Simplifying input structure (was Re: Issue 3720: Built-in templates for SATB vocal scores (issue 41990043))

[Devon Schudy]

Janek Warchoł wrote:
> I believe we need to add an abstraction layer that would make it
> conceptually simpler to write \score blocks.
>
> Please take a look at
> https://github.com/openlilylib/snippets/tree/master/templates/predefined-instruments
>
> I believe that this is exactly what LilyPond needs to allow beginner
> users easily create score structures.  Notice how much it shortens the
> \score definition.

These are convenient! It would be nice to have these for all the
common instruments — it would save most users the repetitive trouble
of setting clefs, transpositions, instrumentName, instrumentShortName,
midiInstrument and \dynamicUp. (But would it bother users who are
writing for an instrument that doesn't have a predefined staff?)

Other things that cause complexity in the SATB templates:
 * Repeating \Time and \Key in each staff
 * Putting \Time in parallel with each voice
 * Explicitly creating Staff, and Lyrics contexts

With one voice per staff, VocalStaff etc. take care of the first four.
Users can get away with putting \time (and \repeat, if they don't use
\unfoldRepeats) in just one staff. The hard part is creating contexts.

Anything requiring understanding of contexts is beyond beginners. They
should be able to use implicitly created contexts for almost
everything. There are a few reasons this doesn't work:

 * Music that is lyrics (i.e. a SequentialMusic whose first child is a
LyricEvent) creates a useless staff, not a Lyrics aligned to the
previous voice.

 * Some group-level contexts have predictable children that are
different, so they can't be created correctly automatically.
PianoStaff's children are LH and RH; ChoralStaff's children are SATB.
If implicit contexts handled these (perhaps by making defaultChild a
callback instead of a string?), users wouldn't have to create explicit
staves just to get the normal behavior.

 * Contexts are created even when a suitable context already exists.
Users expect { << c' a >> << d' b >> } to be equivalent to << { c' d'
} { a b } >>, but it isn't when it creates contexts — it makes four
staves instead of two. This means intuitive operations like
concatenating tunes into a medley don't work.

 * Implicit contexts aren't named, so they don't support \lyricsto,
\context, \change, etc.

 * At top-level, StaffGroup isn't implicit when creating multiple staves.

Fixing these problems would make most explicit context creation
unnecessary. With the first two fixes, SATB scores would be simply:

<<
  \new ChoralStaff <<
    \Soprano \SopranoLyrics
    \Alto \AltoLyrics
    \Tenor \TenorLyrics
    \Bass \BassLyrics
  >>
  \new PianoStaff << \RH \LH >>
>>

\new ChoralStaff <<
  \new SopranoAltoStaff << \Soprano \Alto >>
  \VerseOne \VerseTwo %...
  \new TenorBassStaff << \Tenor \Bass >>
>>

...which is simple enough to expect users to type.