Clarity on passing parameter objects to Provide

[jdhaines]

Hi everyone. I'm just getting started with the library and it feels like I'm hitting a wall on a pretty simple and core feature... parameter objects. I've set up a parameter struct as in the help docs...

type ServerParams struct {
	fx.In

	Port int
}

I have a NewHTTPServer function that requires a port int.

func NewHTTPServer(lc fx.Lifecycle, p ServerParams) *http.Server {
	NewServer := &Server{}

	// Declare Server config
	server := &http.Server{
		Addr:         fmt.Sprintf(":%d", p.Port),
		Handler:      NewServer.RegisterRoutes(),
		IdleTimeout:  time.Minute,
		ReadTimeout:  10 * time.Second,
		WriteTimeout: 30 * time.Second,
	}

	lc.Append(fx.Hook{
		OnStart: func(ctx context.Context) error {
			ln, err := net.Listen("tcp", server.Addr)
			if err != nil {
				return err
			}
			fmt.Println("Starting HTTP server at", server.Addr)
			go server.Serve(ln)
			return nil
		},
		OnStop: func(ctx context.Context) error {
			return server.Shutdown(ctx)
		},
	})
	return server
}

Finally, I have the function in main.

func main() {
	port := 3000
	// fx.New(
	// 	// fx.In(fx.Provide(func() int { return port })),
	// 	fx.Provide(NewHTTPServer),
	//
	// 	// fx.Supply(ServerParams{Port: port}),
	// 	fx.Invoke(func(server *http.Server) {
	// 	}),
	// ).Run()

	fx.New(
		fx.Supply(port),
		fx.Provide(func(port int) ServerParams {
			return ServerParams{Port: port}
		}),
		fx.Provide(NewHTTPServer),
		fx.Invoke(func(server *http.Server) {
		}),
	).Run()
}

I'm trying to figure out how to simply pass the port parameter to the NewHTTPServer function. It's not clear how invoke knows that I'm invoking NewHTTPServer. It's also not clear to me how to pass the parameter. ChatGPT got me one working method, (the uncommented code above) but it feels wrong to pass it with fx.Supply(port). I tried the commented code with fx.In and also with fx.Provide and I get errors about the missing port.

Eventually I'll be getting this port from a flag using ff and trying to use fx to make that work. I'd like to be able to parse the flags, and then have those values injected into things like server, database, logger, etc. This feels right in line with the problems this library is trying to solve, but after a few hours with the docs I can't find even basic examples of passing parameters in either manually or through fx properly.

I took a stab at better documentation, but I highly doubt it's the right style or the right content.

First Attempt

Appreciate any help or examples. I'll be using echo, sqlite, zerolog, and ff in case that matters... but don't feel like you have to tailor the examples to my stack. Thanks!