WIP: Add category sorter

org-ql.el

@@ -417,7 +417,10 @@ each priority the newest items would appear first."
        (org-ql--sort-by items (-list sort)))
       ;; Sort by user-given comparator.
       ((pred functionp) (-sort sort items))
-      (_ (user-error "SORT must be either nil, one or a list of the defined sorting methods (see documentation), or a comparison function of two arguments")))))
+      ;; FIXME: Test more carefully for sorters with arguments.  In the meantime, just try to sort.
+      (_ (org-ql--sort-by items (-list sort)))
+      ;; (_ (user-error "SORT must be either nil, one or a list of the defined sorting methods (see documentation), or a comparison function of two arguments"))
+      )))
 
 ;;;###autoload
 (cl-defun org-ql-query (&key (select 'element-with-markers) from where narrow order-by)
@@ -2157,12 +2160,20 @@ PREDICATES is a list of one or more sorting methods, including:
                        ((or 'deadline 'scheduled 'closed)
                         (apply-partially #'org-ql--date-type< (intern (concat ":" (symbol-name symbol)))))
                        ;; TODO: Rename `date' to `planning'.  `date' should be something else.
-                       ('date #'org-ql--date<)
+                       ('category #'org-ql--category<)
+		       ('date #'org-ql--date<)
                        ('priority #'org-ql--priority<)
                        ('random (lambda (&rest _ignore)
                                   (= 0 (random 2))))
                        ;; NOTE: reverse and todo are handled below.
                        ;; TODO: Add more.
+		       (`(tags . ,tags) (lambda (a b)
+					  "Return non-nil if A has certain tags and B doesn't."
+					  (cl-flet ((test-entry
+						     (entry) (org-with-point-at (org-element-property :org-hd-marker entry)
+							       (apply #'org-ql--predicate-tags tags))))
+					    (and (test-entry a)
+						 (not (test-entry b))))))
                        (_ (user-error "Invalid sorting predicate: %s" symbol))))
              (sort-by-todo-keyword (items)
                                    (let* ((grouped-items (--group-by (when-let (keyword (org-element-property :todo-keyword it))
@@ -2185,6 +2196,17 @@ PREDICATES is a list of one or more sorting methods, including:
                     (_ (-sort (sorter pred) items)))))
     items))
 
+(defun org-ql--category< (a b)
+  "Return non-nil if A's category is alphabetically less than B's."
+  (cl-macrolet ((category (element)
+			  `(org-with-point-at (org-element-property :org-hd-marker ,element)
+			     (org-get-category (point)))))
+    (let ((a-category (category a))
+	  (b-category (category b)))
+      (if (and a-category b-category)
+	  (string< a-category b-category)
+	a-category))))
+
 ;; TODO: Rewrite date sorters using `ts'.
 
 (defun org-ql--date-type< (type a b)