问题

Here's a more idiomatic Rust function to extract the primitive value from the netcdf::attribute::AttrValue enum variant:

use netcdf;
use std::env;

fn extract_primitive_value(attr_value: &netcdf::attribute::AttrValue) -> Option<String> {
    if let netcdf::attribute::AttrValue::Str(v) = attr_value {
        Some(v.clone())
    } else {
        None
    }
}

fn main() -> Result<(), netcdf::error::Error> {
    let args: Vec<String> = env::args().collect();
    let file = netcdf::open(&args[1])?; // data/CCMP_Wind_Analysis_19930103_V03.0_L4.0.nc

    let vwnd = &file.variable("vwnd").expect("Could not find variable 'vwnd'");

    if let Some(attr) = vwnd.attribute("units") {
        if let Some(units) = extract_primitive_value(attr.value()) {
            println!("{}", units);
        }
    }

    Ok(())
}

I've created the extract_primitive_value function to handle the extraction of the String value from the AttrValue enum variant. This function makes the code more idiomatic and reusable.

use netcdf;
use std::env;

fn main() -> Result<(), netcdf::error::Error> {
    let args: Vec<String> = env::args().collect();
    let file = netcdf::open(&args[1])?; // data/CCMP_Wind_Analysis_19930103_V03.0_L4.0.nc
  
    let vwnd = &file.variable("vwnd").expect("Could not find variable 'vwnd'");
  
    // help me re-write this hacky block in a nice function that works for any variant:

    let mut units: String = String::from("null");
    let x = vwnd.attribute("units").unwrap().value().unwrap(); // here's the AttrValue enum
    if let netcdf::attribute::AttrValue::Str(v) = x {
        units = v;
    }
    println!("{}",units);

    // hack block complete    

    return Ok(())

}

答案1

得分: 4

你通常会使用 TryFrom 特性来实现这一点。如果库的作者为他的类型提供了 TryFrom 实例，那么你可以简单地编写 String::try_from(v)，然后获得 Ok(string) 或适当的错误。

不幸的是，他们没有这样做。你总是可以使用新类型模式自己完成，但要让所有这些类型正常运行需要相当多的工作。

答案2

得分: 0

Here's the translated code portion:

感谢Silvio - 对于那些寻求明确答案的人，这是我对Silvio的建议的解释：

```rust
use netcdf;
use std::env;
use std::convert::TryFrom;

struct Wrapper {
    s: String
}

impl std::convert::TryFrom<netcdf::attribute::AttrValue> for Wrapper {
    type Error = &'static str;

    fn try_from(value: netcdf::attribute::AttrValue) -> Result<Self, Self::Error> {

        if let netcdf::attribute::AttrValue::Str(v) = value {
            Ok(Wrapper { s: String::from(v) })
        } else {
            Err("nope")
        }
    }
}

fn main() -> Result<(), netcdf::error::Error> {
    let args: Vec<String> = env::args().collect();

    let file = netcdf::open(&args[1])?; // data/CCMP_Wind_Analysis_19930103_V03.0_L4.0.nc
    let vwnd = &file.variable("vwnd").expect("Could not find variable 'vwnd'");

    let units = Wrapper::try_from(vwnd.attribute("units").unwrap().value().unwrap()).unwrap().s;
    println!("{:#?}", units);

    return Ok(())
}

我有点惊讶需要这么多工作来提取一个简单的属性，但那是完全不同的问题。

请注意，翻译是根据您提供的代码内容进行的，如果有任何问题，请告诉我。

<details>
<summary>英文:</summary>

Thanks Silvio - for those looking for an explicit answer, here&#39;s how I interpreted Silvio&#39;s advice:

use netcdf;
use std::env;
use std::convert::TryFrom;

struct Wrapper{
s: String
}

impl std::convert::TryFrom<netcdf::attribute::AttrValue> for Wrapper {
type Error = &'static str;

fn try_from(value: netcdf::attribute::AttrValue) -> Result<Self, Self::Error> {

    if let netcdf::attribute::AttrValue::Str(v) = value {
        Ok(Wrapper{s: String::from(v)} )
    } else {
        Err("nope")
    }
}

}

fn main() -> Result<(), netcdf::error::Error> {
let args: Vec<String> = env::args().collect();

let file = netcdf::open(&args[1])?; // data/CCMP_Wind_Analysis_19930103_V03.0_L4.0.nc
let vwnd = &file.variable("vwnd").expect("Could not find variable 'vwnd'");

let units = Wrapper::try_from(vwnd.attribute("units").unwrap().value().unwrap()).unwrap().s;
println!("{:#?}", units);

return Ok(())

}

I'm a little surprised so much work would be needed to extract a simple attribute, but that's a different question entirely.

</details>