Subject: bug#70155: 29.3; Several Emacs Lisp list functions accept non-list arguments

From: <[email-masked]>
Date: Tue, 02 Apr 2024 19:15:28 -0400

The built-in Emacs Lisp functions ‘last’, ‘nthcdr’, ‘take’,
and ‘ntake’ are functions that accept lists as an argument.
However, they also accept non-list arguments without
signaling an error.  This is not documented in their
docstrings or in the Emacs Lisp reference manual.  The
behavior of the related list functions ‘butlast’ and
‘nbutlast’ is that an error is signaled when the function’s
list argument is not a list.

If it is intended that the functions ‘last’, ‘nthcdr’,
‘take’, and ‘ntake’ should accept non-list arguments without
signaling an error, then this should be documented.
Otherwise, these functions should be changed to behave
consistent with other list functions by signaling an error
when an expected list argument is not a list.

This behavior can be seen by following these steps:

1. Start Emacs at a shell prompt with option ‘-Q’: $ emacs -Q

2. Evaluate the following expressions in the *scratch*
   buffer.  Note that ‘last’ does not signal an error when a non-list
   argument is provided:

(last '(a b c))
;;=> (c)

(last 'a)
;;=> a

(last 3.14)
;;=> 3.14

(last "a string")
;;=> "a string"

3. Evaluate following expressions with the related function ‘butlast’.
   Note that the function signals an error when provided a non-list
   argument.

(butlast '(a b c))
;;=> (a b)

(butlast 'a)
;;=> *** Eval error ***  Wrong type argument: sequencep, a

(butlast 3.14)
;;=> *** Eval error ***  Wrong type argument: sequencep, 3.14

(butlast "a string")
;;=> *** Eval error ***  Wrong type argument: listp, "a string"

4. Evaluate the following expressions for ‘nthcdr’, ‘take’, and ‘ntake’.

As expected, an error is signaled when the (first) number argument is
non-zero and the list argument is a non-list.

But no error is signaled when the (first) number argument is zero and
the list argument is a non-list.

(nthcdr 0 '(a b c))
;;=> (a b c) (correct, as documented)

(nthcdr 1 'a)
;;=> *** Eval error ***  Wrong type argument: listp, a

(nthcdr 0 'a)
;;=> a (expect an error, but got the argument returned instead)

(take 0 '(a b c))
;;=> nil (correct, as documented)

(take 1 'a) => nil
;;=> *** Eval error ***  Wrong type argument: listp, a

(take 0 'a)
;;=> nil (expect an error, but got the argument returned instead)

(ntake 0 '(a b c))
;;=> nil (correct, as documented)

(ntake 1 'a) => nil
;;=> *** Eval error ***  Wrong type argument: listp, a

(ntake 0 'a)
;;=> nil (expect an error, but got the argument returned instead)

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