Ability to use fx.As and fx.ResultTags for the same Provider

[Dan6erbond]

Ability to use fx.As and fx.ResultTags for the same Provider

[JacobOaks]

Hey @Dan6erbond thanks for the issue.

With the way that ResultTags works, the code you showed is applying the router name to both the http.Handler and the chi.Router. Named types are considered entirely separate from their unnamed counterparts from Fx's perspective. That's why you see Fx complaining about not having an unnamed http.Handler:

missing dependencies <...> missing type: http.Handler.

For now, you'll have to workaround this by either doing two returns and annotating them independently like:

func NewRouter() (chi.Router, http.Handler) {
	/* logic to create the router */
	return router, router
}

fx.Provide(
	fx.Annotate(
 		NewRouter,
		fx.ResultTags(`name:"router"`),
	)
)

Or if you can't change NewRouter, you can sort of manually annotate using "constructors" to add the 2nd representation.

fx.Provide(
	NewRouter, // let's say this returns a *routerImpl
	func(r *routerImpl) http.Handler { return r },
	fx.Annotate(
		func(r *routerImpl) chi.Router { return r },
		fx.ResultTags(`name:"router"`),
	),
)

This does unfortunately expose the underlying type to the container. If you can provide the exact definitions of chiRouter and the return types of NewRouter I can help you with an example to avoid this if that's an isue.

This is admittedly not very ergonomic and is an unfortunate consequence of how annotations work, which is why #899 is on our radar. Nested annotations would help a little bit allowing you to do something like this to keep the handler unnamed but the router named:

fx.Provide(
	fx.Annotate(
		fx.Annotate(
			NewRouter,
			fx.As(new(http.Handler)),
		),
	fx.As(new(chi.Router)),
	fx.ResultTags(`name:"router"`),
)