Subject: Re: bug#27177: 26.0.50: Macroexpanding cl-loop and friends (make-symbol usage)

From: Michael Heerdegen <[email-masked]>
Date: Thu, 01 Jun 2017 01:51:42 +0200

Alex <[email-masked]> writes:

> Consider the following simple cl-loop form:
>
> (cl-loop for x in '(1 2 3)
>          for y in '(a b c)
>          collect (list x y))
>
>
> The macroexpanded result is:
>
> (cl-block nil
>   (let*
>       ((--cl-var--
>         '(1 2 3))
>        (x nil)
>        (--cl-var--
>         '(a b c))
>        (y nil)
>        (--cl-var-- nil))
>     (while
>         (and
>          (consp --cl-var--)
>          (progn
>            (setq x
>                  (car --cl-var--))
>            (consp --cl-var--)))
>       (setq y
>             (car --cl-var--))
>       (push
>        (list x y)
>        --cl-var--)
>       (setq --cl-var--
>             (cdr --cl-var--))
>       (setq --cl-var--
>             (cdr --cl-var--)))
>     (nreverse --cl-var--)))
>
>
> It's easy to verify that this expansion doesn't do the same job by
> noticing that the macroexpanded form always returns nil.
>
> Note that in Common Lisp (at least in SBCL), macroexpanding and then
> evaluating the result works as expected.

Also in Elisp:

(eval (macroexpand '(cl-loop for x in '(1 2 3)
                        for y in '(a b c)
                        collect (list x y))))
==>
((1 a)
 (2 b)
 (3 c))

> This is because cl-macs.el uses make-symbol instead of gensym, like SBCL
> does.

Note that `make-symbol' doesn't return an interned symbol - what is
printed as "--cl-var--" above are actually different symbols.  You need
to enable `print-gensym' to make that visible when printing the
macroexpansion.  If you print with print-gensym bound to nil, you don't
get a correct printed representation.

So I think there is not a bug, unless your complaint is about human
readability or the default value of `print-gensym'.

Michael.