Hook change hook?

[Neil Jerram]

When working with hooks, I often find that I have to wrap access to
the hook rather than making the hook available directly, because I
need to do something extra whenever something is added or removed from
that hook.

For example:

(define trace-hook (make-hook))

(define-public (add-trace-hook! proc)
  (add-hook! trace-hook proc)
  (if (not (hook-empty? trace-hook))
      (debug-enable 'trace)))

(define-public (remove-trace-hook! proc)
  (add-hook! trace-hook proc)
  (if (hook-empty? trace-hook)
      (debug-disable 'trace)))

Suppose instead that a hook could have an observer associated with it,
which would be called whenever the hook was changed.  Then one could
expose the hook directly, and not require the user to use unusual
procedures to change it:

(define-public trace-hook (make-hook))

(define (trace-observer proc action)
  ((if (hook-empty? trace-hook)
       debug-disable
       debug-enable) 'trace))

(set-hook-observer trace-hook trace-observer)

[Although unused in this example, PROC would be the procedure that was
just added or removed, and ACTION would be a keyword, one of
`#:appended', `#:prepended' and `#:removed'.]

This strikes me as much nicer.  Would it be good to add this to Guile?

        Neil