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

[Geoffrey K. <ge...@kn...>]

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

Geoffrey
--
Geoffrey S. Knauth | http://knauth.org/gsk

On Nov 3, 2004, at 16:32, Ken Anderson wrote:

> Here's a proposal.
>
> I think (and i know Rusty really does) think JScheme handling of #null 
> is awk weird.  What if in conditionals we treated #null as #f.  This 
> would make code more scheme like, here are 2 examples.
>
> #null is currently treated at #t in conditionals but this change may 
> not break much code.
>
>
> (define (default  value default)
>   ;; Before
>   (if (isNull value) default
>       value))
>
> (define (default value default)
>   ;; After.
>   (or value default))
>
> (define (memoize key table computation)
>   ;; Before.
>   (let ((it (.get key table)))
>     (if (not (isNull it)) it
> 	(let ((it (computation key)))
> 	  (.put table key it)
> 	  it))))
>
> (define (memoize key table computation)
>   ;; After.
>   (or (.get key table)
>       (let ((it (computation key)))
> 	  (.put table key it)
> 	  it)))