rust/src/doc/guide-error-handling.md

% Error Handling in Rust

The best-laid plans of mice and men Often go awry

"Tae a Moose", Robert Burns

Sometimes, things just go wrong. It's important to have a plan for when the inevitable happens. Rust has rich support for handling errors that may (let's be honest: will) occur in your programs.

There are two main kinds of errors that can occur in your programs: failures, and panics. Let's talk about the difference between the two, and then discuss how to handle each. Then, we'll discuss upgrading failures to panics.

Failure vs. Panic

Rust uses two terms to differentiate between two forms of error: failure, and panic. A failure is an error that can be recovered from in some way. A panic is an error that cannot be recovered from.

What do we mean by 'recover'? Well, in most cases, the possibility of an error is expected. For example, consider the from_str function:

from_str("5");

This function takes a string argument and converts it into another type. But because it's a string, you can't be sure that the conversion actually works. For example, what should this convert to?

from_str("hello5world");

This won't work. So we know that this function will only work properly for some inputs. It's expected behavior. We call this kind of error 'failure.'

On the other hand, sometimes, there are errors that are unexpected, or which we cannot recover from. A classic example is an assert!:

assert!(x == 5);

We use assert! to declare that something is true. If it's not true, something is very wrong. Wrong enough that we can't continue with things in the current state. Another example is using the unreachable!() macro

enum Event {
    NewRelease,
}

fn probability(_: &Event) -> f64 {
    // real implementation would be more complex, of course
    0.95
}

fn descriptive_probability(event: Event) -> &'static str {
    match probability(&event) {
        1.00          => "certain",
        0.00          => "impossible",
        0.00 ... 0.25 => "very unlikely",
        0.25 ... 0.50 => "unlikely",
        0.50 ... 0.75 => "likely",
        0.75 ... 1.00  => "very likely",
    }
}

fn main() {
    std::io::println(descriptive_probability(NewRelease));
}

This will give us an error:

error: non-exhaustive patterns: `_` not covered [E0004]

While we know that we've covered all possible cases, Rust can't tell. It doesn't know that probability is between 0.0 and 1.0. So we add another case:

use Event::NewRelease;

enum Event {
    NewRelease,
}

fn probability(_: &Event) -> f64 {
    // real implementation would be more complex, of course
    0.95
}

fn descriptive_probability(event: Event) -> &'static str {
    match probability(&event) {
        1.00          => "certain",
        0.00          => "impossible",
        0.00 ... 0.25 => "very unlikely",
        0.25 ... 0.50 => "unlikely",
        0.50 ... 0.75 => "likely",
        0.75 ... 1.00  => "very likely",
        _ => unreachable!()
    }
}

fn main() {
    println!("{}", descriptive_probability(NewRelease));
}

We shouldn't ever hit the _ case, so we use the unreachable!() macro to indicate this. unreachable!() gives a different kind of error than Result. Rust calls these sorts of errors 'panics.'

Handling errors with Option and Result

The simplest way to indicate that a function may fail is to use the Option<T> type. Remember our from_str() example? Here's its type signature:

pub fn from_str<A: FromStr>(s: &str) -> Option<A>

from_str() returns an Option<A>. If the conversion succeeds, it will return Some(value), and if it fails, it will return None.

This is appropriate for the simplest of cases, but doesn't give us a lot of information in the failure case. What if we wanted to know why the conversion failed? For this, we can use the Result<T, E> type. It looks like this:

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

This enum is provided by Rust itself, so you don't need to define it to use it in your code. The Ok(T) variant represents a success, and the Err(E) variant represents a failure. Returning a Result instead of an Option is recommended for all but the most trivial of situations.

Here's an example of using Result:

#[deriving(Show)]
enum Version { Version1, Version2 }

#[deriving(Show)]
enum ParseError { InvalidHeaderLength, InvalidVersion }

fn parse_version(header: &[u8]) -> Result<Version, ParseError> {
    if header.len() < 1 {
        return Err(ParseError::InvalidHeaderLength);
    }
    match header[0] {
        1 => Ok(Version::Version1),
        2 => Ok(Version::Version2),
        _ => Err(ParseError::InvalidVersion)
    }
}

let version = parse_version(&[1, 2, 3, 4]);
match version {
    Ok(v) => {
        println!("working with version: {}", v);
    }
    Err(e) => {
        println!("error parsing header: {}", e);
    }
}

This function makes use of an enum, ParseError, to enumerate the various errors that can occur.

Non-recoverable errors with panic!

In the case of an error that is unexpected and not recoverable, the panic! macro will induce a panic. This will crash the current task, and give an error:

panic!("boom");

gives

task '<main>' panicked at 'boom', hello.rs:2

when you run it.

Because these kinds of situations are relatively rare, use panics sparingly.

Upgrading failures to panics

In certain circumstances, even though a function may fail, we may want to treat it as a panic instead. For example, io::stdin().read_line() returns an IoResult<String>, a form of Result, when there is an error reading the line. This allows us to handle and possibly recover from this sort of error.

If we don't want to handle this error, and would rather just abort the program, we can use the unwrap() method:

io::stdin().read_line().unwrap();

unwrap() will panic! if the Option is None. This basically says "Give me the value, and if something goes wrong, just crash." This is less reliable than matching the error and attempting to recover, but is also significantly shorter. Sometimes, just crashing is appropriate.

There's another way of doing this that's a bit nicer than unwrap():

let input = io::stdin().read_line()
                       .ok()
                       .expect("Failed to read line");

ok() converts the IoResult into an Option, and expect() does the same thing as unwrap(), but takes a message. This message is passed along to the underlying panic!, providing a better error message if the code errors.