programming-interface.itely: Improve `\applyOutput' documentation.

[lilypond.git] / Documentation / extending / programming-interface.itely

@c -*- coding: utf-8; mode: texinfo; -*-

@ignore
    Translation of GIT committish: FILL-IN-HEAD-COMMITTISH

    When revising a translation, copy the HEAD committish of the
    version that you are working on.  For details, see the Contributors'
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@end ignore

@c \version "2.12.0"

@node Interfaces for programmers
@chapter Interfaces for programmers

Advanced tweaks may be performed by using Scheme.  If you are
not familiar with Scheme, you may wish to read our
@ref{Scheme tutorial}.

@menu
* Music functions::
* Markup functions::
* Contexts for programmers::
* Callback functions::
* Inline Scheme code::
* Difficult tweaks::
@end menu


@node Music functions
@section Music functions

@emph{Music functions} are scheme procedures that can create music
expressions automatically, and can be used to greatly simplify the
input file.

@menu
* Music function syntax::
* Simple substitution functions::
* Intermediate substitution functions::
* Mathematics in functions::
* Functions without arguments::
* Void functions::
@end menu


@node Music function syntax
@subsection Music function syntax

The general form for music functions is:

@example
function =
#(define-music-function
     (parser location @var{arg1} @var{arg2} @dots{})
     (@var{type1?} @var{type2?} @dots{})
   @var{music})
@end example

@noindent
where

@multitable @columnfractions .33 .66
@item @code{@var{argN}}
@tab @var{n}th argument

@item @code{@var{typeN?}}
@tab a scheme @emph{type predicate} for which @code{@var{argN}}
must return @code{#t}.

@item @code{@var{music}}
@tab A music expression, optionally written in scheme, with any
LilyPond code enclosed in hashed braces
(@tie{}@w{@code{#@{@dots{}#@}}}@tie{}).  Within LilyPond code
blocks, use @code{$} to reference function arguments (eg.,
@samp{$arg1}) or to start an inline scheme expression containing
function arguments (eg., @w{@samp{$(cons arg1 arg2)}}).

@end multitable

@noindent
For a list of available type predicates, see
@ruser{Predefined type predicates}.  User-defined type predicates
are also allowed.


@seealso

Notation Reference:
@ruser{Predefined type predicates}.

Installed Files:
@file{lily/music-scheme.cc},
@file{scm/c++.scm},
@file{scm/lily.scm}.


@node Simple substitution functions
@subsection Simple substitution functions

Simple substitution functions are music functions whose output
music expression is written in LilyPond format and contains
function arguments in the output expression.  They are described
in @ruser{Substitution function examples}.


@node Intermediate substitution functions
@subsection Intermediate substitution functions

Intermediate substitution functions involve a mix of Scheme code
and LilyPond code in the music expression to be returned.

Some @code{\override} commands require an argument consisting of
a pair of numbers (called a @code{cons cell} in Scheme).

The pair can be directly passed into the music function,
using a @code{pair?} variable:

@example
manualBeam =
#(define-music-function
     (parser location beg-end)
     (pair?)
   #@{
     \once \override Beam #'positions = $beg-end
   #@})

\relative c' @{
  \manualBeam #'(3 . 6) c8 d e f
@}
@end example

Alternatively, the numbers making up the pair can be
passed as separate arguments, and the Scheme code
used to create the pair can be included in the
music expression:

@lilypond[quote,verbatim,ragged-right]
manualBeam =
#(define-music-function
     (parser location beg end)
     (number? number?)
   #{
     \once \override Beam #'positions = $(cons beg end)
   #})

\relative c' {
  \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
}

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

\relative c' {
  c2 \withAlt #0.5 { c4 c }
  \withAlt #1.5 { c c } c2
}
@end lilypond


@node Functions without arguments
@subsection Functions without arguments

In most cases a function without arguments should be written
with a variable,

@example
dolce = \markup@{ \italic \bold dolce @}
@end example

However, in rare cases it may be useful to create a music function
without arguments,

@example
displayBarNum =
#(define-music-function
     (parser location)
     ()
   (if (eq? #t (ly:get-option 'display-bar-numbers))
       #@{ \once \override Score.BarNumber #'break-visibility = ##f #@}
       #@{#@}))
@end example

To actually display bar numbers where this function is called,
invoke @command{lilypond} with

@example
lilypond -d display-bar-numbers FILENAME.ly
@end example


@node Void functions
@subsection Void functions

A music function must return a music expression, but sometimes we
may want to have a function that 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
@w{@code{(make-music @dots{})}}.  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 Markup functions
@section Markup functions

Markups are implemented as special Scheme functions which produce a
@code{Stencil} object given a number of arguments.

@menu
* Markup construction in Scheme::
* How markups work internally::
* New markup command definition::
* New markup list command definition::
@end menu


@node Markup construction in Scheme
@subsection Markup construction in Scheme

@cindex defining markup commands

The @code{markup} macro builds markup expressions in Scheme while
providing a LilyPond-like syntax.  For example,
@example
(markup #:column (#:line (#:bold #:italic "hello" #:raise 0.4 "world")
                  #:larger #:line ("foo" "bar" "baz")))
@end example

@noindent
is equivalent to:
@example
\markup \column @{ \line @{ \bold \italic "hello" \raise #0.4 "world" @}
                  \larger \line @{ foo bar baz @} @}
@end example

@noindent
This example demonstrates the main translation rules between regular
LilyPond markup syntax and Scheme markup syntax.

@quotation
@multitable @columnfractions .3 .3
@item @b{LilyPond} @tab @b{Scheme}
@item @code{\markup markup1} @tab @code{(markup markup1)}
@item @code{\markup @{ markup1 markup2 ... @}} @tab
        @code{(markup markup1 markup2 ... )}
@item @code{\markup-command} @tab @code{#:markup-command}
@item @code{\variable} @tab @code{variable}
@item @code{\center-column @{ ... @}} @tab @code{#:center-column ( ... )}
@item @code{string} @tab @code{"string"}
@item @code{#scheme-arg} @tab @code{scheme-arg}
@end multitable
@end quotation

The whole Scheme language is accessible inside the
@code{markup} macro.  For example, You may use function calls inside
@code{markup} in order to manipulate character strings.  This is
useful when defining new markup commands (see
@ref{New markup command definition}).


@knownissues

The markup-list argument of commands such as @code{#:line},
@code{#:center}, and @code{#:column} cannot be a variable or
the result of a function call.

@lisp
(markup #:line (function-that-returns-markups))
@end lisp

@noindent
is invalid.  One should use the @code{make-line-markup},
@code{make-center-markup}, or @code{make-column-markup} functions
instead,

@lisp
(markup (make-line-markup (function-that-returns-markups)))
@end lisp


@node How markups work internally
@subsection How markups work internally

In a markup like

@example
\raise #0.5 "text example"
@end example

@noindent
@code{\raise} is actually represented by the @code{raise-markup}
function.  The markup expression is stored as

@example
(list raise-markup 0.5 (list simple-markup "text example"))
@end example

When the markup is converted to printable objects (Stencils), the
@code{raise-markup} function is called as

@example
(apply raise-markup
       @var{\layout object}
       @var{list of property alists}
       0.5
       @var{the "text example" markup})
@end example

The @code{raise-markup} function first creates the stencil for the
@code{text example} string, and then it raises that Stencil by 0.5
staff space.  This is a rather simple example; more complex examples
are in the rest
of this section, and in @file{scm/@/define@/-markup@/-commands@/.scm}.


@node New markup command definition
@subsection New markup command definition

This section discusses the definition of new markup commands.

@menu
* Markup command definition syntax::
* On properties::
* A complete example::
* Adapting builtin commands::
@end menu

@node Markup command definition syntax
@unnumberedsubsubsec Markup command definition syntax

New markup commands can be defined using the
@code{define-markup-command} Scheme macro, at top-level.

@lisp
(define-markup-command (@var{command-name} @var{layout} @var{props} @var{arg1} @var{arg2} ...)
    (@var{arg1-type?} @var{arg2-type?} ...)
    [ #:properties ((@var{property1} @var{default-value1})
                    ...) ]
  ..command body..)
@end lisp

The arguments are

@table @var
@item command-name
the markup command name
@item layout
the @q{layout} definition.
@item props
a list of associative lists, containing all active properties.
@item argi
@var{i}th command argument
@item argi-type?
a type predicate for the i@var{th} argument
@end table

If the command uses properties from the @var{props} arguments, the
@code{#:properties} keyword can be used to specify which properties are
used along with their default values.

Arguments are distinguished according to their type:
@itemize
@item a markup, corresponding to type predicate @code{markup?};
@item a list of markups, corresponding to type predicate
@code{markup-list?};
@item any other scheme object, corresponding to type predicates such as
@code{list?}, @code{number?}, @code{boolean?}, etc.
@end itemize

There is no limitation on the order of arguments (after the standard
@var{layout} and @var{props} arguments).  However, markup functions
taking a markup as their last argument are somewhat special as you can
apply them to a markup list, and the result is a markup list where the
markup function (with the specified leading arguments) has been applied
to every element of the original markup list.

Since replicating the leading arguments for applying a markup function
to a markup list is cheap mostly for Scheme arguments, you avoid
performance pitfalls by just using Scheme arguments for the leading
arguments of markup functions that take a markup as their last argument.

@node On properties
@unnumberedsubsubsec On properties

The @code{layout} and @code{props} arguments of markup commands bring a
context for the markup interpretation: font size, line width, etc.

The @code{layout} argument allows access to properties defined in
@code{paper} blocks, using the @code{ly:output-def-lookup} function.
For instance, the line width (the same as the one used in scores) is
read using:

@example
(ly:output-def-lookup layout 'line-width)
@end example

The @code{props} argument makes some properties accessible to markup
commands.  For instance, when a book title markup is interpreted, all
the variables defined in the @code{\header} block are automatically
added to @code{props}, so that the book title markup can access the book
title, composer, etc.  It is also a way to configure the behaviour of a
markup command: for example, when a command uses font size during
processing, the font size is read from @code{props} rather than having a
@code{font-size} argument.  The caller of a markup command may change
the value of the font size property in order to change the behaviour.
Use the @code{#:properties} keyword of @code{define-markup-command} to
specify which properties shall be read from the @code{props} arguments.

The example in next section illustrates how to access and override
properties in a markup command.

@node A complete example
@unnumberedsubsubsec A complete example

The following example defines a markup command to draw a double box
around a piece of text.

Firstly, we need to build an approximative result using markups.
Consulting the @ruser{Text markup commands} shows us the @code{\box}
command is useful:

@lilypond[quote,verbatim,ragged-right]
\markup \box \box HELLO
@end lilypond

Now, we consider that more padding between the text and the boxes is
preferable.  According to the @code{\box} documentation, this command
uses a @code{box-padding} property, which defaults to 0.2.  The
documentation also mentions how to override it:

@lilypond[quote,verbatim,ragged-right]
\markup \box \override #'(box-padding . 0.6) \box A
@end lilypond

Then, the padding between the two boxes is considered too small, so we
override it too:

@lilypond[quote,verbatim,ragged-right]
\markup \override #'(box-padding . 0.4) \box \override #'(box-padding . 0.6) \box A
@end lilypond

Repeating this lengthy markup would be painful.  This is where a markup
command is needed.  Thus, we write a @code{double-box} markup command,
taking one argument (the text).  This draws the two boxes, with some
padding.

@lisp
#(define-markup-command (double-box layout props text) (markup?)
  "Draw a double box around text."
  (interpret-markup layout props
    (markup #:override '(box-padding . 0.4) #:box
            #:override '(box-padding . 0.6) #:box text)))
@end lisp

@code{text} is the name of the command argument, and @code{markup?} its
type: it identifies it as a markup.  The @code{interpret-markup}
function is used in most of markup commands: it builds a stencil, using
@code{layout}, @code{props}, and a markup.  Here, this markup is built
using the @code{markup} scheme macro, see @ref{Markup construction in Scheme}.
The transformation from @code{\markup} expression to scheme
markup expression is straight-forward.

The new command can be used as follow:

@example
\markup \double-box A
@end example

It would be nice to make the @code{double-box} command customizable:
here, the @code{box-padding} values are hard coded, and cannot be
changed by the user.  Also, it would be better to distinguish the
padding between the two boxes, from the padding between the inner box
and the text.  So we will introduce a new property,
@code{inter-box-padding}, for the padding between the two boxes.  The
@code{box-padding} will be used for the inner padding.  The new code is
now as follows:

@lisp
#(define-markup-command (double-box layout props text) (markup?)
  #:properties ((inter-box-padding 0.4)
                (box-padding 0.6))
  "Draw a double box around text."
  (interpret-markup layout props
    (markup #:override `(box-padding . ,inter-box-padding) #:box
            #:override `(box-padding . ,box-padding) #:box text)))
@end lisp

Here, the @code{#:properties} keyword is used so that the
@code{inter-box-padding} and @code{box-padding} properties are read from
the @code{props} argument, and default values are given to them if the
properties are not defined.

Then, these values are used to override the @code{box-padding}
properties used by the two @code{\box} commands.  Note the backquote and
the comma in the @code{\override} argument: they allow you to introduce
a variable value into a literal expression.

Now, the command can be used in a markup, and the boxes padding be
customized:

@lilypond[quote,verbatim,ragged-right]
#(define-markup-command (double-box layout props text) (markup?)
  #:properties ((inter-box-padding 0.4)
                (box-padding 0.6))
  "Draw a double box around text."
  (interpret-markup layout props
    (markup #:override `(box-padding . ,inter-box-padding) #:box
            #:override `(box-padding . ,box-padding) #:box text)))

\markup \double-box A
\markup \override #'(inter-box-padding . 0.8) \double-box A
\markup \override #'(box-padding . 1.0) \double-box A
@end lilypond

@node Adapting builtin commands
@unnumberedsubsubsec Adapting builtin commands

A good way to start writing a new markup command, is to take example on
a builtin one.  Most of the markup commands provided with LilyPond can be
found in file @file{scm/@/define@/-markup@/-commands@/.scm}.

For instance, we would like to adapt the @code{\draw-line} command, to
draw a double line instead.  The @code{\draw-line} command is defined as
follow (documentation stripped):

@lisp
(define-markup-command (draw-line layout props dest)
  (number-pair?)
  #:category graphic
  #:properties ((thickness 1))
  "..documentation.."
  (let ((th (* (ly:output-def-lookup layout 'line-thickness)
               thickness))
        (x (car dest))
        (y (cdr dest)))
    (make-line-stencil th 0 0 x y)))
@end lisp

To define a new command based on an existing one, copy the definition,
and change the command name.  The @code{#:category} keyword can be
safely removed, as it is only used for generating LilyPond
documentation, and is of no use for user-defined markup commands.

@lisp
(define-markup-command (draw-double-line layout props dest)
  (number-pair?)
  #:properties ((thickness 1))
  "..documentation.."
  (let ((th (* (ly:output-def-lookup layout 'line-thickness)
               thickness))
        (x (car dest))
        (y (cdr dest)))
    (make-line-stencil th 0 0 x y)))
@end lisp

Then, a property for setting the gap between two lines is added, called
@code{line-gap}, defaulting e.g. to 0.6:

@lisp
(define-markup-command (draw-double-line layout props dest)
  (number-pair?)
  #:properties ((thickness 1)
                (line-gap 0.6))
  "..documentation.."
  ...
@end lisp

Finally, the code for drawing two lines is added.  Two calls to
@code{make-line-stencil} are used to draw the lines, and the resulting
stencils are combined using @code{ly:stencil-add}:

@lilypond[quote,verbatim,ragged-right]
#(define-markup-command (my-draw-line layout props dest)
  (number-pair?)
  #:properties ((thickness 1)
                (line-gap 0.6))
  "..documentation.."
  (let* ((th (* (ly:output-def-lookup layout 'line-thickness)
                thickness))
         (dx (car dest))
         (dy (cdr dest))
         (w (/ line-gap 2.0))
         (x (cond ((= dx 0) w)
                  ((= dy 0) 0)
                  (else (/ w (sqrt (+ 1 (* (/ dx dy) (/ dx dy))))))))
         (y (* (if (< (* dx dy) 0) 1 -1)
               (cond ((= dy 0) w)
                     ((= dx 0) 0)
                     (else (/ w (sqrt (+ 1 (* (/ dy dx) (/ dy dx))))))))))
     (ly:stencil-add (make-line-stencil th x y (+ dx x) (+ dy y))
                     (make-line-stencil th (- x) (- y) (- dx x) (- dy y)))))

\markup \my-draw-line #'(4 . 3)
\markup \override #'(line-gap . 1.2) \my-draw-line #'(4 . 3)
@end lilypond


@node New markup list command definition
@subsection New markup list command definition
Markup list commands are defined with the
@code{define-markup-list-command} Scheme macro, which is similar to the
@code{define-markup-command} macro described in
@ref{New markup command definition}, except that where the latter returns
a single stencil, the former returns a list of stencils.

In the following example, a @code{\paragraph} markup list command is
defined, which returns a list of justified lines, the first one being
indented.  The indent width is taken from the @code{props} argument.
@example
#(define-markup-list-command (paragraph layout props args) (markup-list?)
   #:properties ((par-indent 2))
   (interpret-markup-list layout props
     (make-justified-lines-markup-list (cons (make-hspace-markup par-indent)
                                             args))))
@end example

Besides the usual @code{layout} and @code{props} arguments, the
@code{paragraph} markup list command takes a markup list argument, named
@code{args}.  The predicate for markup lists is @code{markup-list?}.

First, the function gets the indent width, a property here named
@code{par-indent}, from the property list @code{props}.  If the
property is not found, the default value is @code{2}.  Then, a
list of justified lines is made using the
@code{make-justified-lines-markup-list} function, which is related
to the @code{\justified-lines} built-in markup list command.  A
horizontal space is added at the beginning using the
@code{make-hspace-markup} function.  Finally, the markup list is
interpreted using the @code{interpret-markup-list} function.

This new markup list command can be used as follows:
@example
\markuplines @{
  \paragraph @{
    The art of music typography is called \italic @{(plate) engraving.@}
    The term derives from the traditional process of music printing.
    Just a few decades ago, sheet music was made by cutting and stamping
    the music into a zinc or pewter plate in mirror image.
  @}
  \override-lines #'(par-indent . 4) \paragraph @{
    The plate would be inked, the depressions caused by the cutting
    and stamping would hold ink.  An image was formed by pressing paper
    to the plate.  The stamping and cutting was completely done by
    hand.
  @}
@}
@end example

@node Contexts for programmers
@section Contexts for programmers

@menu
* Context evaluation::
* Running a function on all layout objects::
@end menu

@node Context evaluation
@subsection Context evaluation

@cindex calling code during interpreting
@funindex \applyContext

Contexts can be modified during interpretation with Scheme code.  The
syntax for this is
@example
\applyContext @var{function}
@end example

@var{function} should be a Scheme function that takes a single
argument: the context in which the @code{\applyContext} command is
being called.  The following code will print the current bar
number on the standard output during the compile:

@example
\applyContext
  #(lambda (x)
    (format #t "\nWe were called in barnumber ~a.\n"
     (ly:context-property x 'currentBarNumber)))
@end example



@node Running a function on all layout objects
@subsection Running a function on all layout objects


@cindex calling code on layout objects
@funindex \applyOutput


The most versatile way of tuning an object is @code{\applyOutput} which
works by inserting an event into the specified context
(@rinternals{ApplyOutputEvent}).  Its syntax is
@example
\applyOutput @var{context} @var{proc}
@end example

@noindent
where @var{proc} is a Scheme function, taking three arguments.

When interpreted, the function @var{proc} is called for every layout
object found in the context @var{context} at the current time step, with
the following arguments:
@itemize
@item the layout object itself,
@item the context where the layout object was created, and
@item the context where @code{\applyOutput} is processed.
@end itemize


In addition, the cause of the layout object, i.e., the music
expression or object that was responsible for creating it, is in the
object property @code{cause}.  For example, for a note head, this is a
@rinternals{NoteHead} event, and for a stem object,
this is a @rinternals{Stem} object.

Here is a function to use for @code{\applyOutput}; it blanks
note-heads on the center-line and next to it:

@lilypond[quote,verbatim,ragged-right]
#(define (blanker grob grob-origin context)
   (if (and (memq 'note-head-interface (ly:grob-interfaces grob))
            (< (abs (ly:grob-property grob 'staff-position)) 2))
       (set! (ly:grob-property grob 'transparent) #t)))

\relative c' {
  a'4 e8 <<\applyOutput #'Voice #blanker a c d>> b2
}
@end lilypond


@node Callback functions
@section Callback functions

Properties (like @code{thickness}, @code{direction}, etc.) can be
set at fixed values with @code{\override}, e.g.

@example
\override Stem #'thickness = #2.0
@end example

Properties can also be set to a Scheme procedure,

@lilypond[fragment,verbatim,quote,relative=2]
\override Stem #'thickness = #(lambda (grob)
    (if (= UP (ly:grob-property grob 'direction))
        2.0
        7.0))
c b a g b a g b
@end lilypond

@noindent
In this case, the procedure is executed as soon as the value of the
property is requested during the formatting process.

Most of the typesetting engine is driven by such callbacks.
Properties that typically use callbacks include

@table @code
@item stencil
  The printing routine, that constructs a drawing for the symbol
@item X-offset
  The routine that sets the horizontal position
@item X-extent
  The routine that computes the width of an object
@end table

The procedure always takes a single argument, being the grob.

If routines with multiple arguments must be called, the current grob
can be inserted with a grob closure.  Here is a setting from
@code{AccidentalSuggestion},

@example
`(X-offset .
  ,(ly:make-simple-closure
    `(,+
        ,(ly:make-simple-closure
           (list ly:self-alignment-interface::centered-on-x-parent))
      ,(ly:make-simple-closure
           (list ly:self-alignment-interface::x-aligned-on-self)))))
@end example

@noindent
In this example, both @code{ly:self-alignment-interface::x-aligned-on-self} and
@code{ly:self-alignment-interface::centered-on-x-parent} are called
with the grob as argument.  The results are added with the @code{+}
function.  To ensure that this addition is properly executed, the whole
thing is enclosed in @code{ly:make-simple-closure}.

In fact, using a single procedure as property value is equivalent to

@example
(ly:make-simple-closure (ly:make-simple-closure (list @var{proc})))
@end example

@noindent
The inner @code{ly:make-simple-closure} supplies the grob as argument
to @var{proc}, the outer ensures that result of the function is
returned, rather than the @code{simple-closure} object.

From within a callback, the easiest method for evaluating a markup is
to use grob-interpret-markup.  For example:

@example
my-callback = #(lambda (grob)
                 (grob-interpret-markup grob (markup "foo")))
@end example

@node Inline Scheme code
@section Inline Scheme code

The main disadvantage of @code{\tweak} is its syntactical
inflexibility.  For example, the following produces a syntax error.

@example
F = \tweak #'font-size #-3 -\flageolet

\relative c'' @{
  c4^\F c4_\F
@}
@end example

@noindent
In other words, @code{\tweak} doesn't behave like an articulation
regarding the syntax; in particular, it can't be attached with
@code{^} and @code{_}.

Using Scheme, this problem can be avoided.  The route to the
result is given in @ref{Adding articulation to notes (example)},
especially how to use @code{\displayMusic} as a helping guide.

@example
F = #(let ((m (make-music 'ArticulationEvent
                          'articulation-type "flageolet")))
       (set! (ly:music-property m 'tweaks)
             (acons 'font-size -3
                    (ly:music-property m 'tweaks)))
       m)

\relative c'' @{
  c4^\F c4_\F
@}
@end example

@noindent
Here, the @code{tweaks} properties of the flageolet object
@code{m} (created with @code{make-music}) are extracted with
@code{ly:music-property}, a new key-value pair to change the
font size is prepended to the property list with the
@code{acons} Scheme function, and the result is finally
written back with @code{set!}.  The last element of the
@code{let} block is the return value, @code{m} itself.



@node Difficult tweaks
@section Difficult tweaks

There are a few classes of difficult adjustments.

@itemize


@item
One type of difficult adjustment involves the appearance of
spanner objects, such as slurs and ties.  Usually, only one
spanner object is created at a time, and it can be adjusted with
the normal mechanism.  However, occasionally a spanner crosses a
line break.  When this happens, the object is cloned.  A separate
object is created for every system in which the spanner appears.
The new objects are clones of the original object and inherit all
properties, including @code{\override}s.


In other words, an @code{\override} always affects all pieces of a
broken spanner.  To change only one part of a spanner at a line break,
it is necessary to hook into the formatting process.  The
@code{after-line-breaking} callback contains the Scheme procedure that
is called after the line breaks have been determined and layout
objects have been split over different systems.

In the following example, we define a procedure
@code{my-callback}.  This procedure

@itemize
@item
determines if the spanner has been split across line breaks
@item
if yes, retrieves all the split objects
@item
checks if this grob is the last of the split objects
@item
if yes, it sets @code{extra-offset}.
@end itemize

This procedure is installed into @rinternals{Tie}, so the last part
of the broken tie is repositioned.

@lilypond[quote,verbatim,ragged-right]
#(define (my-callback grob)
   (let* (
          ;; have we been split?
          (orig (ly:grob-original grob))

          ;; if yes, get the split pieces (our siblings)
          (siblings (if (ly:grob? orig)
                        (ly:spanner-broken-into orig)
                        '())))

     (if (and (>= (length siblings) 2)
              (eq? (car (last-pair siblings)) grob))
         (ly:grob-set-property! grob 'extra-offset '(-2 . 5)))))

\relative c'' {
  \override Tie #'after-line-breaking =
  #my-callback
  c1 ~ \break
  c2 ~ c
}
@end lilypond

@noindent
When applying this trick, the new @code{after-line-breaking} callback
should also call the old one, if such a default exists.  For example,
if using this with @code{Hairpin}, @code{ly:spanner::kill-zero-spanned-time}
should also be called.


@item
Some objects cannot be changed with @code{\override} for
technical reasons.  Examples of those are @code{NonMusicalPaperColumn}
and @code{PaperColumn}.  They can be changed with the
@code{\overrideProperty} function, which works similar to @code{\once
\override}, but uses a different syntax.

@example
\overrideProperty
#"Score.NonMusicalPaperColumn"  % Grob name
#'line-break-system-details     % Property name
#'((next-padding . 20))         % Value
@end example

Note, however, that @code{\override}, applied to
@code{NonMusicalPaperColumn} and @code{PaperColumn}, still works as
expected within @code{\context} blocks.

@end itemize

@node LilyPond Scheme interfaces
@chapter LilyPond Scheme interfaces

This chapter covers the various tools provided by LilyPond to help
Scheme programmers get information into and out of the music streams.

TODO -- figure out what goes in here and how to organize it