Re: [Jscheme-user] treating #null as #f in conditionals?

[Timothy J. H. <ti...@cs...>]

On Nov 4, 2004, at 2:50 PM, Michael Thome wrote:

> I'm not sure I agree:  what is wrong with defining #f and #null as 
> semantically equivalent?
I would side with Rusty and Geoffrey.

One problem with identifying #f and #null is that they have different 
semantics in Java
and hence would create problems when calling Java methods/constructors 
etc.
e.g. to create a Boolean array of size 4 and store two Boolean values 
in it, we could use

   >   (define f (list->array Boolean.class  '( #f #t #null #null)))

But if #f and #null were identified, we would not be able to represent 
#null



>   Even if #null printed as #f, it might be better than the current 
> situation.
Again, it would create problems when working with the Java interface. 
You would like to know
when a Boolean function is returning #null or #f.

>  Bottom line is that #null is a projection of a non-scheme thing into 
> scheme space - it is up to jscheme to manage that projection in 
> whatever way makes the most sense.
True, but we want the Scheme space to embed into Java space as well.


>   What if we got rid of #null altogether?... or make #null a variable 
> bound to #f.
>
> cheers,
>    -mik
>
> P.S.  what bugs me is having to do (equal? foo #null):
> > (or (null? #null) (not #null))
> #f
I agree with Rusty that macros would provide a mechanism for getting 
this kind of behavior.

>
> Geoffrey Knauth wrote:
>
>> As a practical matter, it certainly seems convenient to be able to 
>> treat #null the same as #f in conditionals--I agree it is 
>> counterintuitive to have #null be not-false.
>>
>> But I'm concerned about breaking R5RS 6.3.1., "only #f counts as 
>> false in conditional expressions,"
>>
>> http://www.swiss.ai.mit.edu/~jaffer/r5rs_8.html#SEC58
>>
>> note:  (not '())  ==>  #f
>>
>> and 3.2, "no object satisfies more than one of the following 
>> predicates: boolean? pair? symbol? number? char? string? vector? 
>> port? procedure?"
>>
>> http://www.swiss.ai.mit.edu/~jaffer/r5rs_5.html#SEC20
>>
>> So we'd have to caveat the language standard, and surprise newcomers 
>> to JScheme from other Schemes.  On the principle of least surprise, 
>> we'd probably be 50-50 between surprising newcomes to JScheme from 
>> other languages vs. other Schemes.
>>
>> In Java you can't do:  Object o = NULL; if (o) { true-stuff} else { 
>> false-stuff }
>> you have to do:  if (o == NULL) ...
>> It's a pain in the butt, but it's been drilled into us.
>>
>> Reluctantly, I suggest leaving #null and the semantics of `if' alone, 
>> and offering a "new" `if', e.g., `jif', that would treat #null as #f.
>>
>> By the way, I tried to figure out what #null is in JScheme, and it 
>> isn't anything:
>>
>> > (symbol? #null)
>> #f
>> > (boolean? #null)
>> #f
>> > (pair? #null)
>> #f
>> > (vector? #null)
>> #f
>> > (procedure? #null)
>> #f
>> > (string? #null)
>> #f
>> > (port? #null)
>> (port? '#null)
>>   ====================================
>> SchemeException: ERROR: undefined variable "port?"
>> > (number? #null)
>> #f
>> > (char? #null)
>> #f
>>
>> Then I remembered that the empty list '() isn't anything either, but 
>> it also isn't #null:
>>
>> > (eq? '() #null)
>> #f
>> > (eqv? '() #null)
>> #f
>> > (equal? '() #null)
>> #f
>
> and, of course,
>
> > (null? #null)
> #f
>
> As an alternative, what about defining #f and #null as the same object?
>
>
>
>
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