Rust Essentials - Second Edition

Result and Option

Here we look at two kinds of enum that are used everywhere in Rust code. A Result is a special kind of enum that is defined in the standard library. It is used whenever something is executed, that can either end:

• Successfully, then a value Ok (of a certain type T) is returned
• With an error, then an Err value (of type E) is returned

Because this situation is so common, provision is made that the value T and error E types can be as general, or generic, as possible. The Result enum is defined as:

enum Result<T, E> { 
    Ok(T), 
    Err(E) 
} 

An Option is another enum that is defined in the standard library. It is used whenever there is a value, but there can also be a possibility that there is no value. For example, suppose our program ...