2.3.6 Functions without arguments

In most cases a function without arguments should be written with a variable,

dolce = \markup{ \italic \bold dolce }

However, in rare cases it may be useful to create a music function without arguments,

displayBarNum =
#(define-music-function
     (parser location)
     ()
   (if (eq? #t (ly:get-option 'display-bar-numbers))
       #{ \once \override Score.BarNumber.break-visibility = ##f #}
       #{#}))

To actually display bar numbers where this function is called, invoke lilypond with

lilypond -d display-bar-numbers FILENAME.ly

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LilyPond — Extending v2.19.14 (development-branch).