Rust returns a result error from fn: mismatched types

问题

I want this function to return an error result:

fn get_result() -> Result<String, std::io::Error> {
     // Ok(String::from("foo")) <- works fine
     Result::Err(String::from("foo"))
}

Error Message

error[E0308]: mismatched types
 --> src/main.rs:3:17
  |
3 |     Result::Err(String::from("foo"))
  |                 ^^^^^^^^^^^^^^^^^^^ expected struct `std::io::Error`, found struct `std::string::String`
  |
  = note: expected type `std::io::Error`
             found type `std::string::String`

I'm confused how I can print out an error message when using the expected struct.

回答1:

The error message is quite clear. Your return type for get_result is Result<String, std::io::Error>, meaning that in the Result::Ok case, the inner value of the Ok variant is of type String, whereas in the Result::Err case, the inner value of the Err variant is of type std::io::Error.

Your code attempted to create an Err variant with an inner value of type String, and the compiler rightfully complains about a type mismatch. To create a new std::io::Error, you can use the new method on std::io::Error. Here's an example of your code using the correct types:

fn get_result() -> Result<String, std::io::Error> {
    Err(std::io::Error::new(std::io::ErrorKind::Other, "foo"))
}

回答2:

You might want to do something like this, if I get it right...

fn get_result() -> Result<String, String> {
   // Ok(String::from("foo")) <- works fine
   Result::Err(String::from("Error"))
}

fn main(){
    match get_result(){
        Ok(s) => println!("{}",s),
        Err(s) => println!("{}",s)
    };
}

I wouldn't recommend doing this though.

来源：https://stackoverflow.com/questions/45583246/rust-returns-a-result-error-from-fn-mismatched-types

标签

rust

rust-result