[taos-glug] Re: nil vs '() using Guile and PLT Scheme
- From: Jonathan Bartlett <johnnyb@xxxxxxxxxx>
- To: taos-glug@xxxxxxxxxxxxx
- Date: Thu, 14 Aug 2003 13:38:42 -0700 (PDT)
First of all, on PLT Scheme, be sure what version you are in. I think you
want advanced mode. (MrEd doesn't matter either way). Second of all, I
think nil may have gone away in the Scheme standard (R5RS uses it in an
example, but doesn't define it, I believe). With standard scheme, use
'().
Of course, you can always (define nil '())
Also, on some schemes, false is '(), and on others #f is it's own entity.
I liked #f being '(), but I guess it makes sense for it not to be. I
think R5RS is when #f got it's own entity, and '() was no longer false.
Jon
On Thu, 14 Aug 2003, Philip Ansteth wrote:
>
> I'm not sure why, but Guile wants you to use '() instead
> of nil. For example, in the first version of scale-list
> in the section headed "Mapping of lists", try '() instead of
> nil.
>
> Under PLT Scheme, nil doesn't work at all. But '() does work.
>
>
> For example, using Guile,
>
> (define (scale-list items factor)
> (if (null? items)
> nil
> (cons (* (car items) factor) (scale-list (cdr items) factor))))
>
> (scale-list (list 1 2 3 4 5) 10)
>
> yields
> (10 20 30 40 50 . nil)
>
> But, changing nil to '(), the following.
>
> (define (scale-list items factor)
> (if (null? items)
> '()
> (cons (* (car items) factor) (scale-list (cdr items) factor))))
>
> (scale-list (list 1 2 3 4 5) 10)
>
> yields
> (10 20 30 40 50)
>
> Using PLT Scheme (= mzScheme?) on my Windows laptop, nil doesn't work at all,
> giving the message: "reference to undefined identifier: nil"
>
>
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