Error Handling

• The Result enum is used to indicate success or failure.

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

• This is very useful especially in production code, I think we will use more custom error enum.

• The ? operator simplifies error propagation.

  • If the result is Err, it propagates the error.

  • If Ok, it unwraps the value.

• When your function can produce multiple error types, we can use Box<dyn std::error::Error> to erase the concrete error type.

• This simplifies the return type while still maintaining flexibility.

• References:

  • Error Handling

  • Generics

  • Result

  • Boxing Errors

errors1.rs

// This function refuses to generate text to be printed on a nametag if
// you pass it an empty string. It'd be nicer if it explained what the problem
// was instead of just returning `None`. Thankfully, Rust has a similar
// construct to `Option` that can be used to express error conditions. Change
// the function signature and body to return `Result<String, String>` instead
// of `Option<String>`.
fn generate_nametag_text(name: String) -> Result<String, String> {
    if name.is_empty() {
        // Empty names aren't allowed
        Err("Empty names aren't allowed".to_string())
    } else {
        Ok(format!("Hi! My name is {name}"))
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn generates_nametag_text_for_a_nonempty_name() {
        assert_eq!(
            generate_nametag_text("Beyoncé".to_string()).as_deref(),
            Ok("Hi! My name is Beyoncé"),
        );
    }

    #[test]
    fn explains_why_generating_nametag_text_fails() {
        assert_eq!(
            generate_nametag_text(String::new())
                .as_ref()
                .map_err(|e| e.as_str()),
            Err("Empty names aren't allowed"),
        );
    }
}

• In this exercise we need to fix the return type in function generate_nametag_text to use Result<String, String> enum.

• The Result enum is used to indicate success or failure.

  • Use Ok if success.

  • Use Err if not.

• Then if given name is empty we return Err("Empty names aren't allowed".to_string()) instead of None.

• If not empty then we should return Ok(format!("Hi! My name is {name}")).

errors2.rs

use std::num::ParseIntError;

fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
    let processing_fee = 1;
    let cost_per_item = 5;

    // Add ? to propagate the error.
    let qty = item_quantity.parse::<i32>()?;

    Ok(qty * cost_per_item + processing_fee)
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::num::IntErrorKind;

    #[test]
    fn item_quantity_is_a_valid_number() {
        assert_eq!(total_cost("34"), Ok(171));
    }

    #[test]
    fn item_quantity_is_an_invalid_number() {
        assert_eq!(
            total_cost("beep boop").unwrap_err().kind(),
            &IntErrorKind::InvalidDigit,
        );
    }
}

• In this exercise we just need to propagate the error.

• We can use the ? operator.

  • If the result is Err, it propagates the error.

  • If Ok, it unwraps the value.

• So it will be like this:

  let qty = item_quantity.parse::<i32>()?;

errors3.rs

// This is a program that is trying to use a completed version of the
// `total_cost` function from the previous exercise. It's not working though!
// Why not? What should we do to fix it?

use std::num::ParseIntError;

// Don't change this function.
fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
    let processing_fee = 1;
    let cost_per_item = 5;
    let qty = item_quantity.parse::<i32>()?;

    Ok(qty * cost_per_item + processing_fee)
}

// Add return here
fn main() -> Result<(), ParseIntError> { 
    let mut tokens = 100;
    let pretend_user_input = "8";

    // Don't change this line.
    let cost = total_cost(pretend_user_input)?;

    if cost > tokens {
        println!("You can't afford that many!");
    } else {
        tokens -= cost;
        println!("You now have {tokens} tokens.");
    }
    Ok(()) // call Ok
}

• In this exercise we need to fix the error propagation.

• Inside the main function as we can se it also propagate the error from calling function total_cost.

• But the main function doesn't return anything so it got compile error.

• To fix this we can add Result<(), ParseIntError> as return type in main function.

• We also need to add Ok(()) at the end to mark as the main function is done without any error.

• Reference: https://stackoverflow.com/a/50459909/903350

errors4.rs

#[derive(PartialEq, Debug)]
enum CreationError {
    Negative,
    Zero,
}

#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);

impl PositiveNonzeroInteger {
    fn new(value: i64) -> Result<Self, CreationError> {
        // This function shouldn't always return an `Ok`.
        match value.cmp(&0) {
            std::cmp::Ordering::Less => Err(CreationError::Negative),
            std::cmp::Ordering::Equal => Err(CreationError::Zero),
            std::cmp::Ordering::Greater => Ok(Self(value as u64)),
        }
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_creation() {
        assert_eq!(
            PositiveNonzeroInteger::new(10),
            Ok(PositiveNonzeroInteger(10)),
        );
        assert_eq!(
            PositiveNonzeroInteger::new(-10),
            Err(CreationError::Negative),
        );
        assert_eq!(PositiveNonzeroInteger::new(0), Err(CreationError::Zero));
    }
}

• In this exercise we need complete the function PositiveNonzeroInteger::new.

  • If the value is < 0 return CreationError::Negative.

  • If the value is == 0 return CreationError::Negative.

  • Else return Self(value as u64).

• We can do this in multiple ways.

• We can use classic if syntax like this:

  if value < 0 {
    Err(CreationError::Negative)
  } else if value == 0 {
    Err(CreationError::Zero)
  } else {
    Ok(Self(value as u64))
  }

• Or we can use match syntax like this:

  match value.cmp(&0) {
    std::cmp::Ordering::Less => Err(CreationError::Negative),
    std::cmp::Ordering::Equal => Err(CreationError::Zero),
    std::cmp::Ordering::Greater => Ok(Self(value as u64)),
  }

• Or we also can combine both match and if.

• All of them should fix the code.

errors5.rs

use std::error::Error;
use std::fmt;

#[derive(PartialEq, Debug)]
enum CreationError {
    Negative,
    Zero,
}

// This is required so that `CreationError` can implement `Error`.
impl fmt::Display for CreationError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let description = match *self {
            CreationError::Negative => "number is negative",
            CreationError::Zero => "number is zero",
        };
        f.write_str(description)
    }
}

impl Error for CreationError {}

#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);

impl PositiveNonzeroInteger {
    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
        match value {
            x if x < 0 => Err(CreationError::Negative),
            0 => Err(CreationError::Zero),
            x => Ok(PositiveNonzeroInteger(x as u64)),
        }
    }
}

// Add the correct return type `Result<(), Box<dyn ???>>`. What can we
// use to describe both errors? Is there a trait which both errors implement?
fn main() -> Result<(), Box<dyn Error>> {
    let pretend_user_input = "42";
    let x: i64 = pretend_user_input.parse()?;
    println!("output={:?}", PositiveNonzeroInteger::new(x)?);
    Ok(())
}

• Similar like exercise errors3.rs we want to propagate the error out of the main function.

• But in this case we have multiple kind/variant of Error.

• To make the returned error dynamic we can use Box<dyn Error> like this:

  fn main() -> Result<(), Box<dyn Error>> {
  }

errors6.rs

// Using catch-all error types like `Box<dyn Error>` isn't recommended for
// library code where callers might want to make decisions based on the error
// content instead of printing it out or propagating it further. Here, we define
// a custom error type to make it possible for callers to decide what to do next
// when our function returns an error.

use std::num::ParseIntError;

#[derive(PartialEq, Debug)]
enum CreationError {
    Negative,
    Zero,
}

// A custom error type that we will be using in `PositiveNonzeroInteger::parse`.
#[derive(PartialEq, Debug)]
enum ParsePosNonzeroError {
    Creation(CreationError),
    ParseInt(ParseIntError),
}

impl ParsePosNonzeroError {
    fn from_creation(err: CreationError) -> Self {
        Self::Creation(err)
    }

    // Add another error conversion function here.
    fn from_parse_int(err: ParseIntError) -> Self {
        Self::ParseInt(err)
    }
}

#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);

impl PositiveNonzeroInteger {
    fn new(value: i64) -> Result<Self, CreationError> {
        match value {
            x if x < 0 => Err(CreationError::Negative),
            0 => Err(CreationError::Zero),
            x => Ok(Self(x as u64)),
        }
    }

    fn parse(s: &str) -> Result<Self, ParsePosNonzeroError> {
        // change this to return an appropriate error instead of panicking
        // when `parse()` returns an error.
        let x: i64 = s.parse().map_err(ParsePosNonzeroError::from_parse_int)?;
        Self::new(x).map_err(ParsePosNonzeroError::from_creation)
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn test_parse_error() {
        assert!(matches!(
            PositiveNonzeroInteger::parse("not a number"),
            Err(ParsePosNonzeroError::ParseInt(_)),
        ));
    }

    #[test]
    fn test_negative() {
        assert_eq!(
            PositiveNonzeroInteger::parse("-555"),
            Err(ParsePosNonzeroError::Creation(CreationError::Negative)),
        );
    }

    #[test]
    fn test_zero() {
        assert_eq!(
            PositiveNonzeroInteger::parse("0"),
            Err(ParsePosNonzeroError::Creation(CreationError::Zero)),
        );
    }

    #[test]
    fn test_positive() {
        let x = PositiveNonzeroInteger::new(42).unwrap();
        assert_eq!(x.0, 42);
        assert_eq!(PositiveNonzeroInteger::parse("42"), Ok(x));
    }
}

• In this exercise we need to finish the custom error.

• We need to add from_parse_int function like this:

  fn from_parse_int(err: ParseIntError) -> Self {
    Self::ParseInt(err)
  }

• The we need to change the parse code.

• So instead of unwrap we want to map the error using map_error method and propagate the error like this:

  let x: i64 = s.parse().map_err(ParsePosNonzeroError::from_parse_int)?;