12.3.1 Displaying music expressions

When writing a music function it is often instructive to inspect how a music expression is stored internally. This can be done with the music function \displayMusic

{
  \displayMusic { c'4\f }
}

will display

(make-music
  'SequentialMusic
  'elements
  (list (make-music
          'EventChord
          'elements
          (list (make-music
                  'NoteEvent
                  'duration
                  (ly:make-duration 2 0 1 1)
                  'pitch
                  (ly:make-pitch 0 0 0))
                (make-music
                  'AbsoluteDynamicEvent
                  'text
                  "f")))))

By default, LilyPond will print these messages to the console along with all the other messages. To split up these messages and save the results of \display{STUFF}, redirect the output to a file.

lilypond file.ly >display.txt

With a bit of reformatting, the above information is easier to read,

(make-music 'SequentialMusic
  'elements (list (make-music 'EventChord
                    'elements (list (make-music 'NoteEvent
                                      'duration (ly:make-duration 2 0 1 1)
                                      'pitch (ly:make-pitch 0 0 0))
                                    (make-music 'AbsoluteDynamicEvent
                                      'text "f")))))

A { ... } music sequence has the name SequentialMusic, and its inner expressions are stored as a list in its 'elements property. A note is represented as an EventChord expression, containing a NoteEvent object (storing the duration and pitch properties) and any extra information (in this case, an AbsoluteDynamicEvent with a "f" text property.