Read your tool's typed configuration¶

Get your config out of the App¶

App stores configuration type-erased, so you name the type on the way out:

async fn run(&self, app: App) -> miette::Result<()> {
    let Some(config) = app.typed_config::<MySettings>() else {
        return Err(miette::miette!("mytool: no typed configuration wired"));
    };
    let settings = config.get();
    println!("talking to {}", settings.host);
    Ok(())
}

typed_config::<C>() returns Option<Arc<Config<C>>>, and the returned Arc shares the same allocation as the one inside the App — the downcast is a single Any round-trip, cheap enough to call at the top of every command body without caching it anywhere.

Choose between typed_config and config_as¶

let config = app.config_as::<MySettings>();   // panics if the type does not match

config_as panics with a message naming the type you asked for, and #[track_caller] reports your line rather than one inside rtb-app:

App::config_as::<mytool::Settings>() — no matching typed config wired
(did `Application::builder().config(...)` get called with the right type?)

A library crate that might be used by more than one tool should always use typed_config — the panic is only reasonable when you personally know the config was wired.

Handle "no typed config" gracefully¶

A tool that has not typed its configuration yet has a Config<()> stored, so typed_config::<C>() returns None for every C except (). Commands meant to work in both situations should fall back rather than fail:

let host = app
    .typed_config::<MySettings>()
    .map(|c| c.get().host.clone())
    .unwrap_or_else(|| "localhost".to_string());

Why config_schema() is None when typed_config() is Some¶

This combination catches people out. It means the config was stored, but the schema-and-render bundle was never attached.

App::new erases a Config<C> and nothing else. It cannot build the bundle, because its bound on C is only DeserializeOwned while generating a schema needs JsonSchema and rendering a value needs Serialize. The bundle arrives separately, through App::with_typed_config — which is what rtb_cli::Application::builder().config(...) and TestAppBuilder::config(...) both call for you.

So:

If you are building an App by hand and config show comes back empty, that last row is why.

Read the merged config without naming the type¶

Generic code — anything that has to work across tools — reads JSON instead:

if let Some(value) = app.config_value() {
    println!("{}", serde_json::to_string_pretty(&value)?);
}
if let Some(schema) = app.config_schema() {
    // schema is a &serde_json::Value
}

This is the path rtb-cli's config show / get / schema / validate takes, and it is the reason C never has to appear in rtb-cli's signatures.

Related¶

• App reference — the config accessors
• Typed configuration storage
• Why the config is type-erased