Error handling best-practices

Question

I've been fumbling through Rust's documentation trying to execute a simple esoteric example for my own educational benefit more than practicality. While doing this, I can't seem to wrap my head around how Rust's error handling is meant to be used.

The programming example I'm using is to write a function that runs a command in a shell. From the result of the command I want to retrieve stdout (as a String or &str) and know whether or not the command failed.

The std::process::Command struct gives me the methods I want, but it seems that the only way to combine them is kludgy and awkward:

use std::process::Command;
use std::string::{String, FromUtf8Error};
use std::io::Error;


enum CmdError {
    UtfError(FromUtf8Error),
    IoError(Error),
}


// I would really like to use std::error::Error instead of CmdError,
// but the compiler complains about using a trait in this context.
fn run_cmd(cmd: &str) -> Result<String, CmdError> {
    let cmd_result = Command::new("sh").arg("-c").arg(cmd).output();

    match cmd_result {
        Err(e) => {
            return Err(CmdError::IoError(e));
        }
        Ok(v) => {
            let out_result = String::from_utf8(v.stdout);

            match out_result {
                Err(e) => {
                    return Err(CmdError::UtfError(e));
                }
                Ok(v) => {
                    return Ok(v);
                }
            }
        }
    }
}


fn main() {
    let r = run_cmd("echo 'Hello World!'");

    match r {
        Err(e) => {
            match e {
                CmdError::IoError(e) => {
                    panic!("Failed to run command {:}", e);
                }
                CmdError::UtfError(e) => {
                    panic!("Failed to run command {:}", e);
                }
            }
        }
        Ok(e) => {
            print!("{:}", e);
        }
    }
}

In particular, the nested match blocks inside run_cmd seem really awkward, and the nested match blocks in main are even worse.

What I'd really like to do is be able to use a more general class of error than FromUtf8Error or io::Error which I can type convert into easily from either concrete type, but it doesn't appear the type system is designed in this way, so I had to use the crude CmdError as somewhat of a union type instead.

I'm sure there's an easier way to do this which is more idiomatic, but I haven't found it from the documentation I've read so far.

Any pointers appreciated.

Answer 1

Defining things like this is not a particularly neat thing at present; there are a few things you need to set up with your custom error type, but after you’ve done that things are a lot easier.

First of all, you will want to implement std::error::Error for CmdError (which requires std::fmt::Display and std::fmt::Debug), and then in order that try! can work automatically, std::convert::From<std::string::FromUtf8Error> and std::convert::From<std::io::Error>. Here are the implementations of those:

use std::error::Error;
use std::string::FromUtf8Error;
use std::fmt;
use std::io;

#[derive(Debug)]
enum CmdError {
    UtfError(FromUtf8Error),
    IoError(io::Error),
}

impl From<FromUtf8Error> for CmdError {
    fn from(err: FromUtf8Error) -> CmdError {
        CmdError::UtfError(err)
    }
}

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

impl Error for CmdError {
    fn description(&self) -> &str {
        match *self {
            CmdError::UtfError(ref err) => err.description(),
            CmdError::IoError(ref err) => err.description(),
        }
    }

    fn cause(&self) -> Option<&Error> {
        Some(match *self {
            CmdError::UtfError(ref err) => err as &Error,
            CmdError::IoError(ref err) => err as &Error,
        })
    }
}

impl fmt::Display for CmdError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            CmdError::UtfError(ref err) => fmt::Display::fmt(err, f),
            CmdError::IoError(ref err) => fmt::Display::fmt(err, f),
        }
    }
}

(The description method in the Error implementation could possibly return a string not based on the wrapped error, e.g. “failed to run command”. If one wants the details, they’ll still be there in Error.cause().)

After implementing that lot, things are a lot easier because we can use try!. run_cmd can be written thus:

fn run_cmd(cmd: &str) -> Result<String, CmdError> {
    let output = try!(Command::new("sh").arg("-c").arg(cmd).output());
    Ok(try!(String::from_utf8(output.stdout)))
}

Because try! uses the From infrastructure, this is all a lot simpler; the first line may return an Err(CmdError::IoError(_)) (for Command.output() returns Result<_, io::Error>), and the second line may return an Err(CmdError::UtfError(_)) (for String::from_utf8(…) returns Result<_, FromUtf8Error>).

Your main can also be somewhat simpler then, with the err branch not needing any further matching if you don’t care about the particular error; as it implements fmt::Display now, you can just use it directly.

Incidentally, in a format string, {:} should be written as {}; the : is superfluous if not followed by anything. ({:?} would work for showing Debug output, but you should prefer to use Display if it’s user-facing.)