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Building a CLI tool

Minimal​

fn main() -> Result<(), Error> {
    let args = env::args();
    match args.get(1).map(|s| s.as_str()) {
        Maybe.Some("--help") | Maybe.Some("-h") => print_help(),
        Maybe.Some(name)                        => greet(&name),
        Maybe.None                              => print_help(),
    }
    Result.Ok(())
}

fn greet(name: &Text) {
    print(&f"Hello, {name}!");
}

fn print_help() {
    println(&"usage: greet <name>");
}

With subcommands​

type Command is
    | Build { release: Bool, target: Maybe<Text> }
    | Test  { filter: Maybe<Text> }
    | Version;

fn parse_args(args: &List<Text>) -> Result<Command, Error> {
    match args.get(1).map(|s| s.as_str()) {
        Maybe.Some("build") => {
            let mut release = false;
            let mut target = Maybe.None;
            for a in args.iter().skip(2) {
                match a.as_str() {
                    "--release"              => release = true,
                    s if s.starts_with("--target=")
                        => target = Maybe.Some(s[9..].to_string()),
                    _ => return Result.Err(Error.new(&f"unknown flag {a}")),
                }
            }
            Result.Ok(Command.Build { release, target })
        }
        Maybe.Some("test") => {
            let filter = args.get(2).cloned();
            Result.Ok(Command.Test { filter })
        }
        Maybe.Some("version") => Result.Ok(Command.Version),
        Maybe.Some(c) => Result.Err(Error.new(&f"unknown command: {c}")),
        Maybe.None    => Result.Err(Error.new(&"no command given")),
    }
}

fn main() {
    match parse_args(&env::args()).and_then(dispatch) {
        Result.Ok(()) => (),
        Result.Err(e) => { eprintln(&f"error: {e}"); env::exit(1); }
    }
}

Reading config files​

fn load_config() -> Result<Config, Error> {
    let path = env::var_opt("MYTOOL_CONFIG")
        .unwrap_or_else(|| format!("{}/.mytool/config.toml", env::home_dir().unwrap_or(".".to_string())));

    match fs::read_to_string(&Path.from(&path)) {
        Result.Ok(text) => toml_parse(&text).map_err(Error.from),
        Result.Err(e) if e.kind == IoErrorKind.NotFound => Result.Ok(Config.default()),
        Result.Err(e) => Result.Err(Error.from(e)),
    }
}

Coloured error output​

use term::style::{Color, Style};

fn report_error(e: &Error) {
    let red = Style.new().fg(Color.Red).add_modifier(Modifier.Bold);
    eprint(&red.paint(&"error: "));
    eprintln(&e.to_string());
    for src in e.chain().skip(1) {
        eprintln(&f"  caused by: {src}");
    }
}

Exit codes​

fn main() {
    match run() {
        Result.Ok(())                                  => env::exit_success(),
        Result.Err(e) if e.is_user_error()             => { report_error(&e); env::exit(1); }
        Result.Err(e) if e.is_config_error()           => { report_error(&e); env::exit(78); }  // sysexits EX_CONFIG
        Result.Err(e)                                  => { report_error(&e); env::exit(1); }
    }
}

Progress bars & spinners​

use term::widget::{Spinner, SpinnerFrames};

async fn slow_task() {
    let mut spin = Spinner.new().frames(&SpinnerFrames.Dots);
    spin.start(&"loading");
    let result = heavy_compute().await;
    spin.stop(&f"done in {result.elapsed:?}");
}

See also​