LilyPond Application Usage: 2.6 Writing code to support multiple versions

## 2.6 Writing code to support multiple versions

In some cases, especially when writing library code it is desirable to support multiple LilyPond versions across breaking syntax changes. To do this alternative portions of code can be wrapped into conditional expressions depending on the currently executed LilyPond version. The Scheme function `ly:version?` expects a comparison operator op and a reference version ver passed as a list of integers with up to three elements. Missing elements are ignored so `'(2 20)` is equivalent to any version of the 2.20 line of versions. Constructs like the following are possible:

    #(cond
      ((ly:version? > '(2 20))
       (ly:message "This is code to run for LilyPond after 2.20"))
      ((ly:version? = '(2 19 57))
       (ly:message "This will only be executed with LilyPond 2.19.57"))
      (else (ly:message "This will be executed in any other version")))

Usually this will be integrated in library functions to allow alternative syntax to be used, but it is also possible to use the comparison directly within the music like in the following example:

    {
      c' d' e' f'
      #(if (ly:version? = '(2 21))
           #{ \override NoteHead.color = #red #}
           #{ \override NoteHead.color = #blue #})
      g' a' b' c''
    }

Note: This function has been introduced in LilyPond 2.21.80, so it is not possible to compare with versions earlier than that.

#### LilyPond — Usage v2.22.2 (stable-branch).