GNU bug report logs - #70155
29.3; Several Emacs Lisp list functions accept non-list arguments

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From: Eli Zaretskii <eliz <at> gnu.org>
To: tpeplt <at> gmail.com, Mattias Engdegård <mattiase <at> acm.org>, Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 70155 <at> debbugs.gnu.org
Subject: bug#70155: 29.3; Several Emacs Lisp list functions accept non-list arguments
Date: Sat, 27 Apr 2024 11:22:06 +0300

> From: <tpeplt <at> gmail.com>
> 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)
> 
> --

Mattias, Stefan: any comments on this?  Should we document this, or
should we change the code?

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