if let语句中的类型不匹配：找到期望结果

Question

I am trying to parse a file that has a pipe delimiter between each value and each line is a new record. I am iterating over each line like this:

use std::fs::File;
use std::io::BufReader;
use std::io::prelude::*;

fn main() {
    let source_file = File::open("input.txt").unwrap();
    let reader = BufReader::new(source_file);
    for line in reader.lines() {
        if let Some(channel_line) = line {
            println!("yay");
        }
    }
}

Playground

However, I get an error:

error[E0308]: mismatched types
  --> src/main.rs:9:20
  |
9 |             if let Some(channel_line) = line {
  |                    ^^^^^^^^^^^^^^^^^^ expected enum    `std::result::Result`, found enum `std::option::Option`
  |
  = note: expected type `std::result::Result<std::string::String, std::io::Error>`
  = note:    found type `std::option::Option<_>`

This error is confusing to me as the found type is what I expected namely Option<Result<String, Error>> as indicated by the docs so it would make sense to unwrap the Option before the result assuming I am not missing something.

Answer 1

You linked to the documentation of next , but you're not using next (not directly anyway), you're using a for loop. In a for loop the type of the iteration variable (ie the type of line in your code) is the Item type of the iterator, which in your case is Result<String> .

The type of next is Option<Item> because you might call next on an iterator that already reached the end. The body of a for loop won't ever execute after the end of the iteration, so there's no point in the iteration variable being an option.