Add documentation for define-void-function

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@c \version "2.14.0"

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

@node Lilypond code blocks
@section Lilypond code blocks

Lilypond code blocks look like
@example
  #@{ @var{Lilypond code} #@}
@end example
They can be used anywhere where you can write Scheme code: the Scheme
reader actually is changed for accommodating LilyPond code blocks.  When
the LilyPond code block is being read, it is parsed superficially and
replaced by a call to the LilyPond @code{parser} which is executed at
runtime to interpret the LilyPond code block.

The point of the superficial parsing is the interpretation of @code{$}
signs which can be used for splicing in expressions from the surrounding
lexical Scheme context (like @code{let} variables and function
parameters).  @code{$} can be used in the following ways:

@table @code
@item $$
just passes a single @code{$} to the LilyPond parser.
@item $@var{form}
will evaluate the Scheme form at runtime and splice its value as an
identifier @code{\form} into the LilyPond parser.  Depending on the
value type, it may be interpreted as several different syntactic
entities.
@item #$@var{form}
will evaluate the Scheme form at runtime and splice its value as a
Scheme expression into the LilyPond parser.
@item #@var{form}
Forms in Scheme expressions started with @code{#} are read and parsed
recursively for @code{$} signs.  Those are treated as follows:
@item #@dots{}$@var{variable}
splices the value of the variable into the surrounding expression.
@item #@dots{}($ @var{form} @dots{})
splices the value of the form into the surrounding expression.  As
opposed to a LilyPond level @code{$@var{form}}, you need to separate the
form with a blank, making @code{$} be recognizable as a separate Scheme
symbol.
@end table

A LilyPond code block may contain anything that you can use on the right
side of an assignment.  In addition, an empty LilyPond block corresponds
to a void music expression, and a LilyPond block containing multiple
music events gets turned into a sequential music expression.

@node Scheme functions
@section Scheme functions
@cindex Scheme functions (LilyPond syntax)

@emph{Scheme functions} are Scheme procedures that can create Scheme
expressions from input written in LilyPond syntax.  They can be called
in pretty much all places where using @code{#} for specifying a value in
Scheme syntax is allowed.  While Scheme has functions of its own, this
chapter is concerned with @emph{syntactic} functions, functions that
receive arguments specified in LilyPond syntax.

@menu
* Scheme function definitions::
* Scheme function usage::
* Void scheme functions::
@end menu

@node Scheme function definitions
@subsection Scheme function definitions
@funindex define-scheme-function

The general form for defining scheme functions is:

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

@noindent
where

@multitable @columnfractions .33 .66
@item @code{parser}
@tab needs to be literally @code{parser} in order to give LilyPond code
blocks (@code{#@{}@dots{}@code{#@}}) access to the parser.

@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}.  Some of these predicates are specially
recognized by the parser, see below.  There is also a special form
@code{(@emph{predicate?} @emph{default})} for specifying optional
arguments.  If the actual argument is missing when the function is being
called, the default value is substituted instead.  Default values are
evaluated at definition time (including LilyPond code blocks!), so if
you need a default calculated at runtime, instead write a special value
you can easily recognize.  If you write the predicate in parentheses but
don't follow it with a default value, @code{#f} is used as the default.
Default values are not verified with @emph{predicate?} at either
definition or run time: it is your responsibility to deal with the
values you specify.  Default values that happen to be music expressions
are copied while setting @code{origin} to the @code{location} parameter.

@item @code{@var{body}}
@tab A sequence of Scheme forms evaluated in order, the last one being
used as the return value of the scheme function.  It may contain
LilyPond code blocks enclosed in hashed braces
(@tie{}@w{@code{#@{@dots{}#@}}}@tie{}), like described in @ref{Lilypond
code blocks}.  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)}}).  If your function returns a music expression, it is cloned and
given the correct @code{origin}.
@end multitable

@noindent
Some type predicates are specially recognized by the parser and will
make the parser look for the respective arguments in LilyPond syntax
rather than in Scheme syntax.  Currently these are @code{ly:music?},
@code{markup?}, @code{ly:pitch?}, and @code{ly:duration?}.

If you really want to input one of the special items as a Scheme rather
than a LilyPond expression, you may write them as a Scheme expression
that calls @code{ly:export} at its outermost level.

Other type predicates, including user-defined ones, will make the
respective argument only be accepted as a Scheme expression, usually
introduced with @code{#} or as the result of calling a scheme function
itself.

For a list of available type predicates, see
@ruser{Predefined type predicates}.

@seealso

Notation Reference:
@ruser{Predefined type predicates}.

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

@node Scheme function usage
@subsection Scheme function usage

Scheme functions can be called pretty much anywhere where a Scheme
expression starting with @code{#} can be written.  You call a scheme
function by writing its name preceded by @code{\}, followed by its
arguments.  The last argument can't be an optional argument.  If there
are several optional arguments in a row, they are filled with values
left to right.  Once an optional argument can't match input, it and all
immediately following optional arguments are replaced with their default
values, and the matching continues with the next non-optional argument.

Apart from places where a Scheme value is required, there are a few
places where @code{#} expressions are accepted and evaluated for their
side effects but otherwise ignored.  Mostly those are the places where
an assignment would be acceptable as well.

There are a few special places where an argument matching
@code{ly:music?} has to be either a music identifier or a music
expression enclosed in @code{@{}@dots{}@code{@}} or
@code{<<}@dots{}@code{>>} explicitly, so that possibly following
optional durations or postevents can't be confused with additional
arguments.  One obvious place is before a @code{ly:duration?}
predicate.  Another is as the last argument of a scheme function when it
is used in a place where such optional parts could be considered either
part of the music argument or not.

In those rare cases, you have to delimit your music arguments
appropriately to spare LilyPond from getting confused.

@node Void scheme functions
@subsection Void scheme functions
@funindex define-void-function

Sometimes a procedure is executed in order to perform an action rather
than return a value.  Some programming languages (like C and Scheme)
use functions for either concept and just discard the returned value
(usually by allowing any expression to act as statement, ignoring the
result).  This is clever but error-prone: most C compilers nowadays
offer warnings for various non-``void'' expressions being discarded.
For many functions executing an action, the Scheme standards declare
the return value to be unspecified.  Lilypond's Scheme interpreter
Guile has a unique ``unspecified'' value that it usually (such when
using @code{set!} directly on a variable) but unfortunately not
consistently returns in such cases.

Defining a Lilypond function with @code{define-void-function} makes
sure that this special value (the only value satisfying the predicate
@code{void?}) will be returned.

@example
noPointAndClick =
#(define-void-function
     (parser location)
     ()
   (ly:set-option 'point-and-click #f))
...
\noPointAndClick   % disable point and click
@end example

@node Music functions
@section Music functions

@cindex 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 definitions::
* Music function usage::
* Simple substitution functions::
* Intermediate substitution functions::
* Mathematics in functions::
* Functions without arguments::
* Void music functions::
@end menu


@node Music function definitions
@subsection Music function definitions
@cindex defining music functions
@funindex define-music-function

The general form for defining music functions is:

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

@noindent
quite in analogy to @ref{Scheme function definitions}.  More often than
not, @var{body} will be a @ref{Lilypond code blocks, Lilypond code block}.

For a list of available type predicates, see
@ruser{Predefined type predicates}.

@seealso

Notation Reference:
@ruser{Predefined type predicates}.

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


@node Music function usage
@subsection Music function usage
Music functions may currently be used in three places.  Depending on
where they are used, restrictions apply in order to be able to parse
them unambiguously.  The result a music function returns must be
compatible with the context in which it is called.

@itemize
@item
At top level in a music expression.  There are no special restrictions
on the argument list.

@item
As a post-event, explicitly started with a direction indicator (one of
@code{-}, @code{^}, @w{and @code{_}}).  All trailing arguments of the
music function with the predicate @code{ly:music?} will get parsed also
as post-events (if the last argument is a scheme function, this will
hold for trailing @code{ly:music?} arguments of the scheme function
instead).  Note that returning a post-event will be acceptable for music
functions called as normal music, leading to a result roughly equivalent
to
@example
s 1*0-\fun
@end example

@item
As a chord constituent.  All trailing arguments of the music function
with the predicate @code{ly:music?} will get parsed also as chord
constituents.
@end itemize

@noindent
The special rules for trailing arguments make it possible to write
polymorphic functions like @code{\tweak} that can be applied to
different constructs.

@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 @emph{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 music functions
@subsection Void music functions

A music function must return a music expression.  If you want to
execute a function only for its side effect, you should use
@code{define-void-function}.  But there may be cases where you
sometimes want to produce a music expression, and sometimes not (like
in the previous example).  Returning a @code{void} music expression
via @code{#@{ #@}} will achieve that.

@node Event functions
@section Event functions
@funindex define-event-function
@cindex event functions

To use a music function in the place of an event, you need to write a
direction indicator before it.  But sometimes, this does not quite match
the syntax of constructs you want to replace.  For example, if you want
to write dynamics commands, those are usually attached without direction
indicator, like @code{c'\pp}.  Here is a way to write arbitrary
dynamics:
@lilypond[quote,verbatim,raggedright]
dyn=#(define-event-function (parser location arg) (markup?)
         (make-dynamic-script arg))
\relative c' { c\dyn pfsss }
@end lilypond
You could do the same using a music function, but then you always would
have to write a direction indicator before calling it, like
@code{@w{c-\dyn pfsss}}.


@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.  Using @code{#@{
@dots{} #@}} for entering in LilyPond syntax will often be most
convenient, but we explain how to use the @code{markup} macro to get a
Scheme-only solution.

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

If the command uses properties from the @code{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 @code{layout} and @code{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

or, equivalently 

@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.  In the second case, 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

Again, the equivalent version using the markup macro would be:

@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
     #@{\markuplines \justified-lines @{ \hspace #$par-indent $args @} #@}))
@end example


The version using just Scheme is more complex:
@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 built-in markup list command
@code{\justified-lines}, which is related to the
@code{make-justified-lines-markup-list} function.  A
horizontal space is added at the beginning using @code{\hspace} (or 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

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

When interpreted, the function @code{@var{proc}} is called for
every layout object found in the context @code{@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