Show help info in REPL

[tisonkun]

Here is my parser and repl code:

        // special repl command
        if let Some(input) = input.strip_prefix("\\") {
            let cmd = ReplCommand::try_parse_from(input.split_whitespace())
                .change_context_lazy(|| Error::Internal("failed to parse command".to_string()))?;
            match cmd.cmd {
                ReplSubCommand::Connect(connect) => { ... }
            }
            continue;
        }

#[derive(Debug, Parser)]
#[command(multicall = true)]
pub struct ReplCommand {
    #[command(subcommand)]
    pub cmd: ReplSubCommand,
}

#[derive(Debug, Subcommand)]
pub enum ReplSubCommand {
    /// Connect to another server.
    #[command(name = "connect")]
    Connect(CommandConnect),
}

#[derive(Debug, Parser)]
pub struct CommandConnect {
    /// The endpoint of the server to connect to.
    #[arg(value_name = "ENDPOINT")]
    pub endpoint: String,
}

When running:

> \help
thread 'main' panicked at cmd/xxx/src/main.rs:41:41:
called `Result::unwrap()` on an `Err` value: failed to parse command
├╴at cmd/xxx/src/repl/entrypoint.rs:52:18
│
╰─▶ Usage: <COMMAND>

    Commands:
      connect  Connect to another server
      help     Print this message or the help of the given subcommand(s)
    ╰╴at cmd/xxx/src/repl/entrypoint.rs:52:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

> \connect --help
thread 'main' panicked at cmd/xxx/src/main.rs:41:41:
called `Result::unwrap()` on an `Err` value: failed to parse command
├╴at cmd/xxx/src/repl/entrypoint.rs:52:18
│
╰─▶ Connect to another server

    Usage: connect <ENDPOINT>

    Arguments:
      <ENDPOINT>  The endpoint of the server to connect to

    Options:
      -h, --help  Print help
    ╰╴at cmd/xxx/src/repl/entrypoint.rs:52:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

How can I properly handle help command and print the help info in REPL?

[epage]

You can see our REPL example at https://docs.rs/clap/latest/clap/_derive/_cookbook/repl_derive/index.html

In that case, the difference between an error and help printing doesn't matter because we do the same thing. If you are needing to distinguish, check err.use_stderr().