Re: "defmacro" and "local variables" and "let" and "nconc" strange behaviour...

[jack-mac]

Le dimanche 13 janvier 2013 15:18:46 UTC+1, Oleksandr Gavenko a écrit :
> On 2013-01-13, Oleksandr Gavenko wrote:
> 
> 
> 
> > I wrote simple macro:
> 
> >
> 
> >   (defmacro my-filter (pred list)
> 
> >     "Construct list with elements from LIST which satisfy PRED."
> 
> >     (let ( (r (make-symbol "r_")) )
> 
> >       `(let ( (,r '(nil)) )
> 
> >          (mapc (lambda (item)
> 
> >                  (when (,pred item)
> 
> >                    (nconc ,r (cons item nil))))
> 
> >                ,list)
> 
> >          (cdr ,r))))
> 
> >
> 
> > When I evaluate several times:
> 
> >
> 
> >   (my-filter (lambda (x) x) '(1 nil "a"))
> 
> >
> 
> > I get sequentially:
> 
> >
> 
> >   (1 "a")
> 
> >   (1 "a" 1 "a")
> 
> >   (1 "a" 1 "a" 1 "a")
> 
> >   (1 "a" 1 "a" 1 "a" 1 "a")
> 
> >
> 
> > When I eval:
> 
> >
> 
> >   (pp (macroexpand '(my-filter (lambda (x) x) '(1 nil "a"))))
> 
> >
> 
> > I get:
> 
> >
> 
> >   (let
> 
> >       ((r_
> 
> >         '(nil 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a" 1 "a")))
> 
> >     (mapc
> 
> >      (lambda
> 
> >        (item)
> 
> >        (when
> 
> >            ((lambda
> 
> >               (x)
> 
> >               x)
> 
> >             item)
> 
> >          (nconc r_
> 
> >                 (cons item nil))))
> 
> >      '(1 nil "a"))
> 
> >     (cdr r_))
> 
> >
> 
> > Why instead of '(nil) I get something else?
> 
> 
> 
> I found fix by changing '(nil) to (list nil). Please explain why first variant
> 
> fail?
> 
> 
> 
> -- 
> 
> Best regards!

To understand it, just try:
(defun foo (x)
  (let ((l '(0)))
    (nconc l (list x))))
(foo 1)
(foo 2)
This looks like a closure!