Rustlings Topic: Error Handling

Most errors aren’t serious enough to require the program to stop entirely.
Sometimes, when a function fails, it’s for a reason that you can easily interpret and respond to.

Please take a look at Error Handling / Generics / Result / Boxing Errors

You may find solution code for the topic from my repo.

  1. errors1.rs
  2. errors2.rs
  3. errors3.rs
  4. errors4.rs
  5. errors5.rs
  6. errors6.rs

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 sometimes returning None. The 2nd test currently does not compile or pass, but it illustrates the behavior we would like this function to have.

As I have briefly mentioned in Enums chapter, Result<T, E> is enum that has 2 types.
Ok(T) & Err(E).

In this example, return type of the generate_nametag_text() is Result<String, String>. So the function returns either Ok(String) or Err(String). Change Some to Ok for a valid argument. After that, we only have to implement when the argument is not valid.

The string we have to return can be found in test explains_why_generating_nametag_test_fails(). Implement missing else block to return Err(String).

/* file: "exercises/error_handling/errors1.rs" */
pub fn generate_nametag_text(name: String) -> Result<String, String> {
    if name.len() > 0 {
        Ok(format!("Hi! My name is {}", name))
    } else {
        // Empty names aren't allowed.
        Err("`name` was empty; it must be nonempty.".to_string())
    }
}

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

    // This test passes initially if you comment out the 2nd test.
    // You'll need to update what this test expects when you change
    // the function under test!
    #[test]
    fn generates_nametag_text_for_a_nonempty_name() {
        assert_eq!(
            generate_nametag_text("Beyoncé".into()),
            Ok("Hi! My name is Beyoncé".into())
        );
    }

    #[test]
    fn explains_why_generating_nametag_text_fails() {
        assert_eq!(
            generate_nametag_text("".into()),
            Err("`name` was empty; it must be nonempty.".into())
        );
    }
}

errors2.rs

Right now, this function isn’t handling the error case at all (and isn’t handling the success case properly either). What we want to do is:
If we call the parse function on a string that is not a number, that function will return a ParseIntError, and in that case, we want to immediately return that error from our function and not try to multiply and add.

parse() function returns Result<F, <F as FromStr>::Err>. We have to propagate Error if parse() goes wrong. One way to achieve this is to use match statement like below.

let qty = match item_quantity.parse::<i32>() {
    Ok(val) => val,
    Err(e) => return Err(e),
};

But this is too long and cumbersome. Luckily for us, Rust provides ? operator that immediately returns given Err() if function goes wrong.

So above 3 line can be shorten into one-line code.

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

I recommend you to read about ? operator since it is not always applicable. Current function must return Result<T, E> type, and there must be a way to convert given ERR(E2) to Err(E).

Anyway, full code for the exercise is as below.

/* file: "exercises/error_handling/errors2.rs" */
use std::num::ParseIntError;

pub 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)
}

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

    #[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().to_string(),
            "invalid digit found in string"
        );
    }
}

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?

As I’ve said. Using ? operator comes with limitation. Currently, main() doesn’t return anything. It must return Result<T, E> to have code line like let cost = total_cost(pretend_user_input)?;.

Change main function signature to fn main() -> Result<(), ParseIntError>. And return Ok(()) at the end of the main(). Note that () is used as a placeholder.

/* file: "exercises/error_handling/errors3.rs" */
use std::num::ParseIntError;

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

    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(())
}

pub 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)
}

errors4.rs

PositiveNonzeroInteger::new is always creating a new instance and returning an Ok result. It should be doing some checking, returning an Err result if those checks fail, and only returning an Ok result if those checks determine that everything is… okay :)

It is clear that new() must return CreationError for a invalid argument.

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

match statement combined with Range 1.. will help us clean up the code.

/* file: "exercises/error_handling/errors4.rs" */
#[derive(PartialEq, Debug)]
struct PositiveNonzeroInteger(u64);

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

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

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

errors5.rs

Looking at the main(), we can notice that ? operator is used twice. let x: i64 = pretend_user_input.parse()?; This is Okay. We already know that it may return ParseIntError. main() can handle such Error type. But what about println!("output={:?}", PositiveNonzeroInteger::new(x)?);

error[E0277]: `?` couldn't convert the error to `ParseIntError`
  --> exercises/error_handling/errors5.rs:17:59
   |
14 | fn main() -> Result<(), ParseIntError> {
   |              ------------------------- expected `ParseIntError` because of this
...
17 |     println!("output={:?}", PositiveNonzeroInteger::new(x)?);
   |                                                           ^ the trait `From<CreationError>` is not implemented for `ParseIntError`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = note: required because of the requirements on the impl of `FromResidual<Result<Infallible, CreationError>>` for `Result<(), ParseIntError>`

As error message is saying, PositiveNonzeroInteger::new() returns Result<PositiveNonzeroInteger, CreationError>. So, ? may return Err(CreationError). But Rust compiler has no idea about how to convert Err(CreationError) into ParseIntError!

There is a two ways to solve this problem.

  1. Boxing errors
    Change return type of the main() to Box<dyn error::Error> which can hold any kind of Error type.

    fn main() -> Result<(), Box<dyn error::Error>> {
        let pretend_user_input = "42";
        let x: i64 = pretend_user_input.parse()?;
        println!("output={:?}", PositiveNonzeroInteger::new(x)?);
        Ok(())
    }
    
  2. Implement Trait std::convert::From
    So that compiler knows how to convert given Error type to return Error type.
    This is my personal preference. Because with Box<dyn error::Error>, we cannot know what type of Error it returns. It is determined at the runtime.
    But it is more complicated, and it’s not the intention of the current exercise. So let’s go with the first option.

/* file: "exercises/error_handling/errors5.rs" */
fn main() -> Result<(), Box<dyn error::Error>> {
    let pretend_user_input = "42";
    let x: i64 = pretend_user_input.parse()?;
    println!("output={:?}", PositiveNonzeroInteger::new(x)?);
    Ok(())
}

// Don't change anything below this line.

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

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

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

// This is required so that `CreationError` can implement `error::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::Error for CreationError {}

errors6.rs

Using catch-all error types like Box<dyn error::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.

Look at the enum ParsePosNonzeroError. It can have both CreationError & ParseIntError type. So this enum can be used to express both errors. Implement parse_pos_nonzero() & from_parse_int().

/* file: "exercises/error_handling/errors6.rs" */
use std::num::ParseIntError;

// This is a custom error type that we will be using in `parse_pos_nonzero()`.
#[derive(PartialEq, Debug)]
enum ParsePosNonzeroError {
    Creation(CreationError),
    ParseInt(ParseIntError),
}

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

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

fn parse_pos_nonzero(s: &str) -> Result<PositiveNonzeroInteger, ParsePosNonzeroError> {
    let x: i64 = s.parse().map_err(ParsePosNonzeroError::from_parse_int)?;
    PositiveNonzeroInteger::new(x).map_err(ParsePosNonzeroError::from_creation)
}

// Don't change anything below this line.

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

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

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

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