GNU bug report logs - #33036
Bug with the procedure nil? inside a specific code

Previous Next

Full log

View this message in rfc822 format

From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Mark H Weaver <mhw <at> netris.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#33036: closed (Bug with the procedure nil? inside a specific
 code)
Date: Sat, 20 Oct 2018 01:14:01 +0000

[Message part 1 (text/plain, inline)]

Your message dated Fri, 19 Oct 2018 21:13:14 -0400
with message-id <87zhv94dzp.fsf <at> netris.org>
and subject line Re: bug#33036: Bug with the procedure nil? inside a specific code
has caused the debbugs.gnu.org bug report #33036,
regarding Bug with the procedure nil? inside a specific code
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
33036: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=33036
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: calcium <calcium <at> disroot.org>
To: bug-guile <at> gnu.org
Subject: Bug with the procedure nil? inside a specific code
Date: Sat, 13 Oct 2018 18:17:06 +0200

Order of content:
1] --- Setup information
2] --- what is the procedure nil?
3] --- snippet of code that trigger the bug
4] --- snippet of code that doesn't trigger the bug
5] --- Summary and end

1] ---

Setup :

$ ./config.guess
x86_64-pc-linux-gnu

$ guile --version
guile: warning: failed to install locale
guile (GNU Guile) 2.2.3
etc ...

-> Guile was installed in a foreign Gnu/Linux distribution via Guix.

2] ---

The procedure nil? is not listed in the procedure index :
https://www.gnu.org/software/guile/docs/docs-2.0/guile-ref/Procedure-Index.html

There is a page that speak about nil :
https://www.gnu.org/software/guile/manual/html_node/Nil.html
However it doesn’t mention the nil? Procedure

The only documentation that I found about nil? is this one :

(procedure-documentation nil?)
=> "- Scheme Procedure: nil? x\n     Return `#t' iff X is nil, else
return `#f'."

(procedure-documentation null?)
=> "- Scheme Procedure: null? x\n     Return `#t' iff X is the empty
list, else `#f'."

The procedure documentation is not the same for nil? and null?
They seem nonetheless to be the same.

(null? #nil)
=> #t
(null? ‘())
=> #t

(nil? #nil)
=> #t
(nil? '())
=> #t

3] ---

Here is the bug that I found :

;;; -START- code with the bug -START- ;;;

(define (strange lst)
  (let loop ((lst lst)
             (is-empty '()))
    (cond ((nil? lst)
           (if (nil? is-empty) 'works
             (list 'should-not-occur is-empty)))
          (else
           (loop (cdr lst)
                 is-empty)))))

(strange '())
=> (should-not-occur ())

(strange #nil)
=> (should-not-occur ())

;;; -END- code with the bug -END- ;;;

4] ---

And here follows 3 version that doesn’t trigger this bug.

;;; in this one we remove the call to the loop in the else clause
;;; -START- code that works (1) -START- ;;;

(define (not-strange-1 lst)
  (let loop ((lst lst)
             (is-empty '()))
    (cond ((nil? lst)
           (if (nil? is-empty) 'works
             (list 'should-not-occur is-empty)))
          (else
           'no-more-bug))))

(not-strange-1 '())
=> works
(not-strange-1 #nil)
=> works

;;; -END- code that works (1) -END- ;;;

;;;  we change the ‘() of is-empty to #nil
;;; -START- code that works (2) -START- ;;;

(define (not-strange-2 lst)
  (let loop ((lst lst)
             (is-empty #nil))
    (cond ((nil? lst)
           (if (nil? is-empty) 'works
             (list 'should-not-occur is-empty)))
          (else
           (loop (cdr lst)
                 is-empty)))))

(not-strange-2 '())
=> works
(not-strange-2 #nil)
=> works

;;; -END- code that works (2) -END- ;;;

;;;  if we use null? instead of nil? at the critical point (nil? is-empty)
;;; -START- code that works (3) -START- ;;;

(define (not-strange-3 lst)
  (let loop ((lst lst)
             (is-empty '()))
    (cond ((nil? lst)
           (if (null? is-empty) 'works
             (list 'should-not-occur is-empty)))
          (else
           (loop (cdr lst)
                 is-empty)))))

(not-strange-3 '())
=> works
(not-strange-3 #nil)
=> works

;;; -END- code that works (3) -END- ;;;

5] ---

So in summary :

1) removing the call to the loop in the else clause remove the bug.
This is interesting because we don’t enter the else clause

2) using the #nil instead of ‘() did change the outcome.
But except for this case, it doesn't seem to change the outcome to use
#nil or ‘()

3) using null? Instead of nil? did change the outcome.
This is similar to point 2. except that we change the procedure instead
of the value.


I think that there is 2 way to look at this bug:

The first one is to consider that it is a bug because nil? and null?
should give the same answer.

The second one is to consider that nil? should only answer #t when we
use it with #nil, and otherwise give #f (this include ‘() ).


 ---

If you require any further information or did not understand what I
said, feel free to contact me at calcium <at> disroot.org

[Message part 3 (message/rfc822, inline)]

From: Mark H Weaver <mhw <at> netris.org>
To: Andy Wingo <wingo <at> pobox.com>
Cc: calcium <calcium <at> disroot.org>, 33036-done <at> debbugs.gnu.org
Subject: Re: bug#33036: Bug with the procedure nil? inside a specific code
Date: Fri, 19 Oct 2018 21:13:14 -0400

Hi Andy,

Andy Wingo <wingo <at> pobox.com> writes:
> Thank you, Mark!  That looks great.

Great, thanks for looking it over.  I pushed it to the stable-2.2 branch
as commit c3e14b74e81d0fd3266b97e6bd629cd4e2f98803.

I'm closing this bug now, but feel free to reopen if appropriate.

     Thanks!
       Mark

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.