Dependencies

1 min read

Declare a provider function and inject its value with @dep(provider).

import system;
import system.webapi;

fun request_key(@header("X-Key") key: string): Result<string, HttpStatus> {
    if key.is_empty() {
        return Result.Err(HttpStatus.unauthorized());
    }
    return Result.Ok(key);
}

@router("")
class Routes {
    @get("/key")
    public static fun key(@dep(request_key) value: string): string {
        return value;
    }
}

async fun main(): void {
    let app = WebApp();
    app.include_router(Routes.router()).unwrap();
    app.run("127.0.0.1", 8000).await.unwrap();
}

@dep arguments

Argument	Default	Meaning
provider	Required	The function whose result fills this parameter.
cache	true	Reuse the provider's result for the rest of this request. @dep(provider, false) runs it again for this parameter.

What a provider can return

Provider returns	The parameter receives	On failure
T	T	—
Result<T, HttpStatus>	T	The request stops with that status.
DependencyResource<T>	T, with cleanup registered	— (see resources)

Providers may be synchronous or async. A provider's own parameters work like a handler's: @path, @query, @header, @cookie, and @body values, HttpIncoming, RequestContext, and other @dep values. A missing @header value inside a provider returns 401 instead of 422, so authentication providers fail with the right status. The generator resolves the graph and reports unknown providers and cycles at compile time.

Each request caches provider values by declaration identity. Two parameters using the same provider share one acquisition, including dependencies reached through different providers. Use @dep(provider, false) when an injection must run independently. Cache lifetime ends with that request.

A Result<T, HttpStatus> failure stops the endpoint with the supplied status. Providers returning resources register cleanup as soon as acquisition succeeds. The resource guide explains failure, cancellation, and overrides.

Use dependencies for values an endpoint needs, such as identity, a repository, or a request-scoped resource. Use middleware for behavior surrounding requests and responses.

See the API lookup and complete example.