Add predicate proper-list-p

For discussion, see emacs-devel thread starting at
https://lists.gnu.org/archive/html/emacs-devel/2018-04/msg00460.html.

* lisp/subr.el (proper-list-p): New function.
Implementation suggested by Paul Eggert <eggert@cs.ucla.edu> in
https://lists.gnu.org/archive/html/emacs-devel/2018-06/msg00138.html.
* doc/lispref/lists.texi (List Elements):
* etc/NEWS: Document proper-list-p.
* lisp/org/ob-core.el (org-babel-insert-result):
* lisp/emacs-lisp/byte-opt.el (byte-optimize-if):
* lisp/emacs-lisp/cl-macs.el (cl--make-usage-args): Use proper-list-p.
* lisp/emacs-lisp/ert.el (ert--proper-list-p): Remove.
Replaced by proper-list-p in lisp/subr.el.
(ert--explain-equal-rec): Use proper-list-length.
* lisp/format.el (format-proper-list-p): Remove.
Replaced by proper-list-p in lisp/subr.el.
(format-annotate-single-property-change): Use proper-list-p.
* test/lisp/emacs-lisp/ert-tests.el (ert-test-proper-list-p):
Move from here...
* test/lisp/subr-tests.el (subr-tests--proper-list-length):
...to here, mutatis mutandis.

1 files changed, 6 insertions, 0 deletions

diff --git a/lisp/subr.el b/lisp/subr.el
index ca184d8fc81..c1d90e3fb18 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -555,6 +555,12 @@ If N is omitted or nil, remove the last element."
   (declare (compiler-macro (lambda (_) `(= 0 ,number))))
   (= 0 number))
 
+(defun proper-list-p (object)
+  "Return OBJECT's length if it is a proper list, nil otherwise.
+A proper list is neither circular nor dotted (i.e., its last cdr
+is nil)."
+  (and (listp object) (ignore-errors (length object))))
+
 (defun delete-dups (list)
   "Destructively remove `equal' duplicates from LIST.
 Store the result in LIST and return it.  LIST must be a proper list.