bug#21782: 25.0.50; New functions nfront/front

[Tino Calancha <f92capac@gmail.com>]

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> I think this is already available in core as seq-take, from the seq package.

Thank you, you are right. Indeed, I found seq-take very nice:
its more general because apply to sequences.
There is one small difference: seq-take has no optional arguments.

In addition, seq.el need to be loaded by the user. Yeah, not a big
deal, but IMO such fundamental operation on a list as nfront 
implement, deserve to be available at the starting of the session.
Putting a dedicated function in subr.el we save to load many stuff from
seq.el that maybe we dont need.

I would like nfront/front would behave as nbutlast/butlast concerning
the N argument:
*) N nil giving same output as N = 1.
*) N <= 0 return the full list.
(I choose the name of the function to somehow reflect such symmetry).

I have modified my patch to accomplish such behaviour.

With the new patch

(let ((ltest '(1 2 3 4 5 6)))
   (nbutlast ltest nil))
(1 2 3 4 5)
(let ((ltest '(1 2 3 4 5 6)))
   (nfront ltest nil))
(1)
(require 'seq)
(let ((ltest '(1 2 3 4 5 6)))
   (seq-take ltest nil)); error: second argument is not optional

(let ((ltest '(1 2 3 4 5 6)))
   (nbutlast ltest 0))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
   (nfront ltest 0))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
   (seq-take ltest 0))
nil

(let ((ltest '(1 2 3 4 5 6)))
   (nbutlast ltest -1))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
   (nfront ltest -1))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
   (seq-take ltest -1))
nil

(let ((ltest '(1 2 3 4 5 6)))
   (nbutlast ltest 20))
nil
(let ((ltest '(1 2 3 4 5 6)))
   (nfront ltest 20))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
   (seq-take ltest 20))
(1 2 3 4 5 6)

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diff --git a/lisp/subr.el b/lisp/subr.el
index ea926ae..ef705eb 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -391,13 +391,15 @@ last
 (defun butlast (list &optional n)
   "Return a copy of LIST with the last N elements removed.
 If N is omitted or nil, the last element is removed from the
-copy."
+copy.
+When N is <= 0 LIST is returned."
   (if (and n (<= n 0)) list
     (nbutlast (copy-sequence list) n)))
 
 (defun nbutlast (list &optional n)
   "Modifies LIST to remove the last N elements.
-If N is omitted or nil, remove the last element."
+If N is omitted or nil, remove the last element.
+When N is <= 0 LIST is returned unmodified."
   (let ((m (length list)))
     (or n (setq n 1))
     (and (< n m)
@@ -405,6 +407,21 @@ nbutlast
 	   (if (> n 0) (setcdr (nthcdr (- (1- m) n) list) nil))
 	   list))))
 
+(defun front(list &optional n)
+  "Return a copy of LIST with just the first N elements.
+If is omitted or nil copy just the first element
+When N is <= 0 the full list is copied."
+  (if (and n (<= n 0)) list
+    (nfront (copy-sequence list) n)))
+
+(defun nfront(list &optional n)
+  "Modified LIST to remove all elements but the first N.
+If N is omitted or nil remove all but the first element
+When N is <= 0 LIST is returned unmodified."
+  (or n (setq n 1))
+  (if (<= n 0) list
+    (nreverse (last (nreverse list) n))))
+
 (defun zerop (number)
   "Return t if NUMBER is zero."
   ;; Used to be in C, but it's pointless since (= 0 n) is faster anyway because