ApplicationState

All the Singleton types that your application needs to access at runtime when processing a request are grouped together in a struct called ApplicationState. The ApplicationState is code-generated by Pavex: you don't have to write it yourself!

It is defined in the server SDK crate and exported as a public type.
It is then used in the server crate as an input parameter to the run function, the one that launches the HTTP server to start listening for incoming requests.

Singleton dependencies

Constructors for Singleton types are invoked before the application starts, in the generated build_application_state function.

ApplicationState is designed to be minimal: it only includes the types that are needed at runtime.
If a Singleton type is only needed to build another Singleton type, it won't be included in the ApplicationState. It will be constructed in build_application_state and then discarded.

If Singleton type A is needed to build Singleton type B, you don't have to register a constructor for A.
Pavex will change the signature of build_application_state to require A as input parameter: you're then free to build A however you want in the server crate.