Eshan Singh
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Rust's Result Type is Cool

Beyond the basics with Rust error handling

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Eshan Singh
·Oct 5, 2022·

6 min read

Featured on Hashnode
Rust's Result Type is Cool

Table of contents

  • Combinators
  • Result ❤️ Option
  • Conclusion

If you've worked with Rust before, you know how different its error handling story is from most other languages. The Rust Programming Language explains the two primary ways of raising errors, panicking and the Result type, and how you can propagate the Result type with the ? operator to make recoverable errors explicit without interfering with the happy path in a certain function.

Once you've got the hang of this system, it can cover a your needs for a long time, especially since the ? operator also automatically converts between error types when there's an available From impl. You can even use Box<dyn Error> as a sort of "supertype" error if you're using a bunch of different functions that can error out in different ways.

Here, we're reading a configuration file from a path, that at the moment only consists of a single integer.

fn work_with_config_file(path: &str) 
    -> Result<i32, Box<dyn std::error::Error>> {
    // This error is an std::io::Error
    let file = std::fs::read_to_string(path)?;

    // This one is a ParseIntError
    let number = file.parse()?;

    // No From impl required!

    Ok(number)
}

However, we both know this isn't a particularly "proper" way of doing things. As you progress using Rust in more advanced situations, this will begin to feel janky. You should be take advantage of Rust's type system to create your own Error types, just like the standard library itself does, so that the eventual handler of the error far up the chain can deal with its specific type, instead of just getting a generic boxed Error with their best option being to simply print it out for you to sit there and debug.

Unfortunately, this is hard and involves a TON of boilerplate (enter that link at your own discretion).

With more useful types of errors for a larger function, and with more data and more useful error messages, the size of that code will get even more out of control. While there are libraries that can implement many of these traits automatically, it can be a lot of bloat to bring into a project. More broadly, the concerns here aren't really well-separated - in this case, the function itself should determine how to convert between the error types, not some From impls.

I'll admit to having gotten frustrated with the whole situation and thinking that the system is just fundamentally too much work. And while there's much that can be improved - given there's an entire working group dedicated to fixing it - there are quite a lot of features of the standard library that not a lot of intermediate-level Rustaceans know about that are immensely useful in reducing some of the hassle.

Combinators

The documentation of the Result type lists an absolute treasure trove of methods that can be easy to ignore in the world of unwrap and ?, the most notable of which is map_err.

fn work_with_config_file(path: &str) 
    -> Result<i32, ConfigFileError> {
    let file = std::fs::read_to_string(path)
        .map_err(|e| ConfigFileError::ReadError(e))?;
    let number = file.parse()
        .map_err(|e| ConfigFileError::ParseError(e))?;

    // No From impl required, and no Box either!

    Ok(number)
}

You can make this even simpler by just passing in the variant name instead of a closure to map_err, since it can be used as a function to create the variant, like map_err(ConfigFileError::ReadError)?.

Now the function itself controls how it constructs the error types it returns, and the error type itself simply defines how to print the error in a friendly way. From impls are still appropriate if you find yourself doing this many times or in multiple different places, but map_err can allow potentially complex conversions between error types to happen inline without using match and a bunch of indent levels.

But this is just the beginning. and_then is very useful to chain fallible operations concisely; to group together logically related operations or perform a series of small operations that would clutter the code with ? operators too much otherwise.

fn work_with_config_file(path: &str) 
    -> Result<i32, ConfigFileError> {
    let number = std::fs::read_to_string(path)
        .map_err(ConfigFileError::ReadError)
        .and_then(|f| f.parse()
            .map_err(ConfigFileError::ParseError))?;
    // only a single ?

    Ok(number)
}

Of course, you've got to be worried about trying to be overly concise and ending up with a complicated line. and_then's best use is to be able to easily refactor out each fallible step into a function and make it all look clean.

fn read_config_file(path: &str)
    -> Result<String, ConfigFileError> {
    std::fs::read_to_string(path)
        .map_err(ConfigFileError::ReadError)
}

fn parse_config_file(file: String) 
    -> Result<i32, ConfigFileError> {
    file.parse()
        .map_err(ConfigFileError::ParseError)
}

fn work_with_config_file(path: &str) 
    -> Result<i32, ConfigFileError> {
    // No ? required at all - just directly return
    // the Result, since the types match!
    read_config_file(path).and_then(parse_config_file)
    // .and_then(something_else).and_then(even_more_things) ....
}

Now the reading and parsing steps can be large and hide a lot of implementation details while still allowing the higher level code to work with it seamlessly. [1]

These are the most useful and common combinators, but there are a lot more: or_else, map_or_else, as_deref, etc. I highly encourage you to fully peruse the documentation of Result, because there's a huge variety of incredibly useful operations that can reduce or even totally eliminate the need to manually match on errors.

Result ❤️ Option

The final piece of the puzzle that fully unlocked Rust's default error handling story for me was the fact that while the Option<T> type is defined separately for convenience, conceptually, it is best understood as a special case of Result, equivalent to Result<T, ()>.

Because of this, Option<T> also implements the same useful combinators that a Result<T, ()> would have! and_then, map, they're all here!

More importantly than even this, there are several convenience methods on both Option and Result that allow you to convert between them seamlessly, making them highly connected. ok_or, for instance, allows you to add context to a bare None that you got from somewhere else.

fn get_first(list: Vec<i32>) 
    -> Result<i32, String> {
    // Vec::get returns an Option<i32>.
    list.get(0).ok_or("Empty list!".to_string())
}

You can even use ? on Option values in a function that returns Option, which is a fact I really wish was more highly advertised.

Conclusion

There are many other places to learn about the extensive error handling story in Rust, written by people much better at programming than me. I especially encourage you to read BurntSushi's error handling guide once you run into the limits of these combinators and want to take an even deeper dive. But I hope that these examples brought your attention to the huge amount of stuff available in the Rust standard library that makes writing simple yet robust Rust code easier than it may appear while first learning. It certainly opened up a whole new world to me when I first learned about them!

If you have any feedback about this post, feel free to comment about it; and if you learned something from it, I'd love to hear it too!

[1]: If you're a functional programming nerd, you might recognize these as kind of like monads. They are, but if you understand those, why are you reading this article? Go cure cancer or something.

 
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