string - 如何进一步处理匹配字符串？

Question

我正在逐行读取一些文件，并希望匹配每一行，以验证它是否包含特定字符串。

到目前为止我所拥有的:

// read file line by line
let file = File::open(file_path).expect("Cannot open file");
let buffer = BufReader::new(file);
for line in buffer.lines() {
    // println!("{:?}", line.unwrap());
    parse_line(line.unwrap());

}

fn parse_line(line: String) {
    match line {
        (String) if line.contains("foo") => print!("function head"),
        _ => print!("function body"),
    }
}

这会导致:

error: expected one of `,` or `@`, found `)`
  --> src/main.rs:13:20
   |
13 |             (String) if line.contains("foo") => print!("function head"),
   |                    ^ expected one of `,` or `@` here

我可以使用 match 来检查不同的包含字符串，就像在其他情况下使用 switch 那样吗？

就像这样:

fn parse_line(line: String) {
    match line {
        line.contains("foo") => print!("function foo"),
        line.contains("bar") => print!("function bar"),
        _ => print!("function body"),
    }
}

Answer 1

在匹配中使用if，称为match guard :

use std::fs::File;
use std::io::{self, BufReader, BufRead};

fn main() -> io::Result<()> {
    let file_path = "foo.txt";
    // read file line by line
    let file = File::open(file_path)?;
    let reader = BufReader::new(file);

    for line in reader.lines() {
        parse_line(line?);
    }

    Ok(())
}

fn parse_line(line: String) {
    match &line {
        s if s.contains("foo") => print!("contains foo"),
        s if s.contains("bar") => print!("contains bar"),
        _ => print!("other"),
    }
}

请注意这一行:

(String) if line.contains("foo") => print!("function head");

不是 Rust。 Rust 中没有类似的语法。