How to validate and generate error message based on neighbor option ?

[FirelightFlagboy]

Hello,

I'm working on a CLI for parsec-cloud.

I would like to know if it is possible to validate an argument/option based on another one.

The scenario is like the following:

We have a CLI that take 2 options:

• A config dir
• A device id

The struct opt would look like something:

#[derive(clap::Parser)
struct Opts {
  #[arg(long)]
  config_dir: PathBuf,
  #[arg(long)]
  device_id: Uuid,
}

The dev_id reference a device file present in config_dir and a would like to validate it's value, meaning:

• The ID is in correct format (that is already handled with from_str)
• The device exist in the config dir (otherwise display a list of possible devices based on config_dir)
• If device_id option is not provided, display a list of possible devices based on config_dir

Currently we are able to do that outside of clap, but it is not perfect because:

• We need to define device_id as optional to be able handle those cases after the parsing.
• Since device_id is optional, if we have an additional required option (say --foo) and during usage we don't not provide --device-id nor --foo the error message for missing arguments will be incomplete (missing --device-id).
• Our error message are not formatted the same way than clap.

I've tried to take a look at TypedValueParser but it don't seems to provide a context to get the value for other args to generate error and possible_values isn't given a context to generate it's values.

[epage]

We do not currently have support for this.

TypedValueParser is run too early for this. We process those as we are parsing and so we don't have the full command-line available.

We are exploring more custom validation options in #2763 but overall I suspect doing it on your side will provide the best results.

[FirelightFlagboy]

Thanks for the reply, do you know if it's possible to reuse some part of clap to display our own error message related to CLI's errors ?

We could use that in our custom validation and have an output similar to clap for error message

[epage]

You can but its a bit of work. clap::Error is built of a lot of dynamic content and we don't document the exact combinations of the content to get the right output, see https:/clap-rs/clap/blob/master/clap_builder/src/error/format.rs

You could also construct a PossibleValuesParser and call into it to have it generate the error.