Extending LilyPond: 2.3.6 Functions without arguments

### 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
>          ()
>          ()
>        (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

#### LilyPond — Extending v2.24.4 (stable-branch).