Error Handling

Handle errors correctly in main

error-chain-badge cat-rust-patterns-badge

Handles error that occur when trying to open a file that does not exist. It is achieved by using error-chain, a library that takes care of a lot of boilerplate code needed in order to handle errors in Rust.

Io(std::io::Error) inside foreign_links allows automatic conversion from std::io::Error into error_chain! defined type implementing the Error trait.

The below recipe will tell how long the system has been running by opening the Unix file /proc/uptime and parse the content to get the first number. Returns uptime unless there is an error.

Other recipes in this book will hide the error-chain boilerplate, and can be seen by expanding the code with the ⤢ button.

#[macro_use]
extern crate error_chain;

use std::fs::File;
use std::io::Read;

error_chain!{
    foreign_links {
        Io(std::io::Error);
        ParseInt(::std::num::ParseIntError);
    }
}

fn read_uptime() -> Result<u64> {
    let mut uptime = String::new();
    File::open("/proc/uptime")?.read_to_string(&mut uptime)?;

    Ok(uptime
        .split('.')
        .next()
        .ok_or("Cannot parse uptime data")?
        .parse()?)
}

fn main() {
    match read_uptime() {
        Ok(uptime) => println!("uptime: {} seconds", uptime),
        Err(err) => eprintln!("error: {}", err),
    };
}

Avoid discarding errors during error conversions

error-chain-badge cat-rust-patterns-badge

The error-chain crate makes matching on different error types returned by a function possible and relatively compact. ErrorKind determines the error type.

Uses reqwest to query a random integer generator web service. Converts the string response into an integer. The Rust standard library, reqwest, and the web service can all generate errors. Well defined Rust errors use foreign_links. An additional ErrorKind variant for the web service error uses errors block of the error_chain! macro.

#[macro_use]
extern crate error_chain;
extern crate reqwest;

use std::io::Read;

error_chain! {
    foreign_links {
        Io(std::io::Error);
        Reqwest(reqwest::Error);
        ParseIntError(std::num::ParseIntError);
    }

    errors { RandomResponseError(t: String) }
}

fn parse_response(mut response: reqwest::Response) -> Result<u32> {
    let mut body = String::new();
    response.read_to_string(&mut body)?;
    body.pop();
    body.parse::<u32>()
        .chain_err(|| ErrorKind::RandomResponseError(body))
}

fn run() -> Result<()> {
    let url =
        format!("https://www.random.org/integers/?num=1&min=0&max=10&col=1&base=10&format=plain");
    let response = reqwest::get(&url)?;
    let random_value: u32 = parse_response(response)?;

    println!("a random number between 0 and 10: {}", random_value);

    Ok(())
}

fn main() {
    if let Err(error) = run() {
        match *error.kind() {
            ErrorKind::Io(_) => println!("Standard IO error: {:?}", error),
            ErrorKind::Reqwest(_) => println!("Reqwest error: {:?}", error),
            ErrorKind::ParseIntError(_) => println!("Standard parse int error: {:?}", error),
            ErrorKind::RandomResponseError(_) => println!("User defined error: {:?}", error),
            _ => println!("Other error: {:?}", error),
        }
    }
}

Obtain backtrace of complex error scenarios

error-chain-badge cat-rust-patterns-badge

This recipe shows how to handle a complex error scenario and then print a backtrace. It relies on chain_err to extend errors by appending new errors. The error stack can be unwound, thus providing a better context to understand why an error was raised.

The below recipes attempts to deserialize the value 256 into a u8. An error will bubble up from Serde then csv and finally up to the user code.

# extern crate csv;
#[macro_use]
extern crate error_chain;
# #[macro_use]
# extern crate serde_derive;
#
# use std::fmt;
#
# error_chain! {
#     foreign_links {
#         Reader(csv::Error);
#     }
# }

#[derive(Debug, Deserialize)]
struct Rgb {
    red: u8,
    blue: u8,
    green: u8,
}

impl Rgb {
    fn from_reader(csv_data: &[u8]) -> Result<Rgb> {
        let color: Rgb = csv::Reader::from_reader(csv_data)
            .deserialize()
            .nth(0)
            .ok_or("Cannot deserialize the first CSV record")?
            .chain_err(|| "Cannot deserialize RGB color")?;

        Ok(color)
    }
}

# impl fmt::UpperHex for Rgb {
#     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
#         let hexa = u32::from(self.red) << 16 | u32::from(self.blue) << 8 | u32::from(self.green);
#         write!(f, "{:X}", hexa)
#     }
# }
#
fn run() -> Result<()> {
    let csv = "red,blue,green
102,256,204";

    let rgb = Rgb::from_reader(csv.as_bytes()).chain_err(|| "Cannot read CSV data")?;
    println!("{:?} to hexadecimal #{:X}", rgb, rgb);

    Ok(())
}

fn main() {
    if let Err(ref errors) = run() {
        eprintln!("Error level - description");
        errors
            .iter()
            .enumerate()
            .for_each(|(index, error)| eprintln!("└> {} - {}", index, error));

        if let Some(backtrace) = errors.backtrace() {
            eprintln!("{:?}", backtrace);
        }
#
#         // In a real use case, errors should handled. For example:
#         // ::std::process::exit(1);
    }
}

Backtrace error rendered:

Error level - description
└> 0 - Cannot read CSV data
└> 1 - Cannot deserialize RGB color
└> 2 - CSV deserialize error: record 1 (line: 2, byte: 15): field 1: number too large to fit in target type
└> 3 - field 1: number too large to fit in target type

Run the recipe with RUST_BACKTRACE=1 to display a detailed backtrace associated with this error.