Add new sequence function 'seq-union'

* lisp/emacs-lisp/seq.el (seq-union): New function.
* doc/lispref/sequences.texi (Sequence Functions):
* lisp/emacs-lisp/shortdoc.el (sequence): Document above new
function.
* test/lisp/emacs-lisp/seq-tests.el (test-seq-union): New test.

2 files changed, 13 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index f0dc283f57d..b7dcde87f41 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -467,6 +467,17 @@ negative integer or 0, nil is returned."
         (setq sequence (seq-drop sequence n)))
       (nreverse result))))
 
+(cl-defgeneric seq-union (sequence1 sequence2 &optional testfn)
+  "Return a list of all elements that appear in either SEQUENCE1 or SEQUENCE2.
+Equality is defined by TESTFN if non-nil or by `equal' if nil."
+  (let ((accum (lambda (acc elt)
+                 (if (seq-contains-p acc elt testfn)
+                     acc
+                   (cons elt acc)))))
+    (seq-reverse
+     (seq-reduce accum sequence2
+                 (seq-reduce accum sequence1 '())))))
+
 ;;;###autoload
 (cl-defgeneric seq-intersection (sequence1 sequence2 &optional testfn)
   "Return a list of the elements that appear in both SEQUENCE1 and SEQUENCE2.
diff --git a/lisp/emacs-lisp/shortdoc.el b/lisp/emacs-lisp/shortdoc.el
index adee6be379d..3e0d5aef022 100644
--- a/lisp/emacs-lisp/shortdoc.el
+++ b/lisp/emacs-lisp/shortdoc.el
@@ -809,6 +809,8 @@ There can be any number of :example/:result elements."
    :eval (seq-remove #'numberp '(1 2 c d 5)))
   (seq-group-by
    :eval (seq-group-by #'cl-plusp '(-1 2 3 -4 -5 6)))
+  (seq-union
+   :eval (seq-union '(1 2 3) '(3 5)))
   (seq-difference
    :eval (seq-difference '(1 2 3) '(2 3 4)))
   (seq-intersection