Re: Newbie: Quick Question regarding Disassembling a list

From: Barry Margolin (barmar_at_alum.mit.edu)
Date: 04/30/04 

Date: Fri, 30 Apr 2004 00:43:57 -0400

In article <20040430042115.GX25328@mapcar.org>,
 Matthew Danish <mdanish@andrew.cmu.edu> wrote:

> On Thu, Apr 29, 2004 at 09:10:50PM -0700, Ari Johnson wrote:
> > >AND is a macro. You cannot FUNCALL or APPLY it, that wouldn't make
> > >sense. There is a function named EVERY which does what he wants.
> > >Alternatively, (loop for l in list1 always l).
> >
> > Could you macroexpand it? (eval (macroexpand (cons 'and list1))) seems
> > to work for me.
>
> I strongly discourage this sort of solution. MACROEXPAND and especially
> EVAL are very heavy machinery to invoke, and there is no need for it.
> Essentially, you are creating an entire separate Lisp interpreter or
> even compiler process. Also, EVAL does not operate in the current
> lexical environment (since it is basically running a fresh new program).

Furthermore, unless you know for sure that list1 only contains
self-evaluating objects, the above code has a bug. It should be:

(eval (cons 'and (mapcar '(lambda (x) `',x))))

to prevent the elements of list1 from being evaluated.

The MACROEXPAND is unnecessary -- EVAL will expand macros when necessary. 

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***