5.6 Using music functions

Where tweaks need to be reused with different music expressions, it is often convenient to make the tweak part of a music function. In this section, we discuss only substitution functions, where the object is to substitute a variable into a piece of LilyPond input code. Other more complex functions are described in (extending)Music functions section ‘Music functions’ in Extending.

5.6.1 Substitution function syntax

Making a function that substitutes a variable into LilyPond code is easy. The general form of these functions is

function =
#(define-music-function
     (arg1 arg2 …)
     (type1? type2? …)
   #{
     …music…
   #})

where

The list of type predicates is required. Some of the most common type predicates used in music functions are:

boolean?
cheap-list?  (use instead of ‘list?’ for faster processing)
ly:duration?
ly:music?
ly:pitch?
markup?
number?
pair?
string?
symbol?

For a list of available type predicates, see Predefined type predicates. User-defined type predicates are also allowed.

See also

Notation Reference: Predefined type predicates.

Extending LilyPond: (extending)Music functions section ‘Music functions’ in Extending.

Installed Files: ‘lily/music-scheme.cc’, ‘scm/c++.scm’, ‘scm/lily.scm’.

5.6.2 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:

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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