refactor: make error handling tighter and safer

todo/list.go

@@ -1,10 +1,14 @@
 package todo
 
-import (
- "errors"
-)
+type UserError struct {
+ message string
+}
+
+var ErrorBadId = &UserError{"bad todoItemId"}
 
-var ErrorBadId = errors.New("bad todoItemId")
+func (ue *UserError) Error() string {
+ return ue.message
+}
 
 type Item struct {
  Title string

web/handlers.go

@@ -1,7 +1,6 @@
 package web
 
 import (
- "errors"
  "github.com/xpmatteo/todomvc-golang/todo"
  "html/template"
  "log"
@@ -53,12 +52,8 @@ func NewItemHandler(templ *template.Template, repository Repository) http.Handle
  model.Add(title, nil)
  return nil
  })
- if errors.Is(err, todo.ErrorBadId) {
- badRequest(w, err)
- return
- }
  if err != nil {
- internalServerError(w, err)
+ handleError(w, err)
  return
  }
 
@@ -83,19 +78,25 @@ func ToggleHandler(templ *template.Template, repository Repository) http.Handler
  model, err := with(repository, func(model *todo.List) error {
  return model.Toggle(id)
  })
- if errors.Is(err, todo.ErrorBadId) {
- badRequest(w, err)
- return
- }
  if err != nil {
- internalServerError(w, err)
+ handleError(w, err)
  return
  }
  vm := viewModel(model, r)
  render(w, r, templ, vm)
  })
 }
 
+//goland:noinspection GoTypeAssertionOnErrors
+func handleError(w http.ResponseWriter, err error) {
+ switch err.(type) {
+ case *todo.UserError:
+ badRequest(w, err)
+ default:
+ internalServerError(w, err)
+ }
+}
+
 func EditHandler(templ *template.Template, repository Repository) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
  err := r.ParseForm()
@@ -113,12 +114,8 @@ func EditHandler(templ *template.Template, repository Repository) http.Handler {
  model, err := with(repository, func(model *todo.List) error {
  return model.Edit(id, title)
  })
- if errors.Is(err, todo.ErrorBadId) {
- badRequest(w, err)
- return
- }
  if err != nil {
- internalServerError(w, err)
+ handleError(w, err)
  return
  }
 
@@ -144,7 +141,7 @@ func DestroyHandler(templ *template.Template, repository Repository) http.Handle
  return model.Destroy(id)
  })
  if err != nil {
- internalServerError(w, err)
+ handleError(w, err)
  return
  }