How do I use the Option::ok_or() method correctly?

user_2634045 0
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I'm trying to understand how to use the question mark operator for error handling in Rust. I have this code:

fn main() -> Result<(), &'static str> {
    let foo: i32 = Some("1")
        .ok_or(Err("error 1"))?
        .parse()
        .or(Err("error 2"))?;
    Ok(())
}

This code can not be compiled for some reason:

error[E0277]: the trait bound `&str: std::convert::From<std::result::Result<_, &str>>` is not satisfied
 --> src/main.rs:2:20
  |
2 |       let foo: i32 = Some("1")
  |  ____________________^
3 | |         .ok_or(Err("error 1"))?
  | |_______________________________^ the trait `std::convert::From<std::result::Result<_, &str>>` is not implemented for `&str`
  |
  = note: required by `std::convert::From::from`

The Rust book has an example usage of the question mark operator:

use std::io;
use std::io::Read;
use std::fs::File;

fn read_username_from_file() -> Result<String, io::Error> {
    let mut s = String::new();

    File::open("hello.txt")?.read_to_string(&mut s)?;

    Ok(s)
}

In my opinion, it doesn't differ much from my example in sense of handling errors. I cannot see a reason for my code to be invalid. If the From trait should be implemented for all kinds of Result why does the code from the Rust book work fine?

rust error-handling
1 Answers

Unlike or, ok_or takes an E, not a full Result<T, E> (because it wouldn't have anything to do if passed an Ok). Just pass the error string directly:

fn main() -> Result<(), &'static str> {
   let foo: i32 = Some("1")
       .ok_or("error 1")?
       .parse()
       .or(Err("error 2"))?;
    Ok(())
}

The reason the error message mentions the From trait is because ? implicitly uses From to convert the expression's error type into the return value's error type. If it worked, .ok_or(Err("error 1")) would return a value of Result<&'static str, Result<_, &'static str>> (_ could be almost anything, since Err doesn't specify). The ? operator attempts to find an implementation of From that would convert Result<_, &'static str> (the expression's error type) into &'static str (the return value's error type). Since no such From implementation exists, the compiler emits an error.

user_36503620
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