func query() (email string) {
    db, err := sql.Open("postgres", "user=postgres dbname=test ...")
    if err != nil {
        panic(err)
    }
    err = db.QueryRow(`SELECT email FROM "user" WHERE id = $1`, 1).Scan(&email)
    if err != nil {
        panic(err)
    }
    return email
}

func query(db *sql.DB) (email string) {
    err = db.QueryRow(`SELECT email FROM "user" WHERE id = $1`, 1).Scan(&email)
    if err != nil {
        panic(err)
    }
    return email
}

func TestQuery(t *testing.T) {
    db := mockDB()
    defer db.Close()
    
    email := query(db)
    assert.Equal(t, email, "email@example.com")
}

func main() {
    // Create logger
    logger := log.New(os.Stdout, "[daangn] ", 0)
    
    // Create handlers
    handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    	logger.Println("Handler called")
        io.WriteString(w, "Hello, world!\n")
    })
    
    // Register handlers
    http.Handle("/", handler)
    
    // Start server
    http.ListenAndServe(":8080", nil)
}

type Logger interface{
    Println(v ...interface{})
}

func NewLogger() Logger {
    return log.New(os.Stdout, "[daangn] ", 0)
}

func main() {
    logger := NewLogger()
    handler := NewHandler(logger)
    RegisterHandlers(handler)
    StartServer()
}

package logger

type Logger interface{
    Println(v ...interface{})
}

func NewLogger() Logger {
    return log.New(os.Stdout, "[daangn] ", 0)
}

func main() {
    logger := logger.NewLogger()
    handler := handlers.NewHandler(logger)
    server.RegisterHandlers(handler)
    server.StartServer()
}

func main() {
    ...
    if err != nil {
    	...
    }
    ...
    if err != nil {
    	...
    }
    ...
    if err != nil {
    	...
    }
    ...
}

package logger

var Module = fx.Options(
	fx.Provide(NewLogger),
)
...

func main() {
    app := fx.New(
    	server.Module,
        logger.Module,
        fx.Provide(
        	handlers.NewHandler,
        ),
    )
    app.Run()
}