Timer variable binding

[Johan Andersson <johan.rejeep@gmail.com>]

Hi,

I have some questions regarding timers.

This code will start a timer and the first time the callback runs, the
timer is canceled. Works great!

(let ((timer (run-at-time 0 1 (lambda ()
                                (cancel-timer timer))))))

In the above example I can access the timer variable inside the function
callback. But in this code, I cannot access the variable my-var. Nothing is
printed. In Emacs 24.3.1 I see no error, but in 24.3.50.1 I get
(void-variable my-var). Why is there no error in 24.3.1?

(let* (timer (my-var 10))
  (setq timer (run-at-time 0 1 (lambda ()
                                 (print my-var)
                                 (cancel-timer timer)))))

I can solve this by passing the my-var variable as argument to run-at-time,
but then that same value will be passed to the function callback each time.
Let's say for example that I want to run a timer 10 times, I can't do this
using the code below, because my-var will always have value 10:

(let* (timer (my-var 10))
  (setq timer (run-at-time 0 1 (lambda (my-var)
                                 (setq my-var (1- my-var))
                                 (when (= my-var 0)
                                   (cancel-timer timer)))
                           my-var)))

Something else worth noting is that in the callback function, it is
possible to access globally defined variables.

Can someone please explain these weird behaviors?