Re: "compound music expression" to "music" expression"

[Aaron Hill]

On 2021-04-23 11:32 pm, David Kastrup wrote:
> Gianmaria Lari <gianmarialari@gmail.com> writes:
>
> > Thank you Aaron and Jean, your code works!
> >
> > But I have some trivial questions.
> > If I write:
> >
> > \displayLilyMusic \chordmode {c}
> >
> >
> > I get:
> >
> > { < c' e' g' >4 }
> >
> >
> > It looks a good "compound music expression".
>
> Try \displayMusic rather than \displayLilyMusic for more details.

In case David's hint was not sufficient, there is an additional layer of 
complexity when using \chordmode.  Unfortunately, that layer is not 
visible when using \displayLilyMusic, and this is where the confusion 
arises.

\chordmode produces UnrelativableMusic that contains SequentialMusic and 
then EventChords of NoteEvents.

Jean's first-element assumes that the music argument has a property 
called elements and resolves to the first value within the collection.  
UnrelativableMusic only has an element property, so first-element gets 
stuck.  Here is a modified version of first-element that does not:

%%%%
#(define (first-element music)
  (or (ly:music-property music 'element #f)
      (first (ly:music-property music 'elements))))

% This works, but you get a SequentialMusic.
{ #(first-element #{ \chordmode {c} #}) }

% This grabs the EventChord from the SequentialMusic.
{ #(first-element (first-element #{ \chordmode {c} #})) }
%%%%

My \extract uses the built-in extract-typed-music procedure to do the 
heavy lifting, including the logic necessary to traverse nested music 
expressions with both element and elements properties.


-- Aaron Hill