main returning Result - Set exit code

• Result
• Box
• dyn
• Error
• Ok
• ?

#![allow(unused_variables)]

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let args = std::env::args().collect::<Vec<_>>();

    let filename = &args.get(1).ok_or("Missing file")?;
    let number = args.get(2).ok_or("Param 2 was missing")?.parse::<u8>()?;
    let num = number.checked_add(255).ok_or("overflow")?;

    let content = std::fs::read_to_string(filename)?;

    Ok(())
}

A number of invocations:

cargo run
cargo run data.txt
cargo run data.txt 10
cargo run data.txt abc
cargo run data.txt 0