unknwon/the-way-to-go_ZH_CN · error

err.String()

Error message

err.String()

What it means

err.String() invokes a method that does not exist on Go's built-in error interface. This snippet was written against pre-Go 1 (2011) releases, where errors were the os.Error type carrying a String() method; since Go 1.0 (2012) the error interface exposes only Error() string. Any modern gc toolchain rejects the sample at compile time with `err.String undefined (type error has no field or method String)`, so the sign handler in section 20.6 cannot even build.

Source

Thrown at eBook/20.6.md:53

`

var signTemplate = template.Must(template.New("sign").Parse(signTemplateHTML))

func init() {
	http.HandleFunc("/", root)
	http.HandleFunc("/sign", sign)
}

func root(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/html")
	fmt.Fprint(w, guestbookForm)
}

func sign(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/html")
	err := signTemplate.Execute(w, r.FormValue("content"))
	if err != nil {
		http.Error(w, err.String(), http.StatusInternalServerError)
	}
}
```

## 链接

- [目录](directory.md)
- 上一节：[使用用户服务和探索其 API](20.5.md)
- 下一节：[使用数据存储](20.7.md)

View on GitHub (pinned to 7a54d34d36)

Solutions

1. Replace err.String() with err.Error() in the http.Error call at eBook/20.6.md:53 — that is the only method the error interface has.
2. Migrate the sibling pre-Go 1 APIs in the same chapter so the sample compiles: import google.golang.org/appengine/{http, user, datastore}, and update calls like time.Seconds()/datastore.SecondsToTime to time.Now().
3. Verify with a modern toolchain: gofmt, then go vet / go build on the extracted snippet; add a return after http.Error so the handler stops on template failure.
4. Longer term, generate eBook snippets from real compiled sources and build them in CI so doc code cannot drift from the current Go API.

Example fix

// before
err := signTemplate.Execute(w, r.FormValue("content"))
if err != nil {
	http.Error(w, err.String(), http.StatusInternalServerError)
}
// after
err := signTemplate.Execute(w, r.FormValue("content"))
if err != nil {
	http.Error(w, err.Error(), http.StatusInternalServerError)
	return
}

// Fail the docs build if any snippet still uses the pre-Go 1 error API.
// Run over extracted .go snippets or the markdown source:
func snippetsUseDeadErrorAPI(files []string) error {
	for _, f := range files {
		b, err := os.ReadFile(f)
		if err != nil {
			return err
		}
		if strings.Contains(string(b), "err.String()") {
			return fmt.Errorf("%s: err.String() was removed in Go 1; use err.Error()", f)
		}
	}
	// ultimate check: the snippet must actually compile
	if out, err := exec.Command("go", "vet", "./...").CombinedOutput(); err != nil {
		return fmt.Errorf("snippet does not compile: %s", out)
	}
	return nil
}

// stringerError reports whether err still carries the pre-Go 1 String() method.
func stringerError(err error) bool {
	_, ok := err.(interface{ String() string })
	return ok
}

// Go has no try/catch; check the error inline and format it with Error():
if err := signTemplate.Execute(w, r.FormValue("content")); err != nil {
	http.Error(w, err.Error(), http.StatusInternalServerError)
	return // stop the handler; Execute already wrote partial output
}

When it happens

Trigger: Compiling the sign handler from eBook/20.6.md: `err := signTemplate.Execute(w, r.FormValue("content"))` returns the built-in error interface, then the error branch calls `http.Error(w, err.String(), http.StatusInternalServerError)`. go build / go vet / gopls on this snippet fails immediately with an undefined-method error.

Common situations: Copying code from tutorials or eBooks written before Go 1.0 — this chapter targets the 2011 App Engine SDK (appengine.NewContext, datastore.SecondsToTime, time.Seconds, all likewise removed or changed). Mirrored/vendored course material that was never migrated, and projects resurrecting legacy Google App Engine guestbook samples, all hit this the moment they are compiled with a modern toolchain.

Related errors

• err.String()
• math - square root of negative number
• Not found error
• math: square root of negative number %g
• err.String()

AI-assisted analysis of unknwon/the-way-to-go_ZH_CN@7a54d34d36 (2026-08-15). Data as JSON: /api/errors/06d0c75787bbe340. Report an issue: GitHub.