rust errors

Error handling in Rust is a mess right now. Should I use error_chain or failure? It is complicated.

For real applications I definitely want errors to bubble all the way up to main, and I’ll lay down the boilerplate to wrap all possible errors and output useful error messages.

However, for one-offs I would rather just quit with a simple error.

If you are in main, it will kindly print a Debug formatted error:

use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut file = File::open("foo.txt")?;
    Ok(())
}

$ ./foo
Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Not pretty, but adequate.

What if you aren’t in main or in a function that bubbles Result?

Maybe something like:

let mut file = File::open("foo.txt")
    .unwrap_or_else(|e| {
        eprintln!("{}", e);
        std::process::exit(2);
    });

It quickly gets tedious adding these silly blocks everywhere. I stumbled on an alternative.

Simply implement an extra method on Results:

use std::fmt::Display;

pub trait OrDie<T> {
    fn or_die(self, msg: &str) -> T;
}

impl<T, E: Display> OrDie<T> for Result<T, E> {
    fn or_die(self, msg: &str) -> T {
        match self {
            Ok(t) => t,
            Err(e) =>  {
                eprintln!("{}: {}", msg, e);
                std::process::exit(1);
            }
        }
    }
}

Then you can just chain .or_die("some message") to any Result.

 let mut file = File::open("foo.txt").or_die("foo.txt");

$ ./foo
foo.txt: No such file or directory (os error 2)