+@node Overview of music functions
+@subsection Overview of music functions
+
+Making a function which substitutes a variable into LilyPond
+code is easy. The general form of these functions is
+
+@example
+function =
+#(define-music-function (parser location @var{var1} @var{var2}... )
+ (@var{var1-type?} @var{var2-type?}...)
+ #@{
+ @emph{...music...}
+ #@})
+@end example
+
+@noindent
+where
+
+@multitable @columnfractions .33 .66
+@item @var{argi} @tab @var{i}th variable
+@item @var{argi-type?} @tab type of variable
+@item @var{...music...} @tab normal LilyPond input, using
+ variables as @code{#$var1}.
+@end multitable
+
+There following input types may be used as variables
+in a music function. This list is not exhaustive; see
+other documentation specifically about Scheme for more
+variable types.
+
+@multitable @columnfractions .33 .66
+@headitem Input type @tab @var{argi-type?} notation
+@item Integer @tab @code{integer?}
+@item Float (decimal number) @tab @code{number?}
+@item Text string @tab @code{string?}
+@item Markup @tab @code{markup?}
+@item Music expression @tab @code{ly:music?}
+@item A pair of variables @tab @code{pair?}
+@end multitable
+
+The @code{parser} and @code{location} argument are mandatory,
+and are used in some advanced situations. The @code{parser}
+argument is used to access to the value of another LilyPond
+variable. The @code{location} argument
+is used to set the ``origin'' of the music expression that is built
+by the music function, so that in case of a syntax error LilyPond
+can tell the user an appropriate place to look in the input file.
+
+
+@node Simple substitution functions
+@subsection Simple substitution functions
+
+Here is a simple example,
+
+@lilypond[quote,verbatim,ragged-right]
+padText = #(define-music-function (parser location padding) (number?)
+ #{
+ \once \override TextScript #'padding = #$padding
+ #})
+
+\relative c''' {
+ c4^"piu mosso" b a b
+ \padText #1.8
+ c4^"piu mosso" d e f
+ \padText #2.6
+ c4^"piu mosso" fis a g
+}
+@end lilypond
+
+Music expressions may be substituted as well,
+
+@lilypond[quote,verbatim,ragged-right]
+custosNote = #(define-music-function (parser location note)
+ (ly:music?)
+ #{
+ \once \override Voice.NoteHead #'stencil =
+ #ly:text-interface::print
+ \once \override Voice.NoteHead #'text =
+ \markup \musicglyph #"custodes.mensural.u0"
+ \once \override Voice.Stem #'stencil = ##f
+ $note
+ #})
+
+{ c' d' e' f' \custosNote g' }
+@end lilypond
+
+Multiple variables may be used,
+
+@lilypond[quote,verbatim,ragged-right]
+tempoMark = #(define-music-function (parser location padding marktext)
+ (number? string?)
+#{
+ \once \override Score . RehearsalMark #'padding = $padding
+ \once \override Score . RehearsalMark #'no-spacing-rods = ##t
+ \mark \markup { \bold $marktext }
+#})
+
+\relative c'' {
+c2 e
+\tempoMark #3.0 #"Allegro"
+g c
+}
+@end lilypond
+
+
+@node Paired substitution functions
+@subsection Paired substitution functions
+
+Some @code{\override} commands require a pair of numbers
+(called a @code{cons cell} in Scheme). To pass these numbers
+into a function, either use a @code{pair?} variable, or
+insert the @code{cons} into the music function.
+
+@quotation
+@example
+manualBeam =
+#(define-music-function (parser location beg-end)
+ (pair?)
+#@{
+ \once \override Beam #'positions = #$beg-end
+#@})
+
+\relative @{
+ \manualBeam #'(3 . 6) c8 d e f
+@}
+@end example
+@end quotation
+
+@noindent
+or
+
+@lilypond[quote,verbatim,ragged-right]
+manualBeam =
+#(define-music-function (parser location beg end)
+ (number? number?)
+#{
+ \once \override Beam #'positions = #(cons $beg $end)
+#})
+
+\relative {
+ \manualBeam #3 #6 c8 d e f
+}
+@end lilypond
+
+
+@node Mathematics in functions
+@subsection Mathematics in functions
+
+Music functions can involve Scheme programming in
+addition to simple substitution,
+
+@lilypond[quote,verbatim,ragged-right]
+AltOn = #(define-music-function (parser location mag) (number?)
+ #{ \override Stem #'length = #$(* 7.0 mag)
+ \override NoteHead #'font-size =
+ #$(inexact->exact (* (/ 6.0 (log 2.0)) (log mag))) #})
+
+AltOff = {
+ \revert Stem #'length
+ \revert NoteHead #'font-size
+}
+
+{ c'2 \AltOn #0.5 c'4 c'
+ \AltOn #1.5 c' c' \AltOff c'2 }
+@end lilypond
+
+@noindent
+This example may be rewritten to pass in music expressions,
+
+@lilypond[quote,verbatim,ragged-right]
+withAlt = #(define-music-function (parser location mag music) (number? ly:music?)
+ #{ \override Stem #'length = #$(* 7.0 mag)
+ \override NoteHead #'font-size =
+ #$(inexact->exact (* (/ 6.0 (log 2.0)) (log mag)))
+ $music
+ \revert Stem #'length
+ \revert NoteHead #'font-size #})
+
+{ c'2 \withAlt #0.5 {c'4 c'}
+ \withAlt #1.5 {c' c'} c'2 }
+@end lilypond
+
+@node Void functions
+@subsection Void functions
+
+A music function must return a music expression, but sometimes we
+may want to have a function which does not involve music (such as
+turning off Point and Click). To do this, we return a @code{void}
+music expression.
+
+That is why the form
+that is returned is the @code{(make-music ...)}. With the
+@code{'void} property set to @code{#t}, the parser is told to
+actually disregard this returned music
+expression. Thus the important part of the void music function is the
+processing done by the function, not the music expression that is
+returned.
+
+@example
+noPointAndClick =
+#(define-music-function (parser location) ()
+ (ly:set-option 'point-and-click #f)
+ (make-music 'SequentialMusic 'void #t))
+...
+\noPointAndClick % disable point and click
+@end example
+
+
+@node Programmer interfaces
+@section Programmer interfaces
+
+This section contains information about mixing LilyPond
+and Scheme.
projects / lilypond.git / blobdiff