Re: custom engraver in scheme: accessing nested Music object

[Reinhold Kainhofer]

Am Mittwoch, 10. August 2011, 17:11:44 schrieb Ricardo Wurmus:
> On Aug 10, Neil Puttock wrote:
> > BTW, if you're prepared to wrap the notes in a chord (so you have
> > access to 'articulations), you won't even need a scheme engraver (all
> > the processing can take place in the NoteHead's stencil callback).
> 
> It should be a generic engraver, so I cannot assume that notes will
> always be wrapped in a chord. I'll play around with defining a function
> for process-acknowledged and see where it leads me.

One detail that might be interesting for you is that you can define engraver-
wide variables by wrapping the whole engraver in a (let (..) ...) block.
See e.g. the scheme engraver instance regtest (input/regression/scheme-
engraver-instance.ly in the source code tree):

\header {

  texidoc = "Scheme engravers may be instantiated, with
  instance-scoped slots, by defining a 1 argument procedure which
  shall return the engraver definition as an alist, with the private
  slots defined in a closure.  The argument procedure argument is the
  context where the engraver is instantiated."

}

\version "2.14.0"

\layout {
  \context {
    \Voice
    \consists
    #(let ((instance-counter 0))
       (lambda (context)
         (set! instance-counter (1+ instance-counter))
         (let ((instance-id instance-counter)
               (private-note-counter 0))
           `((listeners
              (note-event
               . ,(lambda (engraver event)
                    (set! private-note-counter (1+ private-note-counter))
                    (let ((text (ly:engraver-make-grob engraver 'TextScript 
event)))
                      (ly:grob-set-property! text 'text
                                             (format #f "~a.~a" instance-id
                                                     private-note-
counter))))))))))
  }
}

<<
  \relative c'' { c4 d e f }
  \\ \relative c' { c4 d e f }
>>


In this example, instance-counter is a global variable, which is reused by 
every voice, while instance-id and private-note-counter are variables that are 
separate for each voice. In this example, instance-id stores the number of the 
voice, while private-note-counter counts the number of notes in each voice 
separately. 
In particular, the engravers used in the first and in the second voice will 
have different private-note-counter variables.

You can use this to collect information from all encountered objects and thin 
in 'process-acknowledged or 'stop-translation-timestep procell all of them at 
once. For a list of all possible "functions" inside the engraver, see 
input/regression/scheme-engraver.ly


Cheers,
Reinhold
-- 
------------------------------------------------------------------
Reinhold Kainhofer, address@hidden, http://reinhold.kainhofer.com/
 * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
 * http://www.fam.tuwien.ac.at/, DVR: 0005886
 * LilyPond, Music typesetting, http://www.lilypond.org