Other uses of ? - Rust By Example
Description: A description

# Other uses of `?`

Notice in the previous example that our immediate reaction to calling `parse` is to `map` the error from a library error into a boxed error:

    .and_then(|s| s.parse::<i32>()
        .map_err(|e| e.into())

Since this is a simple and common operation, it would be convenient if it could be elided. Alas, because `and_then` is not sufficiently flexible, it cannot. However, we can instead use `?`.

`?` was previously explained as either `unwrap` or `return Err(err)`. This is only mostly true. It actually means `unwrap` or `return Err(From::from(err))`. Since `From::from` is a conversion utility between different types, this means that if you `?` where the error is convertible to the return type, it will convert automatically.

Here, we rewrite the previous example using `?`. As a result, the `map_err` will go away when `From::from` is implemented for our error type:

    use std::error;
    use std::fmt;
    use std::num::ParseIntError;

    // Change the alias to `Box<error::Error>`.
    type Result<T> = std::result::Result<T, Box<error::Error>>;

    #[derive(Debug)]
    struct EmptyVec;

    impl fmt::Display for EmptyVec {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "invalid first item to double")
        }
    }

    impl error::Error for EmptyVec {
        fn description(&self) -> &str {
            "invalid first item to double"
        }

        fn cause(&self) -> Option<&error::Error> {
            // Generic error, underlying cause isn't tracked.
            None
        }
    }

    // The same structure as before but rather than chain all `Results`
    // and `Options` along, we `?` to get the inner value out immediately.
    fn double_first(vec: Vec<&str>) -> Result<i32> {
        let first = vec.first().ok_or(EmptyVec)?;
        let parsed = first.parse::<i32>()?;
        Ok(2 * parsed)
    }

    fn print(result: Result<i32>) {
        match result {
            Ok(n)  => println!("The first doubled is {}", n),
            Err(e) => println!("Error: {}", e),
        }
    }

    fn main() {
        let numbers = vec!["42", "93", "18"];
        let empty = vec![];
        let strings = vec!["tofu", "93", "18"];

        print(double_first(numbers));
        print(double_first(empty));
        print(double_first(strings));
    }

This is actually fairly clean now. Compared with the original `panic`, it is very similar to replacing the `unwrap` calls with `?` except that the return types are `Result`. As a result, they must be destructured at the top level.

### See also:

`From::from` and `?`