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3.4.3.2 Substitution function examples

This section introduces some substitution function examples. These are not intended to be exhaustive, but rather to demonstrate some of the possibilities of simple substitution functions.

In the first example, a function is defined that simplifies setting the padding of a TextScript grob:

padText =
#(define-music-function
     (padding)
     (number?)
   #{
     \once \override TextScript.padding = #padding
   #})

\relative {
  c''4^"piu mosso" b a b
  \padText #1.8
  c4^"piu mosso" b a b
  \padText #2.6
  c4^"piu mosso" b a b
}

In addition to numbers, we can use music expressions such as notes for arguments to music functions:

custosNote =
#(define-music-function
     (note)
     (ly:music?)
   #{
     \tweak NoteHead.stencil #ly:text-interface::print
     \tweak NoteHead.text
        \markup \musicglyph "custodes.mensural.u0"
     \tweak Stem.stencil ##f
     #note
   #})

\relative { c'4 d e f \custosNote g }

Both of those functions are simple single expressions where only the last element of a function call or override is missing. For those particular function definitions, there is a simpler alternative syntax, namely just writing out the constant part of the expression and replacing its final missing element with \etc:

padText =
  \once \override TextScript.padding = \etc

\relative {
  c''4^"piu mosso" b a b
  \padText #1.8
  c4^"piu mosso" b a b
  \padText #2.6
  c4^"piu mosso" b a b
}

custosNote =
  \tweak NoteHead.stencil #ly:text-interface::print
  \tweak NoteHead.text
     \markup \musicglyph "custodes.mensural.u0"
  \tweak Stem.stencil ##f
  \etc

\relative { c'4 d e f \custosNote g }

Substitution functions with multiple arguments can be defined:

tempoPadded =
#(define-music-function
     (padding tempotext)
     (number? markup?)
   #{
     \once \override Score.MetronomeMark.padding = #padding
     \tempo \markup { \bold #tempotext }
   #})

\relative {
  \tempo \markup { "Low tempo" }
  c''4 d e f g1
  \tempoPadded #4.0 "High tempo"
  g4 f e d c1
}

---

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