Re: Using one identifier or another

[Dan Eble]

Johan Vromans <jvromans <at> squirrel.nl> writes:

> 
> Hi,
> 
> In a template file I want to check if a particular identifier has been
> defined, and act accordingly.

These two functions should help.  (Suggestions for improvement would be
appreciated.)  You will either need to define your optional music to be empty
when it is not required, as in this example, or provide a function to create
empty music if it is not already defined.  (It's hard to tell from your example
what your needs are.)

\version "2.11.52"

#(define-public (music-empty? m)
   (let ((void-music (ly:music-property m 'void)))
     (or (and (not (null? void-music)) void-music)
         (and (null? (ly:music-property m 'element))
              (null? (ly:music-property m 'elements))))))

ifThenElse =
#(define-music-function
   (parser location if-music then-music else-music)
   (ly:music? ly:music? ly:music?)

   (if (music-empty? if-music)
       else-music
       then-music))


aNotes = \relative c' { e1 }
bNotes = \relative c' { c1 }
cNotes = { }

\book
{
  \score
  {
    \ifThenElse \aNotes
    {
      \time 2/2
      \partcombine \aNotes \bNotes
    } {
      \time 1/1
      \bNotes
    }
  }

  \score
  {
    \ifThenElse \cNotes
    {
      \time 2/2
      \partcombine \cNotes \bNotes
    } {
      \time 1/1
      \bNotes
    }
  }
}