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, 16 insertions, 0 deletions

diff --git a/doc/lispref/lists.texi b/doc/lispref/lists.texi
index 761750eb20c..57cefeac962 100644
--- a/doc/lispref/lists.texi
+++ b/doc/lispref/lists.texi
@@ -153,6 +153,22 @@ considered a list and @code{not} when it is considered a truth value
 @end example
 @end defun
 
+@defun proper-list-p object
+This function returns the length of @var{object} if it is a proper
+list, @code{nil} otherwise (@pxref{Cons Cells}).  In addition to
+satisfying @code{listp}, a proper list is neither circular nor dotted.
+
+@example
+@group
+(proper-list-p '(a b c))
+    @result{} 3
+@end group
+@group
+(proper-list-p '(a b . c))
+    @result{} nil
+@end group
+@end example
+@end defun
 
 @node List Elements
 @section Accessing Elements of Lists