What patterns make HTTP error handling consistent and DRY in Go?

Go HTTP handlers cannot return errors — the function signature is func(http.ResponseWriter, *http.Request). Several patterns solve this: the custom handler type, the handler error interface, or a response helper pattern.

// Pattern 1: custom handler type that returns error
type HandlerFunc func(w http.ResponseWriter, r *http.Request) error

func (h HandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    if err := h(w, r); err != nil {
        handleError(w, r, err)
    }
}

// Typed API errors
type APIError struct {
    Status  int    `json:"-"`
    Code    string `json:"code"`
    Message string `json:"message"`
}
func (e *APIError) Error() string { return e.Message }

// Central error handler
func handleError(w http.ResponseWriter, r *http.Request, err error) {
    var apiErr *APIError
    if errors.As(err, &apiErr) {
        writeJSON(w, apiErr.Status, apiErr)
        return
    }
    if errors.Is(err, ErrNotFound) {
        writeJSON(w, http.StatusNotFound,
            &APIError{Code: "NOT_FOUND", Message: "resource not found"})
        return
    }
    // Unknown error — log and return generic 500
    log.Printf("unhandled error %s %s: %v", r.Method, r.URL.Path, err)
    writeJSON(w, http.StatusInternalServerError,
        &APIError{Code: "INTERNAL", Message: "internal server error"})
}

// Handler using the custom type
mux.Handle("GET /users/{id}", HandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
    id, err := strconv.Atoi(r.PathValue("id"))
    if err != nil {
        return &APIError{Status: 400, Code: "INVALID_ID",
            Message: "id must be an integer"}
    }
    user, err := userService.FindByID(r.Context(), id)
    if err != nil {
        return fmt.Errorf("finding user: %w", err) // wraps ErrNotFound
    }
    return writeJSON(w, http.StatusOK, user)
}))

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