X-Git-Url: https://git.donarmstrong.com/?a=blobdiff_plain;f=Documentation%2Fuser%2Fprogramming-interface.itely;h=3219124d4e08d5a5fefcedba2cd352e702c09d15;hb=2eb63c6977706ba261e4ee0674c734a34b3ee904;hp=753ce05783d8e64f57039084509e13f0ab8088bc;hpb=89fcf03ab65edafeb78957e56923e8b06e97a863;p=lilypond.git

diff --git a/Documentation/user/programming-interface.itely b/Documentation/user/programming-interface.itely
index 753ce05783..3219124d4e 100644
--- a/Documentation/user/programming-interface.itely
+++ b/Documentation/user/programming-interface.itely
@@ -1,39 +1,310 @@
 @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
-@ref{Scheme tutorial}.
+@rlearning{Scheme tutorial}.
 
 @menu
-* Programmer interfaces for input::  
+* 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
 
-@node Programmer interfaces for input
-@section Programmer interfaces for input
+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.
 
-This section discusses how to create functions within LilyPond.
+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::  
-* Extending music syntax::      
-* Manipulating music expressions::  
-* Displaying music expressions::  
-* Using LilyPond syntax inside Scheme::  
 @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
@@ -62,7 +333,7 @@ traLaLa = @{ c'4 d'4 @}
 @noindent
 is internally converted to a Scheme definition
 @example
-(define traLaLa @var{Scheme value of ``@code{... }''})
+(define traLaLa @var{Scheme value of `@code{... }'})
 @end example
 
 This means that input variables and Scheme variables may be freely
@@ -70,30 +341,43 @@ 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}:
-@example
-traLaLa = @{ c'4 d'4 @}
+
+@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 example
+{ \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.
 
-In the above example, music expressions can be `exported' from the
+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
+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
+define some Scheme code in a macro (to be called later), use
+@ref{Void functions}, or
 
 @example
 #(define (nopc)
@@ -105,9 +389,9 @@ define some scheme code in a macro (to be called later), use
 @end example
 
 
-@refbugs
+@knownissues
 
-Mixing Scheme and LilyPond identifiers is not possible with the
+Mixing Scheme and LilyPond variables is not possible with the
 @code{--safe} option.
 
 
@@ -120,29 +404,25 @@ 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
+@itemize
 @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
+available is in the Internals Reference manual, under
 @internalsref{Music expressions}.
 
 @item
-`type' or interface: Each music name has several `types' or
+@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
-Profram reference, under
+Internals Reference, under
 @internalsref{Music classes}.
 
 @item
-C++ object: Each music object is represented by a C++ object.  For
-technical reasons, different music objects may be represented by
-different C++ object types.  For example, a note is @code{Event}
-object, while @code{\grace} creates a @code{Grace_music} object.  We
-expect that distinctions between different C++ types will disappear
-in the future.
+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.
@@ -153,7 +433,7 @@ 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 `child'
+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
@@ -163,193 +443,195 @@ property of @internalsref{RepeatedMusic}, and the alternatives in
 
 
 
+@node Building complicated functions
+@section Building complicated functions
 
-@node Extending music syntax
-@subsection Extending music syntax
-
-@c TODO: rewrite example.
-@c The use of FUNC as example  argument is rather confusing.
-
-The syntax of composite music expressions, like @code{\repeat},
-@code{\transpose}, and @code{\context} follows the general form of
+This section explains how to gather the information necessary
+to create complicated music functions.
 
-@example
-\@code{keyword} @var{non-music-arguments} @var{music-arguments}
-@end example
-
-Such syntax can also be defined as user code.  To do this it is
-necessary to create a @emph{music function}.  This is a specially marked
-Scheme function.  For example, the music function @code{\applyMusic} applies
-a user-defined function to a music expression.  Its syntax is
+@menu
+* Displaying music expressions::  
+* Music properties::            
+* Doubling a note with slurs (example)::  
+* Adding articulation to notes (example)::  
+@end menu
 
-@example
-\applyMusic #@var{func} @var{music}
-@end example
 
-A music function is created with @code{ly:make-music-function},
+@node Displaying music expressions
+@subsection Displaying music expressions
 
-@example
-(ly:make-music-function
-@end example
+@cindex internal storage
+@funindex \displayMusic
+@funindex \displayLilyMusic
 
-@code{\applyMusic} takes a Scheme function and a Music expression as
-arguments.  This is encoded in its parameter list,
+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
-(list procedure? ly:music?)
+@{
+  \displayMusic @{ c'4\f @}
+@}
 @end example
 
-The function itself takes another argument: an Input location
-object.  That object is used to provide error messages with file names
-and line numbers.  The definition is the second argument of
-@code{ly:make-music-function}.  The body simply calls the function
+@noindent
+will display
 
 @example
-(lambda (where func music)
- (func music))
+(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
 
-The above Scheme code only defines the functionality.  The tag
-@code{\applyMusic} is selected by defining
+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
-applyMusic = #(ly:make-music-function
-                (list procedure? ly:music?)
-                (lambda (parser location func music)
-                  (func music)))
+lilypond file.ly >display.txt
 @end example
 
-A @code{define-music-function} macro is introduced on top of
-@code{ly:make-music-function} to ease the definition of music
-functions:
+With a bit of reformatting, the above information is
+easier to read,
 
 @example
-applyMusic = #(define-music-function (parser location func music)
-                (procedure? ly:music?)
-                (func music))
+(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
 
-Examples of the use of @code{\applyMusic} are in the next section.
-
-@seealso
-@file{ly/@/music@/-functions@/-init@/.ly}.
+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 Manipulating music expressions
-@subsection Manipulating music expressions
+@node Music properties
+@subsection Music properties
 
-Music objects and their properties can be accessed and manipulated
-directly through the @code{\applyMusic} mechanism.
-The syntax for @code{\applyMusic} is
+The @code{NoteEvent} object is the first object of the
+@code{'elements} property of @code{someNote}.
 
 @example
-\applyMusic #@var{func} @var{music}
+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
 
-@noindent
-This means that the Scheme function @var{func} is called with
-@var{music} as its argument.  The return value of @var{func} is the
-result of the entire expression.  @var{func} may read and write music
-properties using the functions @code{ly:music-property} and
-@code{ly:music-set-property!}.
-
-An example is a function that reverses the order of elements in
-its argument,
-@lilypond[quote,verbatim,ragged-right]
-#(define (rev-music-1 m)
-  (ly:music-set-property! m 'elements 
-    (reverse (ly:music-property m 'elements)))
-  m)
-
-\applyMusic #rev-music-1 { c'4 d'4 } 
-@end lilypond
-
-The use of such a function is very limited.  The effect of this
-function is void when applied to an argument that does not have
-multiple children.  The following function application has no effect
+The @code{display-scheme-music} function is the function used by
+@code{\displayMusic} to display the Scheme representation of a music
+expression.
 
 @example
-\applyMusic #rev-music-1 \grace @{ c4 d4 @}
+#(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
 
-@noindent
-In this case, @code{\grace} is stored as @internalsref{GraceMusic}, which
-has no @code{elements}, only a single @code{element}.  Every generally
-applicable function for @code{\applyMusic} must -- like music expressions
-themselves -- be recursive.
-
-The following example is such a recursive function: It first extracts
-the @code{elements} of an expression, reverses them and puts them
-back.  Then it recurses, both on @code{elements} and @code{element}
-children.
+Then the note pitch is accessed through the @code{'pitch} property
+of the @code{NoteEvent} object,
 
 @example
-#(define (reverse-music music)
-  (let* ((elements (ly:music-property music 'elements))
-         (child (ly:music-property music 'element))
-         (reversed (reverse elements)))
-
-    ; set children
-    (ly:music-set-property! music 'elements reversed)
-
-    ; recurse
-    (if (ly:music? child) (reverse-music child))
-    (map reverse-music reversed)
-
-    music))
+#(display-scheme-music
+   (ly:music-property (first (ly:music-property someNote 'elements))
+                      'pitch))
+===>
+(ly:make-pitch 0 0 0)
 @end example
 
-A slightly more elaborate example is in
-@inputfileref{input/@/test,reverse@/-music@/.ly}.
-
-Some of the input syntax is also implemented as recursive music
-functions.  For example, the syntax for polyphony
-@example
-<<a \\ b>>
-@end example
+The note pitch can be changed by setting this 'pitch property,
 
-@noindent
-is actually implemented as a recursive function that replaces the
-above by the internal equivalent of
 @example
-<< \context Voice = "1" @{ \voiceOne a @}
-   \context Voice = "2" @{ \voiceTwo b @} >>
+#(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
 
-Other applications of @code{\applyMusic} are writing out repeats
-automatically (@inputfileref{input/@/test,unfold@/-all@/-repeats@/.ly}),
-saving keystrokes (@inputfileref{input/@/test,music@/-box@/.ly}) and
-exporting LilyPond input to other formats
-@c no @inputfileref{} here 
-(eg. @file{input/@/no@/-notation/@/to@/-xml@/.ly}).
-
-@seealso
-
-@file{scm/@/music@/-functions@/.scm}, @file{scm/@/music@/-types@/.scm},
-@inputfileref{input/@/test,add@/-staccato@/.ly},
-@inputfileref{input/@/test,unfold@/-all@/-repeats@/.ly}, and
-@inputfileref{input/@/test,music@/-box@/.ly}.
-
-
-@node Displaying music expressions
-@subsection Displaying music expressions
 
-@cindex internal storage
-@cindex @code{\displayMusic}
-@cindex @code{\displayLilyMusic}
+@node Doubling a note with slurs (example)
+@subsection Doubling a note with slurs (example)
 
-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}
+Suppose we want to create a function which translates
+input like @code{a} into @code{a( a)}.  We begin
+by examining the internal representation of the music
+we want to end up with.
 
 @example
-@{
-  \displayMusic @{ c'4\f @}
-@}
+\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
 
-@noindent
-will display
+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
@@ -363,181 +645,216 @@ will display
                   'duration
                   (ly:make-duration 2 0 1 1)
                   'pitch
-                  (ly:make-pitch 0 0 0))
-                (make-music
-                  'AbsoluteDynamicEvent
-                  'text
-                  "f")))))
+                  (ly:make-pitch 0 5 0))))))
 @end example
 
-Displaying a music expression in LilyPond notation can be
-done using the music function @code{\displayLilyMusic}.  For 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
-@{
-  \displayLilyMusic \transpose c a, @{ c e g a bes @}
-@}
+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
 
-will display
+
+@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
-@{ a, cis e fis g @}
+@{ \music -. -> @}
 @end example
 
+@noindent
+will not work in LilyPond.  We could avoid this problem by attaching
+the articulation to a fake note,
 
-@node Using LilyPond syntax inside Scheme
-@subsection Using LilyPond syntax inside Scheme
+@example
+@{ << \music s1*0-.-> @}
+@end example
 
-Creating music expressions in Scheme can be tedious, as they are
-heavily nested and the resulting Scheme code is large.  For some
-simple tasks this can be avoided by using common LilyPond syntax inside
-Scheme, with the dedicated @code{#@{ ... #@}} syntax.
+@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,
 
-The following two expressions give equivalent music expressions:
 @example
-mynotes = @{ \override Stem #'thickness = #4
-            @{ c'8 d' @} @}
-  
-#(define mynotes #@{ \override Stem #'thickness = #4
-                    @{ c'8 d' @} #@})
+%  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
 
-The content of @code{#@{ ... #@}} is enclosed in an implicit @code{@{
-... @}} block, which is parsed.  The resulting music expression, a
-@code{SequentialMusic} music object, is then returned and usable in Scheme.
+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.
 
-Arbitrary Scheme forms, including variables, can be used in @code{#@{ ... #@}}
-expressions with the @code{$} character (@code{$$} can be used to
-produce a single @code{$} character).  This makes the creation of simple
-functions straightforward.  In the following example, a function
-setting the TextScript's padding is defined:
+To build this function, we begin with
 
-@lilypond[quote,verbatim,ragged-right]
-#(use-modules (ice-9 optargs))
-#(define* (textpad padding #:optional once?)
-  (ly:export   ; this is necessary for using the expression
-               ; directly inside a block
-    (if once?
-        #{ \once \override TextScript #'padding = #$padding #}
-        #{ \override TextScript #'padding = #$padding #})))
-
- {
-   c'^"1"
-   #(textpad 3.0 #t) % only once
-   c'^"2"
-   c'^"3"
-   #(textpad 5.0)
-   c'^"4"
-   c'^"5"
- }
-@end lilypond
+@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
 
-Here, the variable @code{padding} is a number; music expression
-variables may also be used in a similar fashion, as in the following
-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!)
 
-@lilypond[quote,verbatim,ragged-right]
-#(define (with-padding padding)
-  (lambda (music)
-   #{ \override TextScript #'padding = #$padding
-      $music
-      \revert TextScript #'padding #}))
+@example
+"Add a marcato..."
+@end example
 
-{
-  c'^"1"
-  \applyMusic #(with-padding 3) { c'^"2" c'^"3" }
-  c'^"4"
-}
-@end lilypond
+@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
 
-The function created by @code{(with-padding 3)} adds @code{\override} and
-@code{\revert} statements around the music given as an argument, and returns
-this new expression.  Thus, this example is equivalent to:
+@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
-@{
-  c'^"1"
-  @{ \override TextScript #'padding = #3
-    @{ c'^"2" c'^"3"@}
-    \revert TextScript #'padding
-  @}
-  c'^"4"
-@}
+(set! place new-value)
 @end example
 
-This function may also be defined as a music function:
+Here, what we want to set (the @q{place}) is the @q{elements} property of
+@code{result-event-chord} expression.
 
-@lilypond[quote,verbatim,ragged-right]
-withPadding =
-  #(define-music-function (parser location padding music) (number? ly:music?)
-    #{ \override TextScript #'padding = #$padding
-       $music 
-       \revert TextScript #'padding #})
+@example
+(ly:music-property result-event-chord 'elements)
+@end example
 
-{
-  c'^"1"
-  \withPadding #3 { c'^"2" c'^"3"}
-  c'^"4"
-}
-@end lilypond
+@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,
 
-Music functions can involve Scheme programming, in
-addition to simple substitution,
+@example
+(cons (make-music 'ArticulationEvent
+        'articulation-type "marcato")
+      (ly:music-property result-event-chord 'elements))
+@end example
 
-@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))) #})
+@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.
 
-AltOff = {
-  \revert Stem #'length
-  \revert NoteHead #'font-size 
-}
+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.
 
-{ c'2 \AltOn #0.5 c'4 c'
-  \AltOn #1.5 c' c' \AltOff c'2 }
-@end lilypond
+Now we transform the @code{add-marcato} function into a music
+function,
 
-@noindent
-This example may be rewritten
+@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
 
-@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 #})
+We may verify that this music function works correctly,
 
-{ c'2 \withAlt #0.5 {c'4 c'}
-  \withAlt #1.5 {c' c'} c'2 }
-@end lilypond
+@example
+\displayMusic \addMarcato c4
+@end example
 
 
 @node Markup programmer interface
 @section Markup programmer interface
 
-@c Please rewrite the second sentence; I don't understand its meaning. AS
-
-Markups are implemented as special Scheme functions.  When applied
-with as arguments an output definition (@code{\layout} or
-@code{\paper}), and a list of properties and other arguments, produce
-a Stencil object.
+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 ::  
+* 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 
+@cindex defining markup commands
 
 The @code{markup} macro builds markup expressions in Scheme while
 providing a LilyPond-like syntax.  For example,
@@ -561,7 +878,7 @@ LilyPond markup syntax and Scheme markup syntax.
 @multitable @columnfractions .3 .3
 @item @b{LilyPond} @tab @b{Scheme}
 @item @code{\markup markup1} @tab @code{(markup markup1)}
-@item @code{\markup @{ markup1 markup2 ... @}} @tab 
+@item @code{\markup @{ markup1 markup2 ... @}} @tab
         @code{(markup markup1 markup2 ... )}
 @item @code{\command} @tab @code{#:command}
 @item @code{\variable} @tab @code{variable}
@@ -571,17 +888,18 @@ LilyPond markup syntax and Scheme markup syntax.
 @end multitable
 @end quotation
 
-The whole scheme language is accessible inside the
+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
 
-One can not feed the @code{#:line}, @code{#:center}, or
-@code{#:column}) commands with a variable or the result of a function
-call.  Example:
+@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))
@@ -597,8 +915,8 @@ instead,
 @end lisp
 
 
-@node How markups work internally 
-@subsection How markups work internally 
+@node How markups work internally
+@subsection How markups work internally
 
 In a markup like
 
@@ -636,7 +954,7 @@ of this section, and in @file{scm/@/define@/-markup@/-commands@/.scm}.
 @subsection New markup command definition
 
 New markup commands can be defined
-with the @code{define-markup-command} scheme macro.
+with the @code{define-markup-command} Scheme macro.
 
 @lisp
 (define-markup-command (@var{command-name} @var{layout} @var{props} @var{arg1} @var{arg2} ...)
@@ -652,9 +970,9 @@ The arguments are
 @item argi-type?
 a type predicate for the i@var{th} argument
 @item layout
-the `layout' definition
+the @q{layout} definition
 @item props
-a list of alists, containing all active properties. 
+a list of alists, containing all active properties.
 @end table
 
 As a simple example, we show how to add a @code{\smallcaps} command,
@@ -712,7 +1030,7 @@ defined @code{\smallcaps} command:
 #(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 
+  (interpret-markup layout props
    (markup #:hspace 0 #:translate (cons -3 1) #:smallcaps name)))
 @end example
 
@@ -751,7 +1069,7 @@ The final result is as follows:
 #(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 
+  (interpret-markup layout props
    (markup #:hspace 0 #:translate (cons -3 1) #:smallcaps name)))
 
 {
@@ -794,14 +1112,63 @@ to the @code{interpret-markup} function, with the @code{layout} and
 
 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.
+@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::  
@@ -811,7 +1178,7 @@ be used to set text in small caps.  See
 @subsection Context evaluation
 
 @cindex calling code during interpreting
-@cindex @code{\applyContext}
+@funindex \applyContext
 
 Contexts can be modified during interpretation with Scheme code.  The
 syntax for this is
@@ -837,21 +1204,22 @@ current bar number on the standard output during the compile:
 
 
 @cindex calling code on layout objects
-@cindex @code{\applyOutput}
+@funindex \applyOutput
 
 
 The most versatile way of tuning an object is @code{\applyOutput}.  Its
 syntax is
 @example
-\applyOutput @var{proc}
+\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, with the following arguments:
-@itemize @bullet
+object found in the context @var{context}, 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.
@@ -875,3 +1243,73 @@ note-heads on the center-line:
      (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.