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bug#70155: 29.3; Several Emacs Lisp list functions accept non-list argum
From: |
Eli Zaretskii |
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@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?