Re: Scheme question: symbol to music

[Nicolas Sceaux]

Le 12 nov. 08 à 05:48, Carl Sorensen a écrit :

> 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.
>
> 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)))

A more idiomatic way to do it:

ifDefined =
#(define-music-function (parser location sym) (symbol?)
   (let ((music (ly:parser-lookup parser sym)))
     (if (ly:music? music)
         music
         (make-music 'Music 'void #t))))

nicolas