Rust从fn：不匹配的类型返回结果错误

Question

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.

Answer 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"))
}

Answer 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.