Add seq-do-indexed

* lisp/emacs-lisp/seq.el (seq-do-indexed): New function.
* test/lisp/emacs-lisp/seq-tests.el: Add a unit test for seq-do-indexed.

lisp/emacs-lisp/seq.el

@@ -4,7 +4,7 @@
 
 ;; Author: Nicolas Petton <nicolas@petton.fr>
 ;; Keywords: sequences
-;; Version: 2.16
+;; Version: 2.17
 ;; Package: seq
 
 ;; Maintainer: emacs-devel@gnu.org
@@ -117,6 +117,16 @@ Return SEQUENCE."
 
 (defalias 'seq-each #'seq-do)
 
+(defun seq-do-indexed (function sequence)
+  "Apply FUNCTION to each element of SEQUENCE and return nil.
+Unlike `seq-map', FUNCTION takes two arguments: the element of
+the sequence, and its index within the sequence."
+  (let ((index 0))
+    (seq-do (lambda (elt)
+               (funcall function elt index)
+               (setq index (1+ index)))
+             sequence)))
+
 (cl-defgeneric seqp (sequence)
   "Return non-nil if SEQUENCE is a sequence, nil otherwise."
   (sequencep sequence))

test/lisp/emacs-lisp/seq-tests.el

@@ -107,6 +107,21 @@ Evaluate BODY for each created sequence.
                                   '(a b c d))
                  '((a 0) (b 1) (c 2) (d 3)))))
 
+(ert-deftest test-seq-do-indexed ()
+  (let ((result nil))
+    (seq-do-indexed (lambda (elt i)
+                      (add-to-list 'result (list elt i)))
+                    nil)
+    (should (equal result nil)))
+  (with-test-sequences (seq '(4 5 6))
+    (let ((result nil))
+      (seq-do-indexed (lambda (elt i)
+                        (add-to-list 'result (list elt i)))
+                      seq)
+      (should (equal (seq-elt result 0) '(6 2)))
+      (should (equal (seq-elt result 1) '(5 1)))
+      (should (equal (seq-elt result 2) '(4 0))))))
+
 (ert-deftest test-seq-filter ()
   (with-test-sequences (seq '(6 7 8 9 10))
     (should (equal (seq-filter #'test-sequences-evenp seq) '(6 8 10)))