GNU bug report logs - #17296
[PATCH] SRFI-1 'length+' raises an error unless passed a proper or circular list

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Message #16 received at 17296 <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: David Kastrup <dak <at> gnu.org>
Cc: 17296 <at> debbugs.gnu.org
Subject: Re: bug#17296: Uh, wrong?
Date: Tue, 03 Jun 2014 22:55:42 -0400

David Kastrup <dak <at> gnu.org> writes:
> So the behavior for length+ on a dotted list is strictly unspecified.
> It is not even stated "it is an error".

Actually, it is.  At the end of the section that defines the "types"
such a "clist" and "flist", it states:

  It is an error to pass a circular or dotted list to a procedure not
  defined to accept such an argument.

While it is true that we are not required to signal an error, I'm wary
extending 'length+' in this way.  It also effectively extends 'map' and
'for-each' to support things like (map + '() 'foo), and thus potentially
affects many other procedures both inside and outside of Guile that use
'map' and 'for-each'.  Once we've done this, users are likely to grow
dependent on it and we can never go back.

> At any rate, what I am getting at is that I was going to submit the
> following patch as a part of a series fixing other bugs, bugs that I
> need a working "get the length of a dotted list" operator for.  We don't
> have any such operator in GUILE, and that's awkward.

It might be helpful to add such a procedure, but I don't think 'length+'
should be it.

   Regards,
     Mark

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