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22/08 — 2021
108.68 cm   7.5 min

Ergonomic error handling in Rust

There are many ways you can handle errors in Rust programs: panic at every fallible situation, propagate foreign errors up the call stack, craft your own error type.

I’ve recently fallen into a comfortable pattern when dealing with errors in Rust and believe it serves great for small or large projects.

This post will walk through a few different ways to handle errors and then ultimately introduce the snafu library as a clean and ergonomic solution to the elusive task of dealing with erroneous behaviour.

Contents

Panic! at the disco

Let’s say we have some arbitrary text file on disk that we want to read from, and depending on the content of the file, we want to write back some bytes to it. Simple enough?

use std::{
  fs::{self, OpenOptions},
  io::prelude::*
};

const PATH: &str = "file.txt";

fn run() {
  let content = fs::read_to_string(PATH).unwrap();

  let mut file = OpenOptions::new()
    .write(true)
    .append(true)
    .open(PATH)
    .unwrap();

  if content.trim() == "hello" {
    file.write_all(b"world!").unwrap();
  }

  Ok(())
}

fn main() {
  run();
}

This program can fail in a few different ways:

  • The file doesn’t exist.
  • The contents of the file aren’t valid UTF-8.
  • The call to write_all gets interrupted.

We are handling each of these instances with our calls to unwrap on the fallible function calls. However, instead of being able to recover from an error if it occurs, the current thread panics, which, if it is the main thread, terminates all threads and ends the program with code 101.

Recoverable errors with Result<T, E>

Instead of calls to panic! each time the program can fail, we should be writing more robust code by giving our caller a chance to recover from this behaviour.

use std::{
  fs::File,
  io::{self, prelude::*},
};

const PATH: &str = "file.txt";

fn run() -> Result<(), io::Error> {
  let content = fs::read_to_string(PATH)?;

  let mut file = OpenOptions::new()
    .write(true)
    .append(true)
    .open(PATH)?;

  if content.trim() == "hello" {
    file.write_all(b"world!")?;
  }

  Ok(())
}

fn main() {
  match run() {
    Ok(()) => {},
    Err(e) => eprintln!("{}", e)
  }
}

The Result type is an enum which has an Ok and an Err variant. The Ok variant holds a generic value T that gets returned in the case that no errors have occurred. The Err variant holds a generic value E, usually an error type, if an error has occurred.

The ? operator only works when the function we’re applying it to returns a Result in which whose Err variant contains the error type we wish to propagate. It is essentially a shorthand for the following code:

match result {
  Ok(v)  => v,
  Err(e) => Err(e)
}

This implementation allows for our caller to recover from any erroneous behaviour. However, we can only return one type of error from our function. This prompts anyone who wishes to possibly return more than one error type to create their own type that contains multiple variants.

Crafting custom error types

We can craft our own custom error type as a Rust enum and add multiple variants to it. We can then define a Result type which we can use throughout our program which contains our custom error type as the error to propagate.

In order to demonstrate the need for multiple error types, we’ll extend the functionality of this tiny application to include a call to some API endpoint that is written in the text file, using the reqwest library.

use std::{
  fmt::{self, Display},
  fs::{self, OpenOptions},
  io::{self, prelude::*},
};
use reqwest::blocking;

type Result<T, E = Error> = std::result::Result<T, E>;

#[derive(Debug)]
enum Error {
  IoError(io::Error),
  ApiError(reqwest::Error)
}

impl From<io::Error> for Error {
  fn from(err: io::Error) -> Error {
    Error::IoError(err)
  }
}

impl From<reqwest::Error> for Error {
  fn from(err: reqwest::Error) -> Error {
    Error::ApiError(err)
  }
}

impl Display for Error {
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    match self {
      Error::IoError(e) => write!(f, "IO Error: {}", e),
      Error::ApiError(e) => write!(f, "Api Error: {}", e)
    }
  }
}

const PATH: &str = "file.txt";

fn run() -> Result<()> {
  let content = fs::read_to_string(PATH)?;

  let mut file = OpenOptions::new()
    .write(true)
    .append(true)
    .open(PATH)?;

  let response = blocking::get(content.trim())?;

  file.write_all(
    response
      .text()?
      .as_bytes()
  )?;

  Ok(())
}

fn main() {
  match run() {
    Ok(()) => {}
    Err(e) => eprintln!("{}", e),
  }
}

In order to apply our nice shorthand ? operator, we have the implement the From trait for each error type we wish to add to our custom enum. In addition, in order for main to be able to print out the error message, we must implement Display for our custom error type, matching on each variant.

This implementation solves the problem of dealing with multiple error types in a given function. Having our own custom error type also allows for other users of our application to deal with our various custom error type variants using a similar approach. However, as we shall see, we can avoid some of this tedium by bringing in a third party.

Enter snafu

The snafu library allows for the easy assignment of foreign errors into domain-specific errors while also adding contextual information. Whenever a foreign error type is encountered, we can easily pin it to our own custom typing by adding a source attribute.

For instance:

use std::{
  fs::{self, OpenOptions},
  io::{self, prelude::*},
};
use reqwest::{self, blocking};
use snafu::Snafu;

type Result<T, E = Error> = std::result::Result<T, E>;

#[derive(Debug, Snafu)]
enum Error {
  #[snafu(context(false), display("IO Error: {}", source))]
  Io { source: io::Error },

  #[snafu(context(false), display("Api Error: {}", source))]
  Api { source: reqwest::Error },
}

const PATH: &str = "file.txt";

fn run() -> Result<()> {
  let content = fs::read_to_string(PATH)?;

  let mut file = OpenOptions::new()
    .write(true)
    .append(true)
    .open(PATH)?;

  let response = blocking::get(content.trim())?;

  file.write_all(
    response
      .text()?
      .as_bytes()
  )?;

  Ok(())
}

fn main() {
  match run() {
    Ok(()) => {}
    Err(e) => eprintln!("{}", e),
  }
}

The Snafu macro implements all the necessary traits in order to begin using our custom error type in functions.

Crafting our type this way not only let’s us avoid implementing traits ourselves but also gives us the added benefit of easily adding in custom fields to our error types as contextual information.

This tiny example doesn’t show all of the functionality the snafu library has to offer, so I suggest you have a look at the official documentation.

Fin

Quality Rust code should never panic when it doesn’t absolutely need to, and knowing this you should make the best of the situation by crafting your own error types that other people can use in their applications. You can either make it tedious, by doing it by hand, or by using the nifty snafu library to do the job.

Hi.

I'm Liam.

I am a final year Computer Science Student at John Abbott College. Currently learning about rust, crypto and compilers.

Send me a message by email at liam@scalzulli.com or via matrix @worse:matrix.org

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