Re: Returning variable "references" under lexical binding

[Barry Margolin]

In article <bp8sj1hjsoh.fsf@usca1uw-JZWWPM1.sanmateo.corp.akamai.com>,
 Sean McAfee <eefacm@gmail.com> wrote:

> I recently tried writing my first non-toy code that employs lexical
> binding.  It's a routine that sets up a series of idle timers, storing
> each successive timer object into the same lexical variable.
> 
> I want my routine to return an object that can be used to cancel the
> most recently set timer.  If I were writing this code in the days prior
> to lexical binding, I might have dono something like this:
> 
> (defun start-my-timer ()
>   (let ((timer (gensym)))
>     ;; ... (set timer (make-timer ...)) ...
>     timer))
> 
> (defun cancel-my-timer (timer)
>   (cancel-timer (symbol-value timer)))
> 
> The documenentation for lexical variables cautions against treating them
> as symbols, specifically stating that functions like symbol-value will
> not work.  So I wrote my routine to return a closure:
> 
> (defun start-my-timer ()
>   (let (timer)
>     ;; ... (setq timer (make-timer ...)) ...
>     (lambda () timer)))
> 
> (defun cancel-my-timer (timer)
>   (cancel-timer (funcall timer)))
> 
> This works, but is it the "correct" way to do this?

What's wrong with using ordinary variable values:

(defun start-my-timer ()
  (make-timer ...))

(setq my-timer (start-my-timer))

(cancel-timer my-timer)

What do you gain by indirecting through a gensym or a closure?

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