Re: Scheme function to print out active Voice context during interpretation?

[Han-Wen Nienhuys]

On Mon, Dec 15, 2008 at 12:18 AM, Trevor Bača <address@hidden> wrote:

> OK, this explanation is extremely helpful. But there's still something
> deeply wrong. Han-Wen from a bit earlier:
>
> "Note that contexts do not 'nest' as you claim in a message below.
> Voice contexts cannot contain any other context."
>
> But check this out:
>
> %%% VOICE-RESOLUTION EX 3 %%%
>
> \version "2.11.65"
> \include "english.ly"
>
> \new Voice = "foo" {
>    \override Voice.NoteHead #'color = #red
>    \whichContext
>    c'8
>    cs'8
>    \new Voice = "bar" {
>       \override Voice.NoteHead #'color = #blue
>       \whichContext
>       d'8
>       ef'8
>       \new Voice = "blah" {
>          \override Voice.NoteHead #'color = #green
>          \whichContext
>          e'8
>          f'8
>          fs'8
>          g'8
>       }
>       \whichContext
>       af'8
>       a'8
>    }
>    \whichContext
>    bf'8
>    b'8
> }
>
> %%% END %%%
>
> It appears as though the voice contexts are indeed nesting ... perfectly, in
> fact!

The music expressions are nested, as you can easily see from the
syntax.  However, the contexts themselves (that the music expressions
are shipping their events to) are actually running in parallel, and
are siblings in the context tree. When you do


  \new Voice =  foo {
    c4
   // *
    \new Voice = bar { d4 }
    e4
  }

at the location marked by * there is an iterator for the outer
expression (delivering events to Voice=foo); the moment \new Voice =
bar expression is opened, this creates an iterator pointing to
Voice=bar. The outer iterator still points to foo, but it has no
events to deliver, because the inner iterator already sends them to
bar.  The moment the final note (e4) is processed, the relevant
iterator (Event_chord_iterator, if memory serves me) inherits the
destination from its parent, so the e4 goes to foo.

The foo and bar voice contexts themselves are not nested, they are
contained in their respective Staff objects and are independent.

> And, in fact, John's new \whichContext function emits the following during
> interpretation:

-- 
Han-Wen Nienhuys - address@hidden - http://www.xs4all.nl/~hanwen