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2.3.5 Mathematics in functions

Music functions can involve Scheme programming in addition to simple substitution,

AltOn =
#(define-music-function
     (mag)
     (number?)
   #{
     \override Stem.length = #(* 7.0 mag)
     \override NoteHead.font-size =
       #(inexact->exact (* (/ 6.0 (log 2.0)) (log mag)))
   #})

AltOff = {
  \revert Stem.length
  \revert NoteHead.font-size
}

\relative {
  c'2 \AltOn #0.5 c4 c
  \AltOn #1.5 c c \AltOff c2
}

This example may be rewritten to pass in music expressions,

withAlt =
#(define-music-function
     (mag music)
     (number? ly:music?)
   #{
     \override Stem.length = #(* 7.0 mag)
     \override NoteHead.font-size =
       #(inexact->exact (* (/ 6.0 (log 2.0)) (log mag)))
     #music
     \revert Stem.length
     \revert NoteHead.font-size
   #})

\relative {
  c'2 \withAlt #0.5 { c4 c }
  \withAlt #1.5 { c c } c2
}