Re: syntax helper for ly:music-property

[Nicolas Sceaux]

Jan Nieuwenhuizen <address@hidden> writes:

> Han-Wen Nienhuys writes:
>
>>> >> guile> (set! address@hidden (ly:make-pitch -1 4 0))
>
>> No. Do you have a more elaborate example?
>
> Hmm.  I just thought that #@ .. elements.0.elements.2
>
> looked vagely like a class initilizer
>
>    (make <foe> #elements 0 #elements 2 
>
> but then I realized that all names were the same (elements) so I kept
> it extra vague.  I'll have to look into the example again.
>
> Jan.

My first idea was to create a syntactic construct similar to Common
Lisp's WITH-SLOTS, e.g:

  (with-properties (pitch duration) my-music
    (set! pitch (ly:make-pitch ...))
    (display duration)
    ...)

but I was not able to make it work (I'm no guru at define-syntax,
syntax-case and friends, I didn't manage to create symbol-macros in
guile scheme). I came to a second idea (which worked) :

  (with-properties (my-music #{ ... #})
    (display my-music.origin)
    (set-my-music.elements! ...))

but it was horribly slow, and when toying with a more elaborate
example, I find out that it was not a good solution, as music
expressions are often very nested, hence too many with-properties.

Finally, I remembered a post on comp.lang.lisp which introduced a '@'
reader character, used to access class slots, symbol properties,
structure fields, etc, in a fashion similar to
C++/Java/Python/etc. That's what I proposed here. Thus:

  (set! address@hidden (ly:make-pitch -1 4 0))

could have been wrote in, say, Python:

  titi.elements[0].elements[2].elements[0].pitch = ...

or in regular scheme (I may make a mistake):

  (set! (ly:music-property 
          (car (ly:music-property 
                 (caddr (ly:music-property
                          (car (ly:music-property titi 'elements))
                          'elements))
                 'elements)) 
          'pitch)
        (ly:make-pitch -1 4 0)) ;; ouf

nicolas