#clap #repl #command #command-line #clap-parser #user-interface #rustyline


Build nice REPLs using clap and reedline with zero effort

4 releases

0.2.0 May 23, 2024
0.1.2 May 22, 2024
0.1.1 Dec 3, 2023
0.1.0 Jun 22, 2023

#2 in #rustyline


134 lines


Rust crates.io

One of the typical user interfaces for prompting commands is the repl (read eval print loop). One of the best ways of representing commands in a repl is using space separated arguments, which is what terminal shells do. And the way to parse such commands in Rust is the clap crate. This crate uses clap and reedline to provide such user interface in a way that you only focus on your app logic.


Thanks to clap and reedline this crate handles:

  • Parsing the space separated commands into your data structure.
  • Help flag for each command.
  • Verifying the command is valid, generating useful errors and suggestions otherwise.
  • Auto complete and hint for the commands.


use std::path::PathBuf;

use clap::{Parser, ValueEnum};
use clap_repl::ClapEditor;
use reedline::{DefaultPrompt, DefaultPromptSegment, FileBackedHistory, Reedline, Signal};

#[derive(Debug, Parser)]
#[command(name = "")] // This name will show up in clap's error messages, so it is important to set it to "".
enum SampleCommand {
    Download {
        path: PathBuf,
        /// Check the integrity of the downloaded object
        /// Uses SHA256
        check_sha: bool,
    /// A command to upload things.
    /// Login into the system.
    Login {
        /// Optional. You will be prompted if you don't provide it.
        #[arg(short, long)]
        username: Option<String>,
        #[arg(short, long, value_enum, default_value_t = Mode::Secure)]
        mode: Mode,

#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ValueEnum)]
enum Mode {
    /// Encrypt the password
    /// Send the password plain
    /// This paragraph is ignored because there is no long help text for possible values in clap.

fn main() {
    let mut prompt = DefaultPrompt::default();
    prompt.left_prompt = DefaultPromptSegment::Basic("simple-example".to_owned());
    let mut rl = ClapEditor::<SampleCommand>::new_with_prompt(Box::new(prompt), |reed| {
        // Do custom things with `Reedline` instance here
            FileBackedHistory::with_file(10000, "/tmp/clap-repl-simple-example-history".into())
    loop {
        // Use `read_command` instead of `readline`.
        let Some(command) = rl.read_command() else {
        match command {
            SampleCommand::Download { path, check_sha } => {
                println!("Downloaded {path:?} with checking = {check_sha}");
            SampleCommand::Upload => {
            SampleCommand::Login { username, mode } => {
                // You can use another `reedline::Reedline` inside the loop.
                let mut rl = Reedline::create();
                let username = username
                    .unwrap_or_else(|| read_line_with_reedline(&mut rl, "What is your username? "));
                let password = read_line_with_reedline(&mut rl, "What is your password? ");
                println!("Logged in with {username} and {password} in mode {mode:?}");

fn read_line_with_reedline(rl: &mut Reedline, prompt: &str) -> String {
    let Signal::Success(x) = rl
    else {

Screenshot from 2023-06-22 11-32-58 Screenshot from 2023-06-22 11-35-33 image


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