Re: member inconsistency?

[Stephen Berman <stephen.berman@gmx.net>]

On Thu, 28 Jan 2016 11:14:37 +0100 Andreas Schwab <schwab@suse.de> wrote:

> Philipp Stephani <p.stephani2@gmail.com> writes:
>
>> Nicolas Goaziou <mail@nicolasgoaziou.fr> schrieb am Do., 28. Jan. 2016 um
>> 09:41 Uhr:
>>
>>> Hello,
>>>
>>> The following behaviour is surprising
>>>
>>>   (assq 'a 'b)   => #ERROR
>>>   (memq 'a 'b)   => #ERROR
>>>   (assoc 'a 'b)  => #ERROR
>>>
>>
>> For completeness, memql and assq have the same behavior.
>
> memql works like member if looking for a float, like memq otherwise.

The definition of `memq' calls CHECK_LIST (list) and `assq' and `assoc'
return CAR (list), and each of these signals the error in case of a
wrong type.  In contrast, `member' (and `memql' in the float case) has
no check and also uses a for-loop with the condition CONSP (tail), and
when this fails, the function just returns Qnil.  Adding CHECK_LIST
(list) before the for-loop makes (member 'a 'b) signal an error.
(Instead of a for-loop, the other three use while (1), so the loop
always runs and the type checks get executed.)

Steve Berman