Use one argument to determine load environment, before setting defaults

[tgross35]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

4.1.13

Describe your use case

I think there is a fairly common use case for environment variable defaults where you want to load a specific .env file based on a command argument. This means that the process would be: parse single argument, error or default if missing -> read .env based on parsed value -> parse rest of arguments -> do env defaults for those arguments -> do hardcoded defaults for those arguments -> error for missing values.

I don't know a good way to do this with Clap. Example use case:

#[derive(Parser, Debug)]
pub struct Config {
    #[clap(long, env = "MYAPP_ENV", default_value = "production")]
    pub mode: Mode
    #[clap(long, env = "MYAPP_PORT", default_value = "1234")]
    pub port: u16,
    #[clap(long, env = "MYAPP_BIND_ADDR", default_value = "0.0.0.0")]
    pub bind_addr: String,
}

#[derive(Clone, Copy, Debug, ValueEnum)]
pub enum Mode {
    Production,
    Development,
    Test
}

Describe the solution you'd like

Some way to parse a single field of the struct - I'm imagining something like a parse_first attribute:

struct Config {
    #[clap(long, env = "MYAPP_ENV", default_value = "production", parse_first = true)]
    pub mode: Mode
    // ...
}

Which could create

// function to parse only the first values
impl Config { pub fn parse_first() -> ConfigParseFirst }
// dummy struct that holds values marked `parse_first`
struct ConfigParseFirst { pub mode: Mode } 
// continue parsing the rest of the values, then go to their defaults
// move `mode` to `Config` and don't reparse to avoid race condition type thingies
impl ConfigParseFirst { pub fn parse_rest(self) -> Config }

And then be used like:

fn main() {
    load_env(".env");
    let tmp_cfg = Config::parse_first();
    match tmp_cfg.mode {
        Production => load_env(".env.production"),
        Development => load_env(".env.development"),
        Test => load_env(".env.test")
    }
    let cfg = tmp_cfg.parse_remaining();
    todo!()
}

Alternatives, if applicable

A callback method could also work, and maybe be simpler. E.g.

struct Config {
    #[clap(long, env = "MYAPP_ENV", default_value = "production", parse_first_callback = set_the_mode)]
    pub mode: Mode
    // ...
}

fn set_the_mode(mode: &Mode) {
    match mode { /* ... */ }
}

Additional Context

No response

[epage]

I believe you are looking for a mixture of either

• Designing for layered configs #2763
• [modular] User-driven values (defaults, env vars, implied values, etc) #4793

I'm closing in favor of those.

[tgross35]

Good to know those exist, I had no clue what to look for. Thanks!