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* bug#70155: 29.3; Several Emacs Lisp list functions accept non-list arguments
@ 2024-04-02 23:15 tpeplt
  2024-04-03  2:55 ` Drew Adams via Bug reports for GNU Emacs, the Swiss army knife of text editors
  2024-04-27  8:22 ` Eli Zaretskii
  0 siblings, 2 replies; 6+ messages in thread
From: tpeplt @ 2024-04-02 23:15 UTC (permalink / raw)
  To: 70155


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)

--





^ permalink raw reply	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2024-04-27 15:01 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2024-04-02 23:15 bug#70155: 29.3; Several Emacs Lisp list functions accept non-list arguments tpeplt
2024-04-03  2:55 ` Drew Adams via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-04-27  8:22 ` Eli Zaretskii
2024-04-27  9:48   ` Mattias Engdegård
2024-04-27 15:01     ` Drew Adams via Bug reports for GNU Emacs, the Swiss army knife of text editors
2024-04-27 14:38   ` Drew Adams via Bug reports for GNU Emacs, the Swiss army knife of text editors

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