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The Ultimate Guide to Rust Programming

Errors in main

Explore how to implement error handling in Rust's main function by using the Result type and custom error variants. Understand how the ? operator facilitates early exit on errors and how to return appropriate results, enhancing your ability to write robust Rust programs.

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Quite a while ago, we mentioned that main can return a few things besides unit (). Now that we’ve learned about Result, it’s about time we demonstrated that. Behold, our error-inducing main function!

Rust 1.40.0
fn main() -> Result<(), CantRide> {
    let family = [
        Person { name: "Alice".to_owned(), age: 30, height: 160 },
        Person { name: "Bob".to_owned(), age: 35, height: 170 },
        Person { name: "Charlie".to_owned(), age: 8, height: 100 },
        Person { name: "David".to_owned(), age: 12, height: 75 },
    ];
    println!("The price is {}", price_people(&family)?);
    Ok(())
}
struct Person {
    name: String,
    age: u32,    // in years
    height: u32, // in centimeters
}
impl Person {
    // Price to ride the roller coaster
    fn price(&self) -> Result<u32, CantRide> {
        if self.age < 10 {
            return Err(CantRide::TooYoung(self.name.clone()));
        }
        if self.height < 80 {
            return Err(CantRide::TooShort(self.name.clone()));
        }
        Ok(if self.age < 18 {
            5
        } else if self.age < 66 {
            8
        } else {
            7
        })
    }
}
fn price_people(people: &[Person]) -> Result<u32, CantRide> {
    let mut price = 0;
    for person in people {
        price += person.price()?;
    }
    Ok(price)
}
#[derive(Debug)] // for the output later
enum CantRide {
    TooShort(String),
    TooYoung(String),
}

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