Added NewCategory() func. And now NewCategory() method needs a Category as an arg.

pkg/mindflow/mindflow.go

@@ -12,6 +12,13 @@ type Category struct {
 	Name string
 }
 
+func NewCategory(name string) (*Category, error) {
+	if name == "" {
+		return nil, errors.New("category's name should not be empty")
+	}
+	return &Category{Name: name}, nil
+}
+
 type Post struct {
 	ID       int64
 	Category Category
@@ -42,7 +49,7 @@ type Mindflow interface {
 	EditPost(post *Post) error
 	DeletePost(id int64) error
 	Posts() ([]Post, error)
-	NewCategory(name string) (int64, error)
+	NewCategory(category *Category) (int64, error)
 	EditCategory(category *Category) error
 	DeleteCategory(id int64) error
 	DeleteUnusedCategories() error