with-slots and symbol-macros

[Nicolas Sceaux]

Hello,

I am trying to write a variation of CL's WITH-SLOTS for GNU LilyPond,
which uses guile scheme (1.6.4 is installed on my box).

(with-properties (element origin) music-object
  (display origin)
  (set! element (make-music blabla)))

<==>

(let ((#gensym-music# music-object))
  (display (ly:music-property #gensym-music# 'origin))
  (set! (ly:music-property #gensym-music# 'element) (make-music blabla)))

I would like to introduce symbol macros, for instance:
  element
that would expand into:
  (ly:music-property #gensym-music# 'element)
which is setf-able.

I am not familiar with define-syntax and friends. What I have come up
to is:

guile> (define mycar (make-procedure-with-setter car set-car!))
guile> (let-syntax ((car-a (lambda (exp)
                             (syntax-case exp ()
                               (_ (syntax (mycar a)))))))
         (let ((a (list 1 2 3)))
           (+ car-a 3)))
4
guile> (let-syntax ((car-a (lambda (exp)
                             (syntax-case exp ()
                               (_ (syntax (mycar a)))))))
         (let ((a (list 1 2 3)))
           (set! car-a 3)
           a))
ERROR: invalid syntax (set! car-a 3)
ABORT: (misc-error)

That is, set! remains problematic.

In spite of intensive googling, I could not find an example that could
work in guile scheme. A clue is most welcome. Or am I using a wrong
approach?

Please cc: me (nicolas.sceaux AT free.fr) as I'm not subscribed to the
ml.

Best regards,
nicolas