在匹配“Result"并且仍然能够捕获错误时使用 if-let 绑定的惯用方法是什么? [英] What is the idiomatic way of using an if-let binding when matching a `Result` and still being able to capture the error?

问题描述

fn lines_from_file<F>(filename: F) -> Result<io::Lines<BufReader<File>>, io::Error>
where
    F: std::convert::AsRef<std::path::Path>,
{
    let file = File::open(filename)?;
    Ok(io::BufReader::new(file).lines())
}

fn main() {
    let filename: &str = "input.pdl";
    // This works fine
    match lines_from_file(filename) {
        Ok(lines) => {
            for line in lines {
                println!("{:?}", line);
            },
        }
        Err(e) => println!("Error {:?}", e),
    }
}

我想改用这个:

if let lines = Ok(lines_from_file(filename)) {
    for line in lines {
        println!("{:?}", line);
    }
} else {
    println!("Error {:?}" /*what goes here?*/,)
}

但这会导致错误:

| if let lines = Ok(lines_from_file(filename)) {
|                ^^ cannot infer type for `E`

在匹配 Result 并且仍然能够捕获错误时使用 if-let 绑定的惯用方法是什么?

What is the idiomatic way of using an if-let binding when matching a Result and still being able to capture the error?

推荐答案

[...] 在匹配结果时使用 if-let 绑定并且仍然能够捕获错误?

[...] using an if-let binding when matching a Result and still being able to capture the error?

对于一个if let，这从根本上是不可能的.if let 构造的唯一目的是在您只想解构一种模式的情况下使生活更轻松.如果你想解构 Result 的两种情况，你必须使用 match(或多个 if let 或 unwrap()，但这不是一个好的解决方案).为什么你不想首先使用 match?

This is fundamentally impossible with one if let. The if let construct's only purpose is to make life easier in the case where you only want to destructure one pattern. If you want to destructure both cases of a Result, you have to use match (or multiple if let or unwrap(), but this is not a good solution). Why you don't want to use match in the first place?

关于您的编译器错误:您在错误的一侧添加了 Ok():

Regarding your compiler error: you added the Ok() on the wrong side:

if let Ok(lines) = lines_from_file(filename) { ... }

这是使用 if let 的正确方法:左边的解构模式，右边的表达式产生一个值.

This is the correct way of using if let: the destructuring pattern to the left, the expression producing a value to the right.

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