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From: "WJ" <w_a_x_...@yahoo.com>
Newsgroups: comp.lang.lisp
Subject: Re: Loop puzzle
Date: 11 May 2012 03:43:41 GMT
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References: <m2sji6zqn1.fsf@gmail.com> <rNOSPAMon-0339B0.09512019022012@news.albasani.net>
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RG wrote:

> In article <m2sji6zqn1....@gmail.com>,
>  Helmut Eller <eller.hel...@gmail.com> wrote:
> 
> > I tried this code:
> > 
> > (let ((a '(x y)))
> >   (loop for a in '((a b) (0 1))
> >         for b in a
> >         collect (list a b)))
> > 
> > and expected that it evaluates to '(((a b) a) ((0 1) b)) but all the
> > implementations I tried return nil.  
> > 
> > Can somebody give a pointer to the place in the spec that explains why
> > this should/could return nil?
> > 
> > Helmut
> 
> LOOP binds the variables in the FOR clauses in parallel before doing 
> anything else.  It then evaluates all the IN clauses, again in parallel.  
> At that point, A is NIL.  So B is looping over NIL.
> 
> Try something like this:
> 
> (let ((l '((a b) (0 1))))
>   (loop for a in l
>         for b in (car l)
>         collect (list a b)))
> 
> rg

Very bad advice, Ron.  You should tell him to use a Lisp
instead of CL (COBOL-Like).

Racket:

(define lst '((a b) (0 1)))
(map list lst (car lst))
 => '(((a b) a) ((0 1) b))
 
"WE DON'T NEED NO STINKIN' LOOPS!"