[CM] query on references within Loop

Ralf Mattes rm@seid-online.de
Tue, 15 Aug 2006 11:14:45 +0200

On Tue, 2006-08-15 at 16:31 +1200, Adam wrote:
> This has me a little puzzled, in Common Music, 
> The intention is that Loop cycles through a list, and for each entry then 
> collects a list carried in that item. 
> However an error reports that ..  a-ref  is not of type list.  While it 
> appears to be a list, perhaps its outside of the Loop scope somehow ? 
> Can anyone suggest where I am going wrong here ?

I'm far away from my CM-repl, but i think you should be careful with
your quoting ...

> (define a-ref '('a1  (1 2 3)  'a2))
> (define b-ref '('b1  (4 5 6)  'b2))
> (define c-ref '('c1  (7 8 9)  'c2))

As a rule of thumb: you'll most likely never want to quote within a
quoted list. The #\' character is reader magic that expands to
(quote ...) and is done _before_ the evaluator "sees" a form. So the
above is fed to the evaluator as:

 (quote ((quote c1) (7 8 9) (quote c2)))

which evaluates to

 ((quote c1) (7 8 9) (quote c2))

Is this really what you want? This is a list of three lists and
most likely you wanted a list of a symbol, a list and a symbol ...

> (define ref-list '(a-ref b-ref c-ref))

Remeber: quoting stops evaluation, so ref-list is a list of three
symbols. This is why LOOP complains. 

 (define ref-list (list a-ref b-ref c-ref))

Some Lispers will still go for the quoting syntax, but then you need

 (define ref-list `(,a-ref ,b-ref ,c-ref))

HTH, Ralf Mattes

> (loop for refx in ref-list
>       collect 
>       (loop for data in (second refx)
> 	    collect data))
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