Re: proposed changes to handling of false and end-of-list

[Matthieu Vachon <matthieu dot o dot vachon at gmail dot com>]

On Mon, Jul 1, 2013 at 2:20 AM, Per Bothner <per@bothner.com> wrote:
> On 06/30/2013 08:09 PM, Matthieu Vachon wrote:
>>
>> On Sat, Jun 29, 2013 at 2:47 AM, Per Bothner <per@bothner.com> wrote:
>>>
>>> A linked-list may be
>>> terminated by either '() (i.e. LList.Empty) or Java null.  Lists
>>> constructed using the Scheme reader or the 'list' function would
>>> by terminated by LList.Empty as now.
>>
>>
>> I'm less sure about this change, how would this work. If I understand
>> correctly, writing `(1 2 3)` or `(list 1 2 3)` would have a '() as the
>> last
>> element.
>
>
> Not the "last element" - the "tail".  The last element is 3.  The
> last (i.e. 3rd) cons cell (pair) has 3 as the value of its car and '()
> (i.e.  LList.Empty) as the value of its cdr.
>
>
>> For the null part, I would I create such list. Something like
>> this `(list 1 2 3 #!null)` would not end with a '() but rather a #!null ?
>
>
> You're confusing car and cdr.  `(1 2 3 #!null)` is a 4-element list,
> whose last (i.e. 4th) cons cell (pair) has #!null as the value of its
> car and '() (i.e.  LList.Empty) as the value of its cdr.
>
> However, `(1 2 3 . #!null)` (note the dot) is a 3-element list
> last (i.e. 3rd) cons cell (pair) has 3 as the value of its car and #!null
> as the value of its cdr.
>
> `(1 2 3 . ())` is in all respects equivalent to `(1 2 3)`.  However,
> `(1 2 3 . ())` and `(1 2 3 . #!null)` are the same *when viewed as
> a list*, however they would not be equal?  They're not equal? because
> as explained in the "Equality" section here:
> http://www.gnu.org/software/guile/manual/html_node/Nil.html
>
> However, most functions that work on lists, such as length and map,
> would treat `(1 2 3 . ())` and `(1 2 3 . #!null)` the same.
>

You are right, I confused two different things. Now I understand more
the feature.
Since a list would still ends with an empty list, I don't see any
problem with this
feature anymore.

>
>> This seems reasonable. But I'm unsure of all the implications
>> for this change on my existing code base. Do you think this change
>> would be fully backward compatible?
>
>
> No, it will not.  Most obviously:
>   (if #!null 't 'f)
> would return 'f where it currently returns 't.
> Even if you don't use #!null explicitly, it will show up
> if you call a Java method that returns null.
>

Yes, that one was obvious, I was thinking more of the other change,
the one with the
list. But now that I understand more, I don't think there will be any
BC with this new
feature.

> --
>         --Per Bothner
> per@bothner.com   http://per.bothner.com/