Error handling

Introduction#

Rust uses Result<T, E> values to indicate recoverable errors during execution. Unrecoverable errors cause Panics which is a topic of its own.

Remarks#

Details of error handling is described in The Rust Programming Language (a.k.a The Book)

Common Result methods

use std::io::{Read, Result as IoResult};
use std::fs::File;

struct Config(u8);

fn read_config() -> IoResult<String> {
    let mut s = String::new();
    let mut file = File::open(&get_local_config_path())
        // or_else closure is invoked if Result is Err.
        .or_else(|_| File::open(&get_global_config_path()))?;
    // Note: In `or_else`, the closure should return a Result with a matching
    //       Ok type, whereas in `and_then`, the returned Result should have a
    //       matching Err type.
    let _ = file.read_to_string(&mut s)?;
    Ok(s)
}

struct ParseError;

fn parse_config(conf_str: String) -> Result<Config, ParseError> {
    // Parse the config string...
    if conf_str.starts_with("bananas") {
        Err(ParseError)
    } else {
        Ok(Config(42))
    }
}

fn run() -> Result<(), String> {
    // Note: The error type of this function is String. We use map_err below to
    //       make the error values into String type
    let conf_str = read_config()
        .map_err(|e| format!("Failed to read config file: {}", e))?;
    // Note: Instead of using `?` above, we can use `and_then` to wrap the let
    //       expression below.
    let conf_val = parse_config(conf_str)
        .map(|Config(v)| v / 2) // map can be used to map just the Ok value
        .map_err(|_| "Failed to parse the config string!".to_string())?;

    // Run...

    Ok(())
}

fn main() {
    match run() {
        Ok(_) => println!("Bye!"),
        Err(e) => println!("Error: {}", e),
    }
}

fn get_local_config_path() -> String {
    let user_config_prefix = "/home/user/.config";
    // code to get the user config directory
    format!("{}/my_app.rc", user_config_prefix)
}

fn get_global_config_path() -> String {
    let global_config_prefix = "/etc";
    // code to get the global config directory
    format!("{}/my_app.rc", global_config_prefix)
}

If the config files don’t exist, this outputs:

Error: Failed to read config file: No such file or directory (os error 2)

If parsing failed, this outputs:

Error: Failed to parse the config string!

Note: As the project grows, it will get cumbersome to handle errors with these basic methods (docs) without losing information about the origin and propagation path of errors. Also, it is definitely a bad practice to convert errors into strings prematurely in order to handle multiple error types as shown above. A much better way is to use the crate error-chain.

Custom Error Types

use std::error::Error;
use std::fmt;
use std::convert::From;
use std::io::Error as IoError;
use std::str::Utf8Error;

#[derive(Debug)] // Allow the use of "{:?}" format specifier
enum CustomError {
    Io(IoError),
    Utf8(Utf8Error),
    Other,
}

// Allow the use of "{}" format specifier
impl fmt::Display for CustomError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            CustomError::Io(ref cause) => write!(f, "I/O Error: {}", cause),
            CustomError::Utf8(ref cause) => write!(f, "UTF-8 Error: {}", cause),
            CustomError::Other => write!(f, "Unknown error!"),
        }
    }
}

// Allow this type to be treated like an error
impl Error for CustomError {
    fn description(&self) -> &str {
        match *self {
            CustomError::Io(ref cause) => cause.description(),
            CustomError::Utf8(ref cause) => cause.description(),
            CustomError::Other => "Unknown error!",
        }
    }

    fn cause(&self) -> Option<&Error> {
        match *self {
            CustomError::Io(ref cause) => Some(cause),
            CustomError::Utf8(ref cause) => Some(cause),
            CustomError::Other => None,
        }
    }
}

// Support converting system errors into our custom error.
// This trait is used in `try!`.
impl From<IoError> for CustomError {
    fn from(cause: IoError) -> CustomError {
        CustomError::Io(cause)
    }
}
impl From<Utf8Error> for CustomError {
    fn from(cause: Utf8Error) -> CustomError {
        CustomError::Utf8(cause)
    }
}

Iterating through causes

It is often useful for debugging purposes to find the root cause of an error. In order to examine an error value that implements std::error::Error:

use std::error::Error;

let orig_error = call_returning_error();

// Use an Option<&Error>. This is the return type of Error.cause().
let mut err = Some(&orig_error as &Error);

// Print each error's cause until the cause is None.
while let Some(e) = err {
    println!("{}", e);
    err = e.cause();
}

Basic Error Reporting and Handling

Result<T, E> is an enum type which has two variants: Ok(T) indicating successful execution with meaningful result of type T, and Err(E) indicating occurrence of an unexpected error during execution, described by a value of type E.

enum DateError {
    InvalidDay,
    InvalidMonth,
}

struct Date {
    day: u8,
    month: u8,
    year: i16,
}

fn validate(date: &Date) -> Result<(), DateError> {
    if date.month < 1 || date.month > 12 {
        Err(DateError::InvalidMonth)
    } else if date.day < 1 || date.day > 31 {
        Err(DateError::InvalidDay)
    } else {
        Ok(())
    }
}

fn add_days(date: Date, days: i32) -> Result<Date, DateError> {
    validate(&date)?; // notice `?` -- returns early on error
    // the date logic ...
    Ok(date)
}

Also see docs for more details on ? operator.

Standard library contains an Error trait which all error types are recommended to implement. An example implementation is given below.

use std::error::Error;
use std::fmt;

#[derive(Debug)]
enum DateError {
    InvalidDay(u8),
    InvalidMonth(u8),
}

impl fmt::Display for DateError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            &DateError::InvalidDay(day) => write!(f, "Day {} is outside range!", day),
            &DateError::InvalidMonth(month) => write!(f, "Month {} is outside range!", month),
        }
    }
}

impl Error for DateError {
    fn description(&self) -> &str {
        match self {
            &DateError::InvalidDay(_) => "Day is outside range!",
            &DateError::InvalidMonth(_) => "Month is outside range!",
        }
    }

    // cause method returns None by default
}

Note: Generally, Option<T> should not be used for reporting errors. Option<T> indicates an expected possibility of non-existence of a value and a single straightforward reason for it. In contrast, Result<T, E> is used to report unexpected errors during execution, and especially when there are multiple modes of failures to distinguish between them. Furthermore, Result<T, E> is only used as return values. (An old discussion.)