struct - 使用 envy 时如何用匹配结果填充结构体实例？

Question

我想用匹配环境变量的结果填充可变变量:

struct Environment {
    some_env_variable_1: String,
    some_env_variable_2: String,
    some_env_variable_3: String,
    some_env_variable_4: String,
}

match envy::from_env::<Environment>() {
    Ok(environment) => println!("{:#?}", environment),
    Err(error) => panic!("{:#?})", error),
};

//where I'm stuck
let mut e = Environment {
    some_env_variable_1: // want this to be from match
};

Answer 1

您正在调用 envy 方法，该方法会为您完成所有解析和填充。问题是您丢弃结果 - 它仅在匹配中可用:

match envy::from_env::<Environment>() {
    Ok(environment) => println!("{:#?}", environment), // gone now
    Err(error) => panic!("{:#?})", error),
};

你需要不要扔掉它:

let environment = match envy::from_env::<Environment>() {
    Ok(environment) => {
        println!("{:#?}", environment);
        environment // Now the entire `match` evaluates to `environment` 
    }
    Err(error) => panic!("{:#?})", error),
};

或者，更惯用的是，只需使用 expect:

let environment: Environment = envy::from_env()
    .expect("Couldn't parse environment");

println!("{:#?}", environment);

---

完整示例:

extern crate serde;
#[macro_use]
extern crate serde_derive;
extern crate envy;

#[derive(Debug, Deserialize)]
struct Environment {
    username: String,
    port: u16,
}

fn main() {
    let mut environment: Environment = envy::from_env()
        .expect("Couldn't parse environment variables");

    println!("{:#?}", environment);
}

$ USERNAME=overflow PORT=8787 cargo run

Environment {
    username: "overflow",
    port: 8787
}