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1.3.1 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
'NoteEvent
'articulations
(list (make-music
'AbsoluteDynamicEvent
'text
"f"))
'duration
(ly:make-duration 2 0 1/1)
'pitch
(ly:make-pitch 0 0 0))))
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}, you can specify an optional
output port to use:
{
\displayMusic #(open-output-file "display.txt") { c'4\f }
}
This will overwrite a previous output file whenever it is called; if you need to write more than one expression, you would use a variable for your port and reuse it:
{
port = #(open-output-file "display.txt")
\displayMusic \port { c'4\f }
\displayMusic \port { d'4 }
#(close-output-port port)
}
Guile’s manual describes ports in detail. Closing the port is actually only necessary if you need to read the file before LilyPond finishes; in the first example, we did not bother to do so.
A bit of reformatting makes the above information easier to read:
(make-music 'SequentialMusic
'elements (list
(make-music 'NoteEvent
'articulations (list
(make-music 'AbsoluteDynamicEvent
'text
"f"))
'duration (ly:make-duration 2 0 1/1)
'pitch (ly:make-pitch 0 0 0))))
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 a
NoteEvent object (storing the duration and pitch properties) with
attached information (in this case, an AbsoluteDynamicEvent with
a "f" text property) stored in its articulations property.
\displayMusic returns the music it displays, so it will get
interpreted as well as displayed. To avoid interpretation, write
\void before \displayMusic.
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