Re: Scheme question: symbol to music

[Carl Sorensen]

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

> 
> Stupic question, I assume...
> 
> In a scheme function I have a symbol that is the name of a lilypond
> expression. How can I get its music value?
> 
> E.g.
> 
>   ifDefined =
>   #(define-music-function
>    (parser location sym)
>    (symbol?)
> 
>    (if (defined? sym)
>        ??? return the value of the expression ???
>     (make-music 'SequentialMusic 'void #t)))
> 
> So I can write:
> 
>   \ifDefined #'myNotes
> 
> and have it expand to myNotes (if defined), or a void music expression
> otherwise.
> 

You need to use the procedure primitive-eval to evaluate a symbol


(if (defined? sym)
    (primitive-eval sym)
    (make-music 'SequentialMusic 'void #t))

should do the trick.

Here's some test code that worked for me:


%%% Start of test code

\version "2.11.64"

ifDefined = 
  #(define-music-function (parser location sym) (symbol?)
      " Return the music if @code{sym} is defined, and void if not"
      (if (defined? sym)
          (primitive-eval sym)
          (make-music 'SequentialMusic 'void #t)))
  
myNotes = \relative c' { f4 g a b }

{
  \ifDefined #'myNotes
  \ifDefined #'undefinedNotes
} 

%%% End of test code 

HTH,

Carl