error.rs

/**
 *  An example of how we can do error handling in rust.
 */

use std::fmt;
use std::fs::File;
use std::io;
use std::result::Result;


// A custom Error type that has one variant, "Io" representing issues
// with reading from the config file.
#[derive(Debug)]
enum ConfigError {
    Io(io::Error),
}

// Tell rust how to convert an IoError to a ConfigError.
// Essentially, take the original IOError and wrap it with the ConfigError.
impl From<io::Error> for ConfigError {
    fn from(error: io::Error) -> Self {
        ConfigError::Io(error)
    }
}

// Tell rust how to print the ConfigError. For an IO variant, give the
// caller some context that it's config error, that it happenen when trying
// to load the file and print out the original error.
impl fmt::Display for ConfigError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            ConfigError::Io(ref e) =>
                write!(f, "configuration error: problem loading file: {}", e),
        }
    }
}

// Pretend we are getting a config from a file.
fn get_config() -> Result<File, ConfigError> {
    let f = File::open("foobar")?;
    Ok(f)
}
fn main() {
    match get_config() {
        Ok(f) => println!("{:?}", f),
        Err(why) => println!("{}", why),
    }
}