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

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

On Nov 4, 2004, at 5:44 PM, Ken Anderson wrote:

> Thanks for your description of #null.  Its interesting that Scheme, 
> and functional languages can get along without it.  Why is that?  I 
> think Common Lisp had a #null called NIL.
>
> My plan was that #null and #f would be distinct objects, but that 
> #null would be treated as #f.  in conditionals.
That seems more reasonable as it just extends the semantics of some 
primitives and special forms but doesn't change
the data model of the language.
>
> The problem with Rusty and Geof's macros is you need to provide ifj, 
> andj, orj, notj, and condj.  Here's a piece of code that's written in 
> this style:
>
>        (let ((effective (.getNotamEffectiveDtg n))
>              (expire (.getNotamExpireDtg n)))
>          (if (andj effective expire)
>              (prefix " WEF " (string-append (.substring effective 2) 
> "-"
>                                             (.substring expire 2)))
>              (ifj expire (prefix " TIL " (.substring expire 2))
>                   (ifj effective (prefix " WEF " (.substring effective 
> 2)))))))
>
> which i don't like.
I agree, combining if and ifj, or and orj, etc gets too messy and 
creates new sources of error.
.
The alternative of changing the standard definition of if, cond, or, 
and, etc to treate #null as #f
seems reasonable to me. If someone didn't like it they could always use 
a module that
redefines them to their original meanings...

---Tim---

>
> treating #null like #f in conditions lets  java collections return 
> #null but let you write code like (if (member x y) ... or (if (.get 
> table key) ...
>
> k
> At 04:16 PM 11/4/2004 -0500, Alan Donovan wrote:
>> I agree; I think it's much cleaner to keep #null distinct from #f.
>
>
>
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