How to specify the default value for a vector argument with Clap 4

Question

I have the following type:

#[derive(clap::ValueEnum, Clone, Debug)]
pub enum Processor {
    DefaultProcessor,
    SecondaryProcessor,
}

I have a FromStr impl for this struct as well (not shown because it's very simple).

I am currently using this in a struct like this:

#[derive(Parser)]
pub struct RunLocalTestnet {
    /// Processors to run.
    #[clap(long)]
    processors: Vec<Processor>,
}

So far so good, this works great. What I'm trying to do now is add a default value for this vector, for example:

    #[clap(long, default_value_t = vec![Processor::DefaultProcessor])]
    processors: Vec<Processor>,

Unfortunately when I do this I get the following error:

error[E0277]: `Vec<processor::processors::Processor>` doesn't implement `std::fmt::Display`
   --> crates/aptos/src/node/local_testnet.rs:103:18
    |
103 |     #[clap(long, default_value_t = vec![Processor::DefaultProcessor])]
    |                  ^^^^^^^^^^^^^^^ `Vec<processor::processors::Processor>` cannot be formatted with the default formatter
    |
    = help: the trait `std::fmt::Display` is not implemented for `Vec<processor::processors::Processor>`
    = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
    = note: required for `Vec<processor::processors::Processor>` to implement `ToString`

Is there a way to make this work with Clap 4 (specifically 4.3.9) out of the box? I feel like it shouldn't be necessary to make Processor impl Display because it works up until I set the default value.

Answer 1

I believe you want default_values_t (note the s). default_value_t requires the type to implement Display or ValueEnum which of course Vec<T> does not. But default_values_t requires the type to be a Vec<T> where only T has to implement Display or ValueEnum, which is exactly what you have.

Docs page: https://docs.rs/clap/4.3.9/clap/_derive/index.html

• default_value = <str>: Arg::default_value and Arg::required(false)
• default_value_t [= <expr>]: Arg::default_value and Arg::required(false)
  • Requires std::fmt::Display that roundtrips correctly with the Arg::value_parser or #[arg(value_enum)]
  • Without <expr>, relies on Default::default()
• default_values_t = <expr>: Arg::default_values and Arg::required(false)
  • Requires field arg to be of type Vec<T> and T to implement std::fmt::Display or #[arg(value_enum)]
  • <expr> must implement IntoIterator<T>