Fallibility

Let's revisit the Ticket::new function from the previous exercise:

#![allow(unused)]
fn main() {
impl Ticket {
    pub fn new(title: String, description: String, status: Status) -> Ticket {
        if title.is_empty() {
            panic!("Title cannot be empty");
        }
        if title.len() > 50 {
            panic!("Title cannot be longer than 50 bytes");
        }
        if description.is_empty() {
            panic!("Description cannot be empty");
        }
        if description.len() > 500 {
            panic!("Description cannot be longer than 500 bytes");
        }

        Ticket {
            title,
            description,
            status,
        }
    }
}
}

As soon as one of the checks fails, the function panics. This is not ideal, as it doesn't give the caller a chance to handle the error.

It's time to introduce the Result type, Rust's primary mechanism for error handling.

The Result type

The Result type is an enum defined in the standard library:

#![allow(unused)]
fn main() {
enum Result<T, E> {
    Ok(T),
    Err(E),
}
}

It has two variants:

• Ok(T): represents a successful operation. It holds T, the output of the operation.
• Err(E): represents a failed operation. It holds E, the error that occurred.

Both Ok and Err are generic, allowing you to specify your own types for the success and error cases.

No exceptions

Recoverable errors in Rust are represented as values.
They're just an instance of a type, being passed around and manipulated like any other value. This is a significant difference from other languages, such as Python or C#, where exceptions are used to signal errors.

Exceptions create a separate control flow path that can be hard to reason about.
You don't know, just by looking at a function's signature, if it can throw an exception or not. You don't know, just by looking at a function's signature, which exception types it can throw.
You must either read the function's documentation or look at its implementation to find out.

Exception handling logic has very poor locality: the code that throws the exception is far removed from the code that catches it, and there's no direct link between the two.

Fallibility is encoded in the type system

Rust, with Result, forces you to encode fallibility in the function's signature.
If a function can fail (and you want the caller to have a shot at handling the error), it must return a Result.

#![allow(unused)]
fn main() {
// Just by looking at the signature, you know that this function can fail.
// You can also inspect `ParseIntError` to see what kind of failures to expect.
fn parse_int(s: &str) -> Result<i32, ParseIntError> {
    // ...
}
}

That's the big advantage of Result: it makes fallibility explicit.

Keep in mind, though, that panics exist. They aren't tracked by the type system, just like exceptions in other languages. But they're meant for unrecoverable errors and should be used sparingly.

Exercise

The exercise for this section is located in 05_ticket_v2/06_fallibility