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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
  0 siblings, 1 reply; 2+ 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] 2+ messages in thread

* bug#70155: 29.3; Several Emacs Lisp list functions accept non-list arguments
  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
  0 siblings, 0 replies; 2+ messages in thread
From: Drew Adams via Bug reports for GNU Emacs, the Swiss army knife of text editors @ 2024-04-03  2:55 UTC (permalink / raw)
  To: tpeplt@gmail.com, 70155@debbugs.gnu.org

Thanks for filing this bug report.

> 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.

As I mentioned in gnu-emacs-help@gnu.org, this is the case now, but it wasn't the case until Emacs 24.5.  That's when this regression, or design change, occurred (which was it?).

For function `last' this occurred by simply changing `length' to `safe-length'.  If the intention was only to avoid problems with things such as cyclic lists, then this should be fixed by first ensuring that the argument is a cons or nil.

But if the intention was to also have the effect of handling nonlists such as 3.14 then at least this (odd, non-legacy) behavior should be documented.

`last' is one of the oldest Lisp functions...

^ permalink raw reply	[flat|nested] 2+ messages in thread

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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

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