You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: error_example_test.go
    path of file: ./repos/fx
    
    the code of the file until where you have to start completion: // Copyright (c) 2019 Uber Technologies, Inc.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restric
    
func ExampleError() {
	// A module that provides a HTTP server depends on
	// the $PORT environment variable. If the variable
	// is unset, the module returns an fx.Error option.
	newHTTPServer := func() fx.Option {
		port := os.Getenv("PORT")
		if port == "" {
			return fx.Error(errors.New("$PORT is not set"))
		}
		return fx.Provide(&http.Server{
			Addr: fmt.Sprintf("127.0.0.1:%s", port),
		})
	}

	app := fx.New(
		fx.NopLogger,
		newHTTPServer(),
		fx.Invoke(func(s *http.Server) error { return s.ListenAndServe() }),
	)

	fmt.Println(app.Err())

	// Output:
	// $PORT is not set
}