Re: Returning variable "references" under lexical binding

[Sean McAfee <eefacm@gmail.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:
>> (defun start-my-timer ()
>>   (let ((timer (gensym)))
>>     ;; ... (set timer (make-timer ...)) ...
>>     timer))
> [...]
>> (defun cancel-my-timer (timer)
>>   (cancel-timer (symbol-value timer)))

> Why not
>
>   (defun start-my-timer ()
>     (let ((timer (make-timer ...))
>       ...
>       timer))
>   (defun cancel-my-timer (timer)
>     (cancel-timer timer))

Because start-my-timer sets up callbacks that may repeatedly change the
value of the "timer" variable:

;; -*- lexical-binding: t; -*-

(defun start-my-timer ()
  (let (timer)
    (setq timer (make-timer ...))
    ;; ... (later (occasionally (setq timer (make-timer ...)))) ...
    (lambda () timer))

Like I said in my original article:

> It's a routine that sets up a series of idle timers, storing
> each successive timer object into the same lexical variable.

I should have made that more explicit.

>> The documenentation for lexical variables cautions against treating them
>> as symbols, specifically stating that functions like symbol-value will
>> not work.

> In your above code, you're not treating variables as symbols.
> You're just storing a symbol inside a variable, which is fine.

Yes, I was explaining why I assumed this wouldn't work:

;; -*- lexical-binding: t; -*-

(defun start-my-timer ()
  (let (timer)
    (setq timer (make-timer ...))
    ;; ... (later (occasionally (setq timer (make-timer ...)))) ...
    'timer)

(defun cancel-timer (timer)
  (cancel-timer (symbol-value timer)))

If I were not using lexical binding, though, this should work:

(defun start-my-timer ()
  (let ((timer (gensym)))
    (set timer (make-timer ...))
    ;; ... (later (occasionally (set timer (make-timer ...)))) ...
    timer))

;; cancel-timer as before

The question still stands if using a closure as a handle to a varying
lexical variable is the right way to go.