exit 0.2.0

Custom exit status codes with ? in main
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# exit [![crates.io badge](https://img.shields.io/crates/v/exit.svg)](https://crates.io/crates/exit)

This crate exposes a type, `Exit`, which allows using `?` in `main` while also specifying custom exit status codes. 

The goal of this crate was to provide a proof of concept and sample implementation for the ideas discussed in [this blog post](https://www.joshmcguigan.com/blog/custom-exit-status-codes-rust/). 

## Example

```rust
#![feature(try_trait)]
use exit::{Exit, ExitDisplay};

use std::env;
use std::option;

#[derive(Debug)]
enum MyErr {
    MissingArg,
    ParseErrorUserNum,
    ParseErrorGroupNum,
}

impl From<MyErr> for i32 {
    fn from(err: MyErr) -> Self {
        match err {
            MyErr::MissingArg => 2,
            MyErr::ParseErrorUserNum => 3,
            MyErr::ParseErrorGroupNum => 4,
        }
    }
}

// You can optionally implement ExitDisplay for your error type in order to print an error message
// on exit
impl ExitDisplay for MyErr {
    fn display(&self) -> String {
        format!("{:?}", self)
    }
}

impl From<option::NoneError> for MyErr {
    fn from(_: option::NoneError) -> Self {
        MyErr::MissingArg
    }
}

fn main() -> Exit<MyErr> {
    let user_num_string : String = env::args().skip(1).next()?;
    let group_num_string : String = env::args().skip(2).next()?;

    let user_num : u32 = user_num_string.parse()
        .map_err(|_| MyErr::ParseErrorUserNum)?;
    let group_num : u32 = group_num_string.parse()
        .map_err(|_| MyErr::ParseErrorGroupNum)?;

    println!("Hello, user #{} from group #{}!", user_num, group_num);

    Exit::Ok
}
```

## License

Licensed under either of

 * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.