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Node:Manipulating music expressions, Next:Assignments, Previous:Music expressions, Up:Syntactic details

Manipulating music expressions

The \apply mechanism gives you access to the internal representation of music. You can write Scheme-functions that operate directly on it. The syntax is

        \apply #func music

This example replaces the text string of a script. It also shows a dump of the music it processes, which is useful if you want to know more about how music is stored.

#(define (testfunc x)
        (if (equal? (ly-get-mus-property x 'text) "foo")
                (ly-set-mus-property x 'text "bar"))
        ;; recurse
        (ly-set-mus-property x 'elements
          (map testfunc (ly-get-mus-property x 'elements)))
        (display x)
        x
)
\score { \notes
  \apply #testfunc { c'4_"foo" }
}

For more information on what is possible, see the automatically generated documentation.

Directly accessing internal representations is dangerous: the implementation is subject to changes, so you should avoid this feature if possible.

A final example is a function that reverses a piece of music in time:

#(define (reverse-music music)
  (let* ((elements (ly-get-mus-property music 'elements))
         (reversed (reverse elements))
         (span-dir (ly-get-mus-property music 'span-direction)))
    (ly-set-mus-property music 'elements reversed)
    (if (dir? span-dir)
        (ly-set-mus-property music 'span-direction (- span-dir)))
    (map reverse-music reversed)
    music))

music = \notes { c'4 d'4( e'4 f'4 }

\score { \context Voice {
    \music
    \apply #reverse-music \music
  }
}

More examples are given in the distributed example files in input/test/.

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Copyright (c) 1997--2001 Han-Wen Nienhuys and Jan Nieuwenhuizen.

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