feat(tag): Tag feature experimentally

[m-mizutani]

Motivation

There are use cases where we need to adjust the error handling strategy based on the nature of the error. A clear example is an HTTP server, where the status code to be returned varies depending on whether it's an error from a downstream system, a missing resource, or an unauthorized request. To handle this precisely, you could predefine errors for each type and use methods like errors.Is in the error handling section to verify and branch the processing accordingly. However, this approach becomes challenging as the program grows larger and the number and variety of errors increase.

goerr provides a With method that allows parameters to be set on errors. However, specifying parameters as strings each time raises concerns about typos, leaving room for errors during development.

This pull request (PR) introduces a mechanism to prevent typos by predefining Tags and only accepting that type, thus enhancing reliability in development.

Example

var (
	ErrTagSysError   = goerr.NewTag("system_error")
	ErrTagBadRequest = goerr.NewTag("bad_request")
)

func handleError(w http.ResponseWriter, err error) {
	if goErr := goerr.Unwrap(err); goErr != nil {
		switch {
		case goErr.HasTag(ErrTagSysError):
			w.WriteHeader(http.StatusInternalServerError)
		case goErr.HasTag(ErrTagBadRequest):
			w.WriteHeader(http.StatusBadRequest)
		default:
			w.WriteHeader(http.StatusInternalServerError)
		}
	} else {
		w.WriteHeader(http.StatusInternalServerError)
	}
	_, _ = w.Write([]byte(err.Error()))
}

func someAction() error {
	if _, err := http.Get("http://example.com/some/resource"); err != nil {
		return goerr.Wrap(err, "failed to get some resource").WithTags(ErrTagSysError)
	}
	return nil
}

func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		if err := someAction(); err != nil {
			handleError(w, err)
			return
		}
		w.WriteHeader(http.StatusOK)
		_, _ = w.Write([]byte("OK"))
	})

	http.ListenAndServe(":8090", nil)
}