clap/examples/11_only_specific_values.rs

extern crate clap;

use clap::{App, Arg};

fn main() {
    // If you have arguments of specific values you want to test for, you can use the
    // .possible_values() method of Arg
    //
    // This allows you specify the valid values for that argument. If the user does not use one of
    // those specific values, they will receive a graceful exit with error message informing them
    // of the mistake, and what the possible valid values are
    //
    // For this example, assume you want one positional argument of either "fast" or "slow"
    // i.e. the only possible ways to run the program are "myprog fast" or "myprog slow"
    let mode_vals = ["fast", "slow"];
    let matches = App::new("myapp").about("does awesome things")
                        .arg(Arg::with_name("MODE")
                                .help("What mode to run the program in")
                                .index(1)
                                .possible_values(&mode_vals)
                                .required(true))
                        .get_matches();

    // Note, it's safe to call unwrap() because the arg is required
    match matches.value_of("MODE").unwrap() {
        "fast" => {
            // Do fast things...
        },
        "slow" => {
            // Do slow things...
        },
        _      => unreachable!()
    }
}