Default value if $ENV is empty

[flosse]

I can't get the default value for an argument, if the environment value ist empty:

invalid value '' for '--host <HOST>': invalid IP address syntax

This is how i defined the argument:

const DEFAULT_IP_ADDR: IpAddr = IpAddr::V4(Ipv4Addr::LOCALHOST);
const ENV_KEY_ENDPOINT_HOST: &str = "ENDPOINT_HOST";

#[derive(Subcommand)]
pub enum Commands {
    Serve {
        #[arg(
            long,
            env = ENV_KEY_ENDPOINT_HOST,
            default_value_t = DEFAULT_IP_ADDR,
        )]
        host: IpAddr,
    },
    // ...
}

What can I do here?

[epage]

We currently do not have a way to recognize an empty environment variable as "unset". I looked at config for prior art and it does have a global ignore_empty. rust-cli/config-rs#78 mentioned viper did it. That started with only ignoring empty but allowing empty was added in spf13/viper#440. It looks like that was originally going to remove the filtering of empty values but became opt-in to preserve compatibility without any statement on that not being a good direction in general. I'm unsure if its best to always treat it as empty (being bash like) or have it be per data type.

I'm also fairly uncomfortable extending env support because it is a slippery slope to full layered config (#2763) and we have to balance the needs of different clap users while people can pull in their own layer config approach to handle needs like this. If we made env support a plugin (#3476) then it would be easy to play with different approaches in an outside library.

[flosse]

I see, thank you for clarifying that!
Then I'll probably skip the env feature and implement it myself, which will also involve less magic 😉