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

[Carl D. Sorensen]

On 12/14/08 4:34 PM, "Trevor Bača" <address@hidden> wrote:


> 
> I have a pretty big stockpile of tests and sometime in the next couple of days
> I'll find a way to organize them and send them over. But I'd like to start
> with the pair of examples that just seems absolutely pathological to me. Here
> it is:
> 
> 
> %%% VOICE-RESOLUTION EXAMPLE 1 %%%
> 
> \version "2.11.57"
> 
>    {
>       \override Voice.NoteHead #'color = #red
>       c'8
>       d'8
>       e'8
>       f'8
>       \new Voice = "foo" {
>          g'8
>          a'8
>          b'8
>          c''8
>       }
>       d''8
>       e''8
>       f''8
>       g''8
>    }
> 
> %%% END EXAMPLE 1 %%%
> 
> So far, so good. In example 1, LilyPond creates two voices with one nested
> inside the other. We see this because the outer eight notes are red while the
> inner four notes are black.
> 
> But now look at this:
> 
> %%% VOICE-RESOLUTION EXAMPLE 2 %%%
> 
> \version "2.11.57"
> 
>    {
>       \new Voice = "foo" {
>       \override Voice.NoteHead #'color = #red
>          g'8
>          a'8
>          b'8
>          c''8
>       }
>       d''8
>       e''8
>       f''8
>       g''8
>    }
> 
> 
> %%% END EX 2 %%%
> 
> (Stop and think what you'd expect here before looking at the output.)
> 
> Here LilyPond renders a single voice! We see this because all eight notes are
> red.
> 
> Sprinkling the new \whichContext function before the first, fourth (and
> eighth) notes in each example (where possible) makes it clear that the "foo"
> context governs only the middle four notes in the first example while
> governing *all* eight notes in the second example.
> 
> What on earth is going on here?
> 
> What's so odd is that if you look at example 2 and think "OK, this is all just
> a single voice because the explicitly instantiated voice 'leaks' over into the
> following four notes" ... then you're in for a big surprise when you add just
> a single note to the beginning of the example:
> 
> %%% EX 2 + 1 NOTE %%%
> 
>    {
>    c'8
>       \new Voice = "foo" {
>       \override Voice.NoteHead #'color = #red
>          g'8
>          a'8
>          b'8
>          c''8
>       }
>       d''8
>       e''8
>       f''8
>       g''8
>    }
> 
> %%% END EX 2 MOD %%%
> 
> That single note at the beginning now breaks the example into two voices just
> like example 1!
> 
> Can anyone explain what's going on here?

In examples 1 and 2+1 note, the first music occurs without a voice being
defined.  Therefore, LilyPond creates (instantiates) a voice.  Voice foo is
then created, and limited to the notes contained in the brackets.

In example 2, Voice foo is created before any voice is instantiated.  Then,
the music that comes after the voice foo definition is just applied to the
existing voice foo, since there is no explicit instantiation.

This kind of inconsistency shows up lots of times when you use automatic
instantiation of contexts.  In order to avoid these things, I've take to
explicitly instantiating all contexts.  (And if I used multiple books in a
score, I'd explicitly instantiate all of them as well).

Implicit instantiation is beginner-friendly but expert-unfriendly, IMO.  It
makes less for the user to type in building a simple song, but it hides the
structure of LilyPond, and makes it unnecessarily difficult to set complex
music.

A while ago there was a post suggesting that we do away with all implicit
instantiation and require users to explicitly instantiate contexts.  That
went nowhere.  But I think there ought to be a statement somewhere that says
"for complex scores, it is best to explicitly instantiate all voices".  In
my experience, that avoids lots of confusion.

Just my 0.02.

Carl