经完成了后者，但我对第一个有一些问题：对于看起来像这样的请求：

POST http://127.0.0.1:8000/publish_event HTTP/1.1
content-type: application/json

{
    "source_id": "user:asdf",
    "key": "temperature",
    "value": 25
}

我设置了以下结构和枚举：

    use std::option::{Option};
    use serde::{Deserialize};
    
    #[derive(Deserialize, Debug)]
    pub struct EventPayload<T>{
        pub key: String,
        pub tag: Option<String>,
        pub value: Option<T>
    }
    
    #[derive(Deserialize, Debug)]
    #[serde(untagged)]
    pub enum EventValue {
        String(EventPayload<String>),
        Float(EventPayload<f32>),
        Int(EventPayload<i32>),
        Bool(EventPayload<bool>)
    }
    
    #[derive(Deserialize, Debug)]
    pub struct PublishSingleValueEventRequest {
        pub source_id: String,
        #[serde(flatten)]
        pub manifest: EventValue
    }

然后，我将 rocket 用于一个简单的端点。这是出现问题的地方：

#[macro_use] extern crate rocket;
use rocket::serde::json::Json;
use rocket::http::Status;

#[post("/publish_event", format="application/json", data="<request>")]
pub fn publish_event(request: Json<PublishSingleValueEventRequest>) -> Status {
    let event = request.0;
    
    /* This region does not compile
    let evt_val: Option = match event.manifest {
        EventValue::String(x) => x.value,
        EventValue::Float(x) => x.value,
        EventValue::Int(x) => x.value,
        EventValue::Bool(x) => x.value
    };

    match evt_val {
        Some(x) => println!("Event value: {:?}", x),
        None => println!("No event value provided")
    }
    */


    println!("{:?}", event.manifest);
    match event.manifest {
        EventValue::String(x) => {
            println!("String payload: {:?}", x.value);
        }

        EventValue::Float(x) => {
            println!("Float payload: {:?}", x.value);
        }

        EventValue::Int(x) => {
            println!("Int payload: {:?}", x.value);
        }
        
        EventValue::Bool(x) => {
            println!("Bool payload: {:?}", x.value);
        }
    }

    Status::Accepted
}

#[launch]
fn rocket() -> _ {
    rocket::build().mount("/", routes![publish_event])
}

我也希望这样更像

match event.manifest.value {
    Some(x) => ...,
    None => ...
}

...而不是手动展开 .value Option<> 字段。

Answer 1

您可以模式匹配嵌套结构：

fn main() {
    let payload: EventPayload<i32> = EventPayload {
        key: "foo".to_string(),
        tag: None,
        value: Some(10),
    };
    let value = EventValue::Int(payload);
    
    match value {
        EventValue::Int(EventPayload {value: Some(x), ..}) => {
            println!("Some value: {}", x);
        }
        EventValue::String(EventPayload {value: Some(message), ..}) => {
            println!("Some message: {}", message);
        }
        _ => {
            println!("whatever");
        }
    }
}

Playground

Answer 2

这里真正的问题是：您真正想用 evt_val 做什么？如果你只想显示它，那么你可以映射到 Option<&dyn Debug>:

let evt_val: Option::<&dyn Debug> = match event.manifest {
    EventValue::String(x) => x.value.as_ref().map (|v| v as &dyn Debug),
    EventValue::Float(x) => x.value.as_ref().map (|v| v as &dyn Debug),
    EventValue::Int(x) => x.value.as_ref().map (|v| v as &dyn Debug),
    EventValue::Bool(x) => x.value.as_ref().map (|v| v as &dyn Debug),
};

match evt_val {
    Some(x) => println!("Event value: {:?}", x),
    None => println!("No event value provided")
}

一次检查多个嵌套枚举中的 Some, None

Check Some, None in multiple nested enums at once

json

rust

rocket

serde