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Re: member inconsistency?

From: Stephen Berman
Subject: Re: member inconsistency?
Date: Thu, 28 Jan 2016 22:57:49 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux)

On Thu, 28 Jan 2016 20:35:40 +0200 Eli Zaretskii <address@hidden> wrote:

>> From: Stephen Berman <address@hidden>
>> Cc: address@hidden,  address@hidden,  address@hidden,  address@hidden
>> Date: Thu, 28 Jan 2016 18:30:04 +0100
>> diff --git a/src/fns.c b/src/fns.c
>> index 86ad333..17c4a75 100644
>> --- a/src/fns.c
>> +++ b/src/fns.c
>> @@ -1349,7 +1349,7 @@ The value is actually the tail of LIST whose car is 
>> ELT.  */)
>>    (register Lisp_Object elt, Lisp_Object list)
>>  {
>>    register Lisp_Object tail;
>> -  for (tail = list; CONSP (tail); tail = XCDR (tail))
>> +  for (tail = list; CONSP (tail) || !NILP (tail); tail = XCDR (tail))
>>      {
>>        register Lisp_Object tem;
>>        CHECK_LIST_CONS (tail, list);
>> Or is having the check before the loop better it catches the error sooner?
> Before the loop sounds slightly less obscure to me.

With just
   CHECK_LIST_CONS (list, list);
before the loop, the build fails with "wrong-type-argument: nil, listp"
in subr.el.  With 
   if (!NILP (list))
     CHECK_LIST_CONS (list, list);
before the loop, the build succeeds (same with Fmemql).  But isn't this
then effectively the same thing as having CHECK_LIST (list) before the
loop?  With that, the build also succeeds and I didn't see any problems
with calls to member.  I'm confused.

Two other things I don't get: (1) I wondered why Fmember can't be
defined with a while (1) loop analogously to Fmemq, Fassq und Fassoc,
but when I tried that the build failed at
   temacs --batch --load loadup bootstrap
even though I added a !NILP (list) check.
(2) Why are there three occurrences in a row of these lines in Fmemq
(and similar repetitions in Fassq and Fassoc)?
   if (!CONSP (list) || EQ (XCAR (list), elt))
   list = XCDR (list);
I commented out two of the occurrences and built emacs successfully and
all my tests of memq went as expected.  So why the repetitions?

Steve Berman

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