Re: using compensations stacks with Guile

[Marco Maggi]

"Ludovic Courts" wrote:
> I haven't read the paper you mention there but that
> looks very similar to what `dynamic-wind' does.

There are differences between using 'with-compensations'
and using 'dynamic-wind' with alloc forms in the in-guard
and the free forms in the out-guard:

1. 'dynamic-wind' separates the alloc code from the free
   code, while 'compensate' allows them to be one near
   the other;

2. when an error occurs inside the in-guard or inside
   the out-guard the free functions are not invoked;
   with:

   (dynamic-wind
       (lambda ()
         (display 'alloc-a)(newline)
         (display 'alloc-b)(newline)
         (display 'alloc-c)(newline))
       (lambda ()
         (format #t "ciao~%"))
       (lambda ()
         (display 'free-c)(newline)
         (display 'free-b)(newline)
         (display 'free-a)(newline)))

   no error, everything is fine; with:

   (dynamic-wind
    (lambda ()
      (display 'alloc-a)(newline)
      (display 'alloc-b)(newline)
      (display 'alloc-c)(newline))
    (lambda ()
      (format #t "ciao~%")
      (throw 'misc-error))
    (lambda ()
      (display 'free-c)(newline)
      (display 'free-b)(newline)
      (display 'free-a)(newline)))

   'dynamic-wind' does its job, all right; with:

   (dynamic-wind
    (lambda ()
      (display 'alloc-a)(newline)
      (display 'alloc-b)(newline)
      (display 'alloc-c)(newline))
    (lambda ()
      (format #t "ciao~%"))
    (lambda ()
      (display 'free-c)(newline)
      (throw 'misc-error)
      (display 'free-b)(newline)
      (display 'free-a)(newline)))

   'free-b' and 'free-a' are not displayed: a and b are
   leaked; with:

   (dynamic-wind
    (lambda ()
      (display 'alloc-a)(newline)
      (display 'alloc-b)(newline)
      (throw 'misc-error)
      (display 'alloc-c)(newline))
    (lambda ()
      (format #t "ciao~%"))
    (lambda ()
      (display 'free-c)(newline)
      (display 'free-b)(newline)
      (display 'free-a)(newline)))

   'free-a' and 'free-b' are not displayed: a and b are
   leaked; with a compensations stack these cases are
   handled correctly.

--
Marco Maggi

"They say jump!, you say how high?"
Rage Against the Machine - "Bullet in the Head"