I would like to be able to compute the possible values of an argument dynamically, based on the values provided for some other argument(s).

Use case

I'm working on a CLI which has a goal similar to make.

$ make --help
Usage: make [options] [target] ...
Options:
  -f FILE, --file=FILE, --makefile=FILE
                              Read FILE as a makefile.

The file lists the available targets. Given a value of FILE, I am able to load a list of available target names. I would like to use that list as the possible values for target.

This would affect the validation of target. Ideally, this would also affect its autocompletion.

Currently, possible_values is expected to be provided as an array slice (&[&str]) as the App is being built.

Wanted solution

I would like instead to be able to provide a closure that would compute the possible values for my argument. This closure would take the values of the other app arguments as a parameter.

Workaround

It is already possible to obtain this behavior by parsing the arguments twice (the second time with refined validation):

fn get_app(allowed_target_names: Option<Vec<&str>>) -> App {
    let target_arg = Arg::with_name("target");

    let target_arg = if let Some(allowed_target_names) = allowed_target_names {
        target_arg.possible_values(&allowed_target_names)
    } else {
        target_arg
    };

    App::new("make")
        .arg(target_arg)
        [...]
}

fn main() {
    let arg_matches = get_app(None).get_matches();

    let config = Config::load(arg_matches.value_of("config_file"));
    let all_target_names = config.list_target_names();

    let app_args = get_app(Some(all_target_names)).get_matches();

    [...]
}

However, this workaround doubles the overhead of parsing arguments. Also, this solution does not cover the autocompletion part.

Related issues

• completions: Support adding additional code to complete values #568
• Add hinting of arg value types for zsh/fish completion #1793