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The Option struct

The Problem
Consider that you want to implement a function that returns a username, given the corresponding user identifier. The signature of the function could be as follows:
fn get_username_by_id(id: u32) -> String {
    // function body
}
In a success case, you pass the user identifier as a parameter and the function returns the corresponding username. However, what happens if the function does not have a username for the given identifier? A possible solution is to return an empty string:
If the function is able to retrieve the data, then the string returned is the username.
If the function is NOT able to retrieve the data, then the string returned is the empty string ('').
Although this is a valid approach, it creates hidden logic that is not visible unless you deep dive into the function code.
The Solution
Rust provides a better way of dealing with these situations by using the Option<T> enum. This enum has two possible values: Some(T) (used when the returned value is present) and None (used when the returned value is not present). Therefore, the previous function can be refactored to:
fn get_username_by_id(id: u32) -> Option<String> {
    // function body
}
Now, the function works as follows:
If the function is able to retrieve the data, then a Some value containing the string is returned.
If the function is NOT able to retrieve the data, then a None value is returned.
Let's complete the body of the function:
fn get_username_by_id(id: u32) -> Option<String> { // 1.
    match(id) {
        1 => Some(String::from("Susan")), // 2.
        2 => Some(String::from("John")), // 3.
        _ => None // 4.
    }
}
1.Given a user identifier, return the corresponding username if it exists.
2.If id == 1, then a Some struct containing the string is returned.
3.If id == 2, then a Some struct containing the string is returned.
4.If id does not match with any of the provided identifiers, then a None struct is returned.
Using Options
The Option<T> struct contains two helper methods to check if the returned type is Some or None: the .is_some() and .is_none() methods. Let's see how to use these methods:
fn get_username_by_id(id: u32) -> Option<String> {
    match(id) {
        1 => Some(String::from("Susan")),
        2 => Some(String::from("John")),
        _ => None
    }
}
​
fn main() {
    let user1 = get_username_by_id(1); // 1.
    let user10 = get_username_by_id(10); // 2.
​
    if (user1.is_some()) { // 3.
        println!("User with id = 1 holds username {}", user1.unwrap())
    }
​
    if (user10.is_none()) { // 4.
        println!("User with id = 10 does not exist")
    }
}
1.Get the user with id == 1.
2.Get the user with id == 10.
3.If the function returned a name for id == 1, then user1.is_some() returns true.
4.If the function did NOT return a name for id == 10, then user1.is_none() returns true.
You can also use pattern matching instead of the helper methods:
fn get_username_by_id(id: u32) -> Option<String> {
    match(id) {
        1 => Some(String::from("Susan")),
        2 => Some(String::from("John")),
        _ => None
    }
}
​
fn main() {
    let user1 = get_username_by_id(1);
    let user10 = get_username_by_id(10);
    
    match (&user1) {
        Some(name) => println!("User with id = 1 holds username {}", &user1.unwrap()),
        None => println!("No user with id = 1 found")
    }
    
    match (&user10) {
        Some(name) => println!("User with id = 10 holds username {}", &user10.unwrap()),
        None => println!("No user with id = 10 found")
    }
}
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