Re: Replacement for Common Lisp's GENSYM in Emacs Lisp

[Helmut Eller]

* Stefan Monnier [2010-03-07 20:06+0100] writes:

>> This is actually a non-trivial problem.  Best approach would probably be
>> to do something similar to how they generate universally unique IDs.
>
> The best approach often is to not use "unique symbols" at all.  A common
> technique is to rely on lexical scoping to precisely control which
> variables are visible to each part of a macro's argument during its
> evaluation.  E.g.:
>
>   (defmacro dotimes (xl e)
>     (let ((x (car xl))
>           (l (cadr xl)))
>       `(let ((body (lambda (,x) ,e))
>              (list ,l))
>          (while (consp list)
>            (funcall body (car list))
>            (setq list (cdr list))))))
>
> Notice how the expressions `l' and `e' are evaluated in a context where
> neither of `body', nor `list' exist.  No need for gensym.

What makes you think that body and list aren't bound in e?

Helmut