identifier => variable.

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

@c -*- coding: utf-8; mode: texinfo; -*-
@c This file is part of lilypond.tely
@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.  See TRANSLATION for details.
@end ignore

@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
@rlearning{Scheme tutorial}.

@menu
* Music functions::             
* Programmer interfaces::       
* Building complicated functions::  
* Markup programmer interface::  
* Contexts for programmers::    
* Scheme procedures as properties::  
@end menu


@node Music functions
@section Music functions

This section discusses how to create music functions within LilyPond.

@menu
* Overview of music functions::  
* Simple substitution functions::  
* Paired substitution functions::  
* Mathematics in functions::    
* Void functions::              
* Functions without arguments::  
* Overview of available music functions::  
@end menu

@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 @q{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 #'extra-spacing-width = #'(+inf.0 . -inf.0)
  \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 Functions without arguments
@subsection Functions without arguments

In most cases a function without arguments should be written
with an 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 lilypond with

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


@node Overview of available music functions
@subsection Overview of available music functions

@c fixme ; this should be move somewhere else?
The following commands are music functions

@include identifiers.tely



@node Programmer interfaces
@section Programmer interfaces

This section contains information about mixing LilyPond
and Scheme.

@menu
* Input variables and Scheme::  
* Internal music representation::  
@end menu


@node Input variables and Scheme
@subsection Input variables and Scheme

The input format supports the notion of variables: in the following
example, a music expression is assigned to a variable with the name
@code{traLaLa}.

@example
traLaLa = @{ c'4 d'4 @}
@end example

@noindent

There is also a form of scoping: in the following example, the
@code{\layout} block also contains a @code{traLaLa} variable, which is
independent of the outer @code{\traLaLa}.
@example
traLaLa = @{ c'4 d'4 @}
\layout @{ traLaLa = 1.0 @}
@end example
@c
In effect, each input file is a scope, and all @code{\header},
@code{\midi}, and @code{\layout} blocks are scopes nested inside that
toplevel scope.

Both variables and scoping are implemented in the GUILE module system.
An anonymous Scheme module is attached to each scope.  An assignment of
the form
@example
traLaLa = @{ c'4 d'4 @}
@end example

@noindent
is internally converted to a Scheme definition
@example
(define traLaLa @var{Scheme value of `@code{... }'})
@end example

This means that input variables and Scheme variables may be freely
mixed.  In the following example, a music fragment is stored in the
variable @code{traLaLa}, and duplicated using Scheme.  The result is
imported in a @code{\score} block by means of a second variable
@code{twice}:

@lilypond[verbatim]
traLaLa = { c'4 d'4 }

%% dummy action to deal with parser lookahead
#(display "this needs to be here, sorry!")

#(define newLa (map ly:music-deep-copy
  (list traLaLa traLaLa)))
#(define twice
  (make-sequential-music newLa))

{ \twice }
@end lilypond

Due to parser lookahead

In this example, the assignment happens after parser has verified that
nothing interesting happens after @code{traLaLa = @{ ... @}}.  Without
the dummy statement in the above example, the @code{newLa} definition
is executed before @code{traLaLa} is defined, leading to a syntax
error.

The above example shows how to @q{export} music expressions from the
input to the Scheme interpreter.  The opposite is also possible.  By
wrapping a Scheme value in the function @code{ly:export}, a Scheme
value is interpreted as if it were entered in LilyPond syntax.
Instead of defining @code{\twice}, the example above could also have
been written as
@example
@dots{}
@{ #(ly:export (make-sequential-music (list newLa))) @}
@end example

Scheme code is evaluated as soon as the parser encounters it.  To
define some Scheme code in a macro (to be called later), use
@ref{Void functions}, or

@example
#(define (nopc)
  (ly:set-option 'point-and-click #f))

...
#(nopc)
@{ c'4 @}
@end example


@refbugs

Mixing Scheme and LilyPond variables is not possible with the
@code{--safe} option.


@node Internal music representation
@subsection Internal music representation

When a music expression is parsed, it is converted into a set of
Scheme music objects.  The defining property of a music object is that
it takes up time.  Time is a rational number that measures the length
of a piece of music in whole notes.

A music object has three kinds of types:
@itemize @bullet
@item
music name: Each music expression has a name.  For example, a note
leads to a @internalsref{NoteEvent}, and @code{\simultaneous} leads to
a @internalsref{SimultaneousMusic}.  A list of all expressions
available is in the Program reference manual, under
@internalsref{Music expressions}.

@item
@q{type} or interface: Each music name has several @q{types} or
interfaces, for example, a note is an @code{event}, but it is also a
@code{note-event}, a @code{rhythmic-event}, and a
@code{melodic-event}.  All classes of music are listed in the
Program reference, under
@internalsref{Music classes}.

@item
C++ object: Each music object is represented by an object of the C++
class @code{Music}.
@end itemize

The actual information of a music expression is stored in properties.
For example, a @internalsref{NoteEvent} has @code{pitch} and
@code{duration} properties that store the pitch and duration of that
note.  A list of all properties available is in the internals manual,
under @internalsref{Music properties}.

A compound music expression is a music object that contains other
music objects in its properties.  A list of objects can be stored in
the @code{elements} property of a music object, or a single @q{child}
music object in the @code{element} object.  For example,
@internalsref{SequentialMusic} has its children in @code{elements},
and @internalsref{GraceMusic} has its single argument in
@code{element}.  The body of a repeat is stored in the @code{element}
property of @internalsref{RepeatedMusic}, and the alternatives in
@code{elements}.



@node Building complicated functions
@section Building complicated functions

This section explains how to gather the information necessary
to create complicated music functions.

@menu
* Displaying music expressions::  
* Music properties::            
* Doubling a note with slurs (example)::  
* Adding articulation to notes (example)::  
@end menu


@node Displaying music expressions
@subsection Displaying music expressions

@cindex internal storage
@funindex \displayMusic
@funindex \displayLilyMusic

When writing a music function it is often instructive to inspect how
a music expression is stored internally.  This can be done with the
music function @code{\displayMusic}

@example
@{
  \displayMusic @{ c'4\f @}
@}
@end example

@noindent
will display

@example
(make-music
  'SequentialMusic
  'elements
  (list (make-music
          'EventChord
          'elements
          (list (make-music
                  'NoteEvent
                  'duration
                  (ly:make-duration 2 0 1 1)
                  'pitch
                  (ly:make-pitch 0 0 0))
                (make-music
                  'AbsoluteDynamicEvent
                  'text
                  "f")))))
@end example

By default, LilyPond will print these messages to the console along
with all the other messages.  To split up these messages and save
the results of @code{\display@{STUFF@}}, redirect the output to
a file.

@example
lilypond file.ly >display.txt
@end example

With a bit of reformatting, the above information is
easier to read,

@example
(make-music 'SequentialMusic
  'elements (list (make-music 'EventChord
                    'elements (list (make-music 'NoteEvent
                                      'duration (ly:make-duration 2 0 1 1)
                                      'pitch (ly:make-pitch 0 0 0))
                                    (make-music 'AbsoluteDynamicEvent
                                      'text "f")))))
@end example

A @code{@{ ... @}} music sequence has the name @code{SequentialMusic},
and its inner expressions are stored as a list in its @code{'elements}
property.  A note is represented as an @code{EventChord} expression,
containing a @code{NoteEvent} object (storing the duration and
pitch properties) and any extra information (in this case, an
@code{AbsoluteDynamicEvent} with a @code{"f"} text property.


@node Music properties
@subsection Music properties

The @code{NoteEvent} object is the first object of the
@code{'elements} property of @code{someNote}.

@example
someNote = c'
\displayMusic \someNote
===>
(make-music
  'EventChord
  'elements
  (list (make-music
          'NoteEvent
          'duration
          (ly:make-duration 2 0 1 1)
          'pitch
          (ly:make-pitch 0 0 0))))
@end example

The @code{display-scheme-music} function is the function used by
@code{\displayMusic} to display the Scheme representation of a music
expression.

@example
#(display-scheme-music (first (ly:music-property someNote 'elements)))
===>
(make-music
  'NoteEvent
  'duration
  (ly:make-duration 2 0 1 1)
  'pitch
  (ly:make-pitch 0 0 0))
@end example

Then the note pitch is accessed through the @code{'pitch} property
of the @code{NoteEvent} object,

@example
#(display-scheme-music
   (ly:music-property (first (ly:music-property someNote 'elements))
                      'pitch))
===>
(ly:make-pitch 0 0 0)
@end example

The note pitch can be changed by setting this 'pitch property,

@example
#(set! (ly:music-property (first (ly:music-property someNote 'elements))
                          'pitch)
       (ly:make-pitch 0 1 0)) ;; set the pitch to d'.
\displayLilyMusic \someNote
===>
d'
@end example


@node Doubling a note with slurs (example)
@subsection Doubling a note with slurs (example)

Suppose we want to create a function which translates
input like @samp{a} into @samp{a( a)}.  We begin
by examining the internal representation of the music
we want to end up with.

@example
\displayMusic@{ a'( a') @}
===>
(make-music
  'SequentialMusic
  'elements
  (list (make-music
          'EventChord
          'elements
          (list (make-music
                  'NoteEvent
                  'duration
                  (ly:make-duration 2 0 1 1)
                  'pitch
                  (ly:make-pitch 0 5 0))
                (make-music
                  'SlurEvent
                  'span-direction
                  -1)))
        (make-music
          'EventChord
          'elements
          (list (make-music
                  'NoteEvent
                  'duration
                  (ly:make-duration 2 0 1 1)
                  'pitch
                  (ly:make-pitch 0 5 0))
                (make-music
                  'SlurEvent
                  'span-direction
                  1)))))
@end example

The bad news is that the @code{SlurEvent} expressions
must be added @q{inside} the note (or more precisely,
inside the @code{EventChord} expression).

Now we examine the input,

@example
(make-music
  'SequentialMusic
  'elements
  (list (make-music
          'EventChord
          'elements
          (list (make-music
                  'NoteEvent
                  'duration
                  (ly:make-duration 2 0 1 1)
                  'pitch
                  (ly:make-pitch 0 5 0))))))
@end example

So in our function, we need to clone this expression (so that we
have two notes to build the sequence), add @code{SlurEvents} to the
@code{'elements} property of each one, and finally make a
@code{SequentialMusic} with the two @code{EventChords}.

@example
doubleSlur = #(define-music-function (parser location note) (ly:music?)
         "Return: @{ note ( note ) @}.
         `note' is supposed to be an EventChord."
         (let ((note2 (ly:music-deep-copy note)))
           (set! (ly:music-property note 'elements)
                 (cons (make-music 'SlurEvent 'span-direction -1)
                       (ly:music-property note 'elements)))
           (set! (ly:music-property note2 'elements)
                 (cons (make-music 'SlurEvent 'span-direction 1)
                       (ly:music-property note2 'elements)))
           (make-music 'SequentialMusic 'elements (list note note2))))
@end example


@node Adding articulation to notes (example)
@subsection Adding articulation to notes (example)

The easy way to add articulation to notes is to merge two music
expressions into one context, as explained in
@ref{Creating contexts}.  However, suppose that we want to write
a music function which does this.

A @code{$variable} inside the @code{#@{...#@}} notation is like
using a regular @code{\variable} in classical LilyPond
notation.  We know that

@example
@{ \music -. -> @}
@end example

@noindent
will not work in LilyPond.  We could avoid this problem by attaching
the articulation to a fake note,

@example
@{ << \music s1*0-.-> @}
@end example

@noindent
but for the sake of this example, we will learn how to do this in
Scheme.  We begin by examining our input and desired output,

@example
%  input
\displayMusic c4
===>
(make-music
  'EventChord
  'elements
  (list (make-music
          'NoteEvent
          'duration
          (ly:make-duration 2 0 1 1)
          'pitch
          (ly:make-pitch -1 0 0))))
=====
%  desired output
\displayMusic c4->
===>
(make-music
  'EventChord
  'elements
  (list (make-music
          'NoteEvent
          'duration
          (ly:make-duration 2 0 1 1)
          'pitch
          (ly:make-pitch -1 0 0))
        (make-music
          'ArticulationEvent
          'articulation-type
          "marcato")))
@end example

We see that a note (@code{c4}) is represented as an @code{EventChord}
expression, with a @code{NoteEvent} expression in its elements list.  To
add a marcato articulation, an @code{ArticulationEvent} expression must
be added to the elements property of the @code{EventChord}
expression.

To build this function, we begin with

@example
(define (add-marcato event-chord)
  "Add a marcato ArticulationEvent to the elements of `event-chord',
  which is supposed to be an EventChord expression."
  (let ((result-event-chord (ly:music-deep-copy event-chord)))
    (set! (ly:music-property result-event-chord 'elements)
          (cons (make-music 'ArticulationEvent
                  'articulation-type "marcato")
                (ly:music-property result-event-chord 'elements)))
    result-event-chord))
@end example

The first line is the way to define a function in Scheme: the function
name is @code{add-marcato}, and has one variable called
@code{event-chord}.  In Scheme, the type of variable is often clear
from its name.  (this is good practice in other programming languages,
too!)

@example
"Add a marcato..."
@end example

@noindent
is a description of what the function does.  This is not strictly
necessary, but just like clear variable names, it is good practice.

@example
(let ((result-event-chord (ly:music-deep-copy event-chord)))
@end example

@code{let} is used to declare local variables.  Here we use one local
variable, named @code{result-event-chord}, to which we give the value
@code{(ly:music-deep-copy event-chord)}.  @code{ly:music-deep-copy} is
a function specific to LilyPond, like all functions prefixed by
@code{ly:}.  It is use to make a copy of a music
expression.  Here we copy @code{event-chord} (the parameter of the
function).  Recall that our purpose is to add a marcato to an
@code{EventChord} expression.  It is better to not modify the
@code{EventChord} which was given as an argument, because it may be
used elsewhere.

Now we have a @code{result-event-chord}, which is a
@code{NoteEventChord} expression and is a copy of @code{event-chord}.  We
add the marcato to its elements list property.

@example
(set! place new-value)
@end example

Here, what we want to set (the @q{place}) is the @q{elements} property of
@code{result-event-chord} expression.

@example
(ly:music-property result-event-chord 'elements)
@end example

@code{ly:music-property} is the function used to access music properties
(the @code{'elements}, @code{'duration}, @code{'pitch}, etc, that we
see in the @code{\displayMusic} output above).  The new value is the
former elements property, with an extra item: the
@code{ArticulationEvent} expression, which we copy from the
@code{\displayMusic} output,

@example
(cons (make-music 'ArticulationEvent
        'articulation-type "marcato")
      (ly:music-property result-event-chord 'elements))
@end example

@code{cons} is used to add an element to a list without modifying the
original list.  This is what we
want: the same list as before, plus the new @code{ArticulationEvent}
expression.  The order inside the elements property is not important here.

Finally, once we have added the marcato articulation to its @code{elements}
property, we can return @code{result-event-chord}, hence the last line of
the function.

Now we transform the @code{add-marcato} function into a music
function,

@example
addMarcato = #(define-music-function (parser location event-chord)
                                     (ly:music?)
    "Add a marcato ArticulationEvent to the elements of `event-chord',
    which is supposed to be an EventChord expression."
    (let ((result-event-chord (ly:music-deep-copy event-chord)))
      (set! (ly:music-property result-event-chord 'elements)
            (cons (make-music 'ArticulationEvent
                    'articulation-type "marcato")
                  (ly:music-property result-event-chord 'elements)))
      result-event-chord))
@end example

We may verify that this music function works correctly,

@example
\displayMusic \addMarcato c4
@end example


@node Markup programmer interface
@section Markup programmer interface

Markups are implemented as special Scheme functions which produce a
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")
                  #:bigger #:line ("foo" "bar" "baz")))
@end example

@noindent
is equivalent to:
@example
\markup \column @{ \line @{ \bold \italic "hello" \raise #0.4 "world" @}
                  \bigger \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{\command} @tab @code{#:command}
@item @code{\variable} @tab @code{variable}
@item @code{\center-align @{ ... @}} @tab @code{#:center-align ( ... )}
@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}).


@refbugs

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

New markup commands can be defined
with the @code{define-markup-command} Scheme macro.

@lisp
(define-markup-command (@var{command-name} @var{layout} @var{props} @var{arg1} @var{arg2} ...)
            (@var{arg1-type?} @var{arg2-type?} ...)
  ..command body..)
@end lisp

The arguments are

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

As a simple example, we show how to add a @code{\smallcaps} command,
which selects a small caps font.  Normally we could select the
small caps font,

@example
\markup @{ \override #'(font-shape . caps) Text-in-caps @}
@end example

@noindent
This selects the caps font by setting the @code{font-shape} property to
@code{#'caps} for interpreting @code{Text-in-caps}.

To make the above available as @code{\smallcaps} command, we must
define a function using @code{define-markup-command}.  The command should
take a single argument of type @code{markup}.  Therefore the start of the
definition should read

@example
(define-markup-command (smallcaps layout props argument) (markup?)
@end example

@noindent

What follows is the content of the command: we should interpret
the @code{argument} as a markup, i.e.,

@example
(interpret-markup layout @dots{} argument)
@end example

@noindent
This interpretation should add @code{'(font-shape . caps)} to the active
properties, so we substitute the following for the @dots{} in the
above example:

@example
(cons (list '(font-shape . caps) ) props)
@end example

@noindent
The variable @code{props} is a list of alists, and we prepend to it by
cons'ing a list with the extra setting.


Suppose that we are typesetting a recitative in an opera and
we would like to define a command that will show character names in a
custom manner.  Names should be printed with small caps and moved a
bit to the left and top.  We will define a @code{\character} command
which takes into account the necessary translation and uses the newly
defined @code{\smallcaps} command:

@example
#(define-markup-command (character layout props name) (string?)
  "Print the character name in small caps, translated to the left and
  top.  Syntax: \\character #\"name\""
  (interpret-markup layout props
   (markup #:hspace 0 #:translate (cons -3 1) #:smallcaps name)))
@end example

There is one complication that needs explanation: texts above and below
the staff are moved vertically to be at a certain distance (the
@code{padding} property) from the staff and the notes.  To make sure
that this mechanism does not annihilate the vertical effect of our
@code{#:translate}, we add an empty string (@code{#:hspace 0}) before the
translated text.  Now the @code{#:hspace 0} will be put above the notes,
and the
@code{name} is moved in relation to that empty string.  The net effect is
that the text is moved to the upper left.

The final result is as follows:

@example
@{
  c''^\markup \character #"Cleopatra"
  e'^\markup \character #"Giulio Cesare"
@}
@end example

@lilypond[quote,ragged-right]
#(define-markup-command (smallcaps layout props str) (string?)
  "Print the string argument in small caps.  Syntax: \\smallcaps #\"string\""
  (interpret-markup layout props
   (make-line-markup
    (map (lambda (s)
          (if (= (string-length s) 0)
              s
              (markup #:large (string-upcase (substring s 0 1))
                      #:translate (cons -0.6 0)
                      #:tiny (string-upcase (substring s 1)))))
         (string-split str #\Space)))))

#(define-markup-command (character layout props name) (string?)
  "Print the character name in small caps, translated to the left and
  top.  Syntax: \\character #\"name\""
  (interpret-markup layout props
   (markup #:hspace 0 #:translate (cons -3 1) #:smallcaps name)))

{
  c''^\markup \character #"Cleopatra" c'' c'' c''
  e'^\markup \character #"Giulio Cesare" e' e' e'
}
@end lilypond

We have used the @code{caps} font shape, but suppose that our font
does not have a small-caps variant.  In that case we have to fake
the small caps font by setting a string in upcase with the first
letter a little larger:

@example
#(define-markup-command (smallcaps layout props str) (string?)
  "Print the string argument in small caps."
  (interpret-markup layout props
   (make-line-markup
    (map (lambda (s)
          (if (= (string-length s) 0)
              s
              (markup #:large (string-upcase (substring s 0 1))
                      #:translate (cons -0.6 0)
                      #:tiny (string-upcase (substring s 1)))))
         (string-split str #\Space)))))
@end example

The @code{smallcaps} command first splits its string argument into
tokens separated by spaces (@code{(string-split str #\Space)}); for
each token, a markup is built with the first letter made large and
upcased (@code{#:large (string-upcase (substring s 0 1))}), and a
second markup built with the following letters made tiny and upcased
(@code{#:tiny (string-upcase (substring s 1))}).  As LilyPond
introduces a space between markups on a line, the second markup is
translated to the left (@code{#:translate (cons -0.6 0) ...}).  Then,
the markups built for each token are put in a line by
@code{(make-line-markup ...)}.  Finally, the resulting markup is passed
to the @code{interpret-markup} function, with the @code{layout} and
@code{props} arguments.

Note: there is now an internal command @code{\smallCaps} which can
be used to set text in small caps.  See
@ref{Overview of text markup commands}, for details.


@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 later returns
a single stencil, the former returns a list 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?)
   (let ((indent (chain-assoc-get 'par-indent props 2)))
     (interpret-markup-list layout props 
       (make-justified-lines-markup-list (cons (make-hspace-markup 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.  An horizontal space is added at the
begining 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 taking a single argument,
being the context to apply it to.  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}.  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}, with the following
arguments:
@itemize @bullet
@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
@internalsref{NoteHead} event, and for a @internalsref{Stem} object,
this is a @internalsref{NoteHead} object.

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

@example
(define (blanker grob grob-origin context)
 (if (and (memq (ly:grob-property grob 'interfaces)
                note-head-interface)
          (eq? (ly:grob-property grob 'staff-position) 0))
     (set! (ly:grob-property grob 'transparent) #t)))
@end example


@node Scheme procedures as properties
@section Scheme procedures as properties

Properties (like thickness, direction, etc.) can be set at fixed values
with \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.