Prioritize CLI parameters over file/env if not set explicitly

[vbrandl]

I want to prioritize CLI parameters over parameters from a config file or the environment but only if the CLI parameters are explicitly set (e.g. not derived via the clap default value).

If I understand correctly, merging with clap parameters only works if every CLI parameter has a default value.

I have the following config struct:

#[derive(Serialize, Deserialize, Debug, Parser)]
#[command(version, about, long_about = None)]
pub struct Config {
    #[arg(long, default_value = "0.0.0.0")]
    pub host: String,
    #[arg(long, default_value = "1337")]
    pub port: u16,
}

And 2 functions to parse the configuration, with a different order for the config sources:

impl Config {
    pub fn load_env_first() -> Result<Self> {
        Ok(Figment::new()
            .merge(Env::prefixed("APP_"))
            .merge(Serialized::defaults(Config::parse()))
            .extract()?)
    }

    pub fn load_cli_first() -> Result<Self> {
        Ok(Figment::new()
            .merge(Serialized::defaults(Config::parse()))
            .merge(Env::prefixed("APP_"))
            .extract()?)
    }
}

My general expectation for CLI tools is, that explicitly passed command line parameters overwrite everything else (e.g. parameters from env or config files). If I want this, i have to use the load_env_first function. But since for this to work, every clap parameter needs a default value, those default values overwrite everything from env and config files, if I place the serialized defaults last.

$ APP_HOST=127.0.0.1 ./app
env first: Config {
    host: "0.0.0.0",
    port: 1337,
}
cli first: Config {
    host: "127.0.0.1",
    port: 1337,
}

Is it possible to achieve the following behavior using figment, clap and configuration files/environment variables?

# default values
$ ./app
=> host = 0.0.0.0
# value from env (and NOT the clap default)
$ APP_HOST=127.0.0.1 ./app
=> host = 127.0.0.1
# value from cli
$ ./app --host 127.0.0.1
=> host = 127.0.0.1
# if both env and cli params are set, the CLI param should be prioritized
$ APP_HOST=127.0.0.1 ./app --host 127.0.0.2
=> host = 127.0.0.2

[phwoo]

I would also like to see a solution for this at the crate level. It seems a strange gap in the library that this isn't directly supported.

In the interim, I've taken to implementing Provider for the command-line argument struct, with the fields being all Option<T>, such as

impl Provider for AppArgs {
    fn metadata(&self) -> Metadata {
        Metadata::named("AppArgs")
    }

    fn data(&self) -> Result<Map<Profile, Dict>, figment::Error> {
        let mut dict = Dict::new();

        insert_if_some(&mut dict, "foo", self.foo);
        insert_if_some(&mut dict, "bar", self.bar);

        let mut profile_map = Map::new();
        profile_map.insert(Profile::default(), dict);

        Ok(profile_map)
    }
}

You can then "iterate" the fields of your struct and use a helper function such as:

fn insert_if_some<T>(dict: &mut Dict, arg_name: &str, arg: Option<T>)
where
    T: Serialize,
{
    if let Some(a) = arg {
        dict.insert(
            arg_name.to_string(),
            Value::serialize(a).expect("Couldn't serialize {arg}"),
        );
    }
}

to parse out the values. Then you just merge as usual.

[nrdxp]

This is already possible fwiw. You just need to expose your config somehow (I map my figment to a static CONFIG)

Then I can do, e.g.:

    #[clap(long, default_value_t = CONFIG.platforms.build.to_string())]
    build_platform: String,

based on claps rules, this will use the configuration value as the default only when not passed by the user. Then you reference the value exclusive from your cli args in app.