How to tell whether boolean flag was set by user?

[lars-berger]

Boolean flags have a great user experience by default, but a problem I'm running into is I can't figure out how to tell whether the user has explicitly specified the flag value. For my use case, the default arg value comes from an external source at runtime, and so I'd need to know whether the user has overridden this default.

My assumption was to change the type to Option<bool>, however, this makes the behavior inconsistent with the other boolean flags throughout our CLI.

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Foo {
    #[arg(short)]
    flag: bool,
    
    #[arg(short)]
    flag_with_option: Option<bool>, // prevents the use of `--flag-with-option` and a value becomes mandatory
}

Is there a way to keep the Option<bool> type and make the behavior the same as a normal boolean flag?

[lars-berger]

Got something working by using default_missing_value:

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Foo {
    #[arg(long)]
    flag: bool,

    #[clap(long, default_missing_value = "true", require_equals = true, num_args = 0..=1)]
    flag_with_option: Option<bool>,
}

With the above, the Option is Some only if the user specifies a value, and the user can continue to do both --flag-with-option and --flag-with-option=true.

The CLI output is different than a normal boolean flag though. If someone knows a way around that, please let me know 👍

Options:
      --flag
      --flag_with_option[=<FLAG_WITH_OPTION>]  [possible values: true, false]