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

[Ken A. <kan...@bb...>]

This is a good point.  This is why i wanted language lawyers/developers to weigh in.

JScheme has taken the policy of "bending the rules toward Java".  My question is, really, is this change worth it from the Java/Scheme perspective and our current code base.

k

At 05:59 PM 11/3/2004 -0500, 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
>
>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)))