Help message in String from Parser

[Tony-Goat]

Recently I've had trouble generating a help message into a string from a parser, however, I found the solution and thought I should share it for anyone else having trouble.

To do this safely, you need to have clap::Command, clap::CommandFactory, clap::ErrorKind and clap::Parser, and std::io::BufWriter in your uses. I wrote this code assuming that your function signature looks similar to

fn function_name(args: String) -> String

This is mostly useful for bots that accept commands from an interface other than CLI (e.g. a Discord bot). Your help parsing my look something akin to this:

//"command" is the string in which the arguments you need to parse are in
let command_tokens = command.split_whitespace();
//"DerivedParser" is the struct which you used the derive api on
let command_args = match DerivedParser::try_parse_from(command_tokens) {
	//Just set command_args to derived_arguments if it all works
	Ok(derived_arguments) => derived_arguments,
	Err(e) => return match e.kind() {
        //Using the --help or making a mistake causes this error in specific during parsing
		ErrorKind::DisplayHelp | ErrorKind::DisplayHelpOnMissingArgumentOrSubcommand => {
			//We have to turn it into a Command struct to get the write_help method
			let mut derived_cmd: Command = DerivedParser::command();
			//This is a buffer we write the help into
			let mut help_buffer = BufWriter::new(Vec::new());

			//This actually writes the help. You should change String::from_utf8_lossy
			//To the appropriate method to fit your needs here
			match derived_cmd.write_help(&mut help_buffer) {
				Ok(()) => String::from_utf8_lossy(help_buffer.get_ref()).to_string(),
				Err(e) => format!("Error writing help buffer: {}", e)
			}
		},
		_ => format!("There was an error parsing the arguments: {}", e)
	}
};

//Rest of your function logic goes here...

If you want to see how I use it specifically, you can see my repository for the Multi-Target Bot Specialist in src/bot_worker/openlegend.rs line 186. I also use Clap to parse the arguments in the bookmark command, however, I am working on porting that from 2.34.

[epage]

Great use case!

btw

let command_tokens = command.split_whitespace();

I'd recommend using shlex::split so you can handle quoting

[epage]

btw the derive reference now explicitly mentions CommandFactory, see a7b7d43