try_harder.rs

#![feature(macro_rules)]

macro_rules! try_harder {
    ($e:expr) => {
        match $e {
            Ok(val) => val,
            Err(e) => return Err(e)
        }
    };
    ($e:expr, $f:expr) => {
        match $e.map_err($f) {
            Ok(val) => val,
            Err(e) => return Err(e)
        }
    }
}

fn baz() -> Result<i32, i64> {
    Err(420)
}

fn bar() -> Result<i32, String> {
    Ok(42)
}

fn foo() -> Result<i32, String> {
    let m = try_harder!(bar());
    let n = try_harder!(baz(), |n| n.to_string()); 
    Ok(m + n)
}

fn main() {
    println!("{}", foo());
}