clap 3.1.9

A simple to use, efficient, and full-featured Command Line Argument Parser
Documentation 
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// Note: this requires the `cargo` feature

use clap::{arg, command};

fn main() {
    let matches = command!()
        .arg(
            arg!(<MODE>)
                .help("What mode to run the program in")
                .possible_values(["fast", "slow"]),
        )
        .get_matches();

    // Note, it's safe to call unwrap() because the arg is required
    match matches
        .value_of("MODE")
        .expect("'MODE' is required and parsing will fail if its missing")
    {
        "fast" => {
            println!("Hare");
        }
        "slow" => {
            println!("Tortoise");
        }
        _ => unreachable!(),
    }
}