Validator usage is unclear

[lberrymage]

I'm trying to use Validator to automatically validate custom, single-line commands. As a test for handling invalid input I set up the following code:

// `use` statements omitted for brevity
struct CommandHelper;

impl Completer for CommandHelper {
    type Candidate = String;
}
impl Helper for CommandHelper {}
impl Highlighter for CommandHelper {}
impl Hinter for CommandHelper {}
impl Validator for CommandHelper {
    fn validate(&self, ctx: &mut ValidationContext) -> rustyline::Result<ValidationResult> {
        let mut words = ctx.input().split_whitespace();

        match words.next() {
            None => Ok(ValidationResult::Valid(None)),
            Some(command) => Ok(ValidationResult::Invalid(Some("invalid command".to_string()))),
            Some(_) => (),
        }
    }
}

fn main() {
    let mut rl = Editor::<CommandHelper>::new();
    loop {
        match rl.readline("prompt >") {
            Ok(line) => println!("Got line: {}", line),
            Err(e) => eprintln!("Error: {}", e),
        }
    }
}

What's curious is that "invalid command" is never printed to the terminal no matter whether the input is empty, contains one "word," or contains multiple "words." Notably, adding rl.set_helper(Some(CommandHelper { })) before the REPL prevents readline() from ever returning without interrupting the process with CTRL-C. The input_validation example suffers from these same problems if the user enters anything but SELECT <anything_goes_here> ; on the first line.

My questions are:

• What is the purpose of Editor::set_helper(), and does it need to exist? If yes to either of these, is this documented?
• Is there documentation for Validator that explains its usage?
• Is the input_validation example truly broken, or am I missing something?

Additionally:

• Why does Validator::validate() return a Result<ValidationError, ReadlineError>, considering its only input is effectively a string slice? When would it return return ReadlineError?
• If Editor::set_helper() does has a use, why does it take Option<impl Helper> instead of just impl Helper? What is the use case of passing it None?

I am very willing to improve documentation on these subjects (once I understand them myself), write/fix examples, etc..

[gwenn]

• set_helper is used to enable (Some) or disable (None) your helper. But I suppose no one uses set_helper(None). Maybe we should have a constructor instead Editor::with_helper(...) ?
• Validator should be able to invoke command(s) which may produce Error but currently cannot.
• And " --< Expect: SELECT stmt" should have been displayed by input_validation example => a regression has been introduced between version 6.0 and version 6.1.0 (#330). See #461.

[gwenn]

Your "invalid command" message should be displayed with rustyline version >= 7.0.0