Re: music function with access to context properties like currentBarNumber?

[Malte Meyn]

Am 16.07.2015 um 09:05 schrieb Jan-Peter Voigt:
> The problem is, that one has to unfold the music expression during
> iteration, so he may need to build a parallel music iterator, that
> creates the needed grobs. That sounds misleading.

I don’t understand this completely (never heard of music iterators) but 
this does simply mean that it’s hard or impossible to change something 
early enough in the compilation process, doesn’t it?

> % non-functional example
> \version "2.18.2"
>
> fun = #(define-music-function (parser location bars)(integer?)
>           (let ((cbn 0)
>                 (ml (ly:make-moment 4 4))
>                 (rest #{ R1 #}))
>             #{
>               \applyContext #(lambda (context)
>                                (set! cbn (ly:context-property context
> 'currentBarNumber))
>                                (ly:message "bar-number: ~A" cbn)
>                                (set! ml (ly:context-property context
> 'measureLength))
>                                (ly:message "measure-length: ~A" ml)
>                                (ly:music-set-property! rest 'duration ml))
>               $rest
>             #}
>             ))
>
> { \time 3/4 bes'4 a' c''8 b' | \fun 3 | bes'4 a' c''8 b' }

I wrote a very simple engraver today which tries to modify the duration 
of rests. It seems to have the same problem: The duration of events can 
be changed but it doesn’t shift the following events.

%%%%
% another non-functional example

\version "2.19.22"

#(define Modify_rests_engraver
   (make-engraver
    (listeners
     ((rest-event engraver event)
      (ly:event-set-property!
       event
       'duration
       (ly:make-duration 3 1 1)) ; 8.*1
      (ly:message "rest in measure ~A"
       (ly:context-property
        (ly:translator-context engraver)
        'currentBarNumber))))))

\new Voice \with {
  \consists #Modify_rests_engraver
} \relative {
  r4 g'16
  a a a a
  b b b b
  c c c c |
  r2 r16 r
}

%%%%

If this worked, one could ‘abuse’ fingering numbers for telling the 
target bar.

\displayMusic { r-125 }