Add new predicates for sequence lengths

* doc/lispref/sequences.texi (Sequence Functions): Document them.
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Mark them as
side-effect-free.

* lisp/emacs-lisp/shortdoc.el (list): Mention them.

* src/fns.c (Flength): Mention them in the doc string.
(length_internal): New function.
(Flength_less, Flength_greater, Flength_equal): New defuns.
(syms_of_fns): Sym them.

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/lispref/sequences.texi b/doc/lispref/sequences.texi
index 952834bd4e3..57b49847e7f 100644
--- a/doc/lispref/sequences.texi
+++ b/doc/lispref/sequences.texi
@@ -116,6 +116,21 @@ If you need to compute the width of a string on display, you should use
 since @code{length} only counts the number of characters, but does not
 account for the display width of each character.
 
+@defun length< sequence length
+Return non-@code{nil} if @var{sequence} is shorter than @var{length}.
+This may be more efficient than computing the length of @var{sequence}
+if @var{sequence} is a long list.
+@end defun
+
+@defun length> sequence length
+Return non-@code{nil} if @var{sequence} is longer than @var{length}.
+@end defun
+
+@defun length= sequence length
+Return non-@code{nil} if the length of @var{sequence} is equal to
+@var{length}.
+@end defun
+
 @defun elt sequence index
 @anchor{Definition of elt}
 @cindex elements of sequences