[racket] TLS "atom?" definition for Scheme does not work in DrRacket

From: Rufus (rlaggren at mail.com)
Date: Tue Mar 3 15:50:02 EST 2015


Well, in fact, I thought it worked for me, too. And then it stopped
working; DrR threw an  "application: not a procedure;" error saying it
was given a null list instead of an application. Removing the parens got
it working again.

Now that just doesn't make any sense at all but that's what I seem to
have observed. Looking at some StackOverflow responses on similar errors
I found that parens in certain positions are interpreted as invoking a
function regardless of what may in fact be inside the parens. IOW, extra
parens will trigger that error. I believe that initially it was working.
Perhaps I had run it in the executable pane instead of loading and
running the defs file? If I have more time somewhere along, I'll try to
recreate things.

When I first defined atom? I was using a very small definitions file
with the " #lang racket" spec; IIRC atom? worked. After trying BSL and
then going back to using the #lang spec and adding some comments and the
list of variable defs - it didn't work.

At the moment I don't have any good ideas, except to chg the code to get
it to work and proceed.


On 03/03/2015 02:21 PM, Alexander D. Knauth wrote:
> This works for me:
> (define atom? (lambda (arg1)
>                 (and (not (pair? arg1))
>                      (not (null? arg1)))))
> On Mar 3, 2015, at 3:16 PM, Rufus <rlaggren at mail.com> wrote:
>> On page 10 of TLS there is a note defining the "atom?" function for the
>> reader to use w/Lisp or Scheme. The definition for Scheme (which is the
>> most likely one to use for DrR) does not work. To fix one must remove
>> the parens that open at the "and". Apparently the DrR
>> compiler/interpreter sees them as an attempt to invoke a function and
>> that's not correct here. I don't know if other Lisp versions would find
>> the code in the book correct.
>> In the text below XXX shows where I removed a set of parens to make the
>> function work.
>> (define atom? (lambda (arg1)
>>               XXX and (not (pair? arg1))
>>                       (not (null? arg1))
>>               XXX
>>              )
>> )
>> Rufus
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