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Re: Weird intermediate context creation

From: David Kastrup
Subject: Re: Weird intermediate context creation
Date: Mon, 06 Jan 2020 16:57:03 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Dan Eble <address@hidden> writes:

> I find it strange that this code in input/regression/
> ends up on two staves rather than one:
> music = \repeat unfold 16 { c''4\bendAfter #-4 }
> sixteens = \repeat unfold 64 { c'16 }
> \new Score {
>   << \music \sixteens >>
> }
> I have a pretty good idea of why it is happening, and I am currently
> testing a change that affects 4 layout regtests including this one.

Why wouldn't it end up on two staves?  << >> does not start new
contexts, it just groups them.

\new Score {
  << { c'4 d' e' f' } { c' d' e' f' } >>

will also produce two staves.

> The question is whether changing it is the right thing to do.

I have no idea what you want to change it to without breaking stuff like
\new StaffGroup << ... >>

> On the one hand,
> the current result is strange

Can you explain why you feel that?

> the NR is not clear as to whether the current result is expected; I
> lean toward reading it as unexpected
> the NR advises against relying on implicit context creation anyway

Implicit context creation is not going away.  Things like Global and
Score context are created implicitly all the time.

> LilyPond will create lower-level contexts automatically if a music
> expression is encountered before a suitable context exists, but this
> is usually successful only for simple scores or music fragments like
> the ones in the documentation.
> The NR defines "bottom-level contexts" to include Voice and to exclude
> Staff, but leaves the reader to draw his own conclusion of what
> "lower-level contexts" are.  I guess it means any required contexts
> that don't yet exist.

Bottom is any context without \defaultchild .  Bottom is needed for most
events, but sets/overrides/applycontexts tend to be delivered to
somewhere in the context hierarchy.

> I'm not sure from this whether to expect \music and \sixteens to end
> up in separate Voice contexts.  My intuition is that after a Voice
> context is created for \music, "a suitable context exists" for
> \sixteens.

The items in << >> do not invade one another automatically.  You can
wrote \context Voice ... to pick up explicitly (and that's basically
what overrides/sets do) but other than that, there is no automatism.  If
I write

<< \new Staff { c' } \new Voice { d' } >>

should the d' insinuate itself into the same staff as the c' ?

> On the other hand, changing this would change how existing scores that
> rely on automatic context creation are engraved.  Is a change
> justified either on the grounds that the current behavior is a bug or
> that the NR gives fair warning not to rely on this feature?

I have no idea what behavior you propose instead.

David Kastrup

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