rename to music-function

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

@node Interfaces for programmers
@appendix Interfaces for programmers



@menu
* Programmer interfaces for input ::  
* Markup programmer interface::  
* Contexts for programmers::    
@end menu

@node Programmer interfaces for input 
@appendixsec Programmer interfaces for input 

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

@node Input variables and Scheme
@appendixsubsec Input variables and Scheme


The input format supports the notion of variable: in the following
example, a music expression is assigned to a variable with the name
@code{traLaLa}.
@example
  traLaLa = \notes @{ c'4 d'4 @}
@end example

@noindent

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

@noindent
is internally converted to a Scheme definition
@example
 (define traLaLa @var{Scheme value of ``@code{\notes ... }''})
@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} by means of a second variable
@code{twice}:
@example
  traLaLa = \notes @{ c'4 d'4 @}

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

  \score @{ \twice @}
@end example

In the above example, music expressions can be `exported' 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{}
  \score @{ #(ly:export (make-sequential-music newLa)) @}
@end example


@node Internal music representation
@appendixsubsec 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 internals manual, under @internalsref{Music
expressions}.

@item
  `type' or interface: Each music name has several `types' or interface,
  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 internals manual, 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.
@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 `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 in @code{element} property of
@internalsref{RepeatedMusic}, and the alternatives in @code{elements}.




@node Extending music syntax
@appendixsubsec Extending music syntax

The syntax of composite music expressions, like
@code{\repeat}, @code{\transpose} and @code{\context}
follows the general form of

@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 @em{music function}. This is a specially marked
Scheme function. For example, the music function @code{\apply} applies
a user-defined function to a music expression.  Its syntax is

@example
\apply #@var{func} @var{music}
@end example

A music function is created with @code{ly:make-music-function}.



@node Manipulating music expressions
@appendixsubsec Manipulating music expressions

Music objects and their properties can be accessed and manipulated
directly, through the @code{\apply} mechanism.
The syntax for @code{\apply} is
@example
\apply #@var{func} @var{music}
@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[verbatim,raggedright]
  #(define (rev-music-1 m)
     (ly:music-set-property! m 'elements (reverse
       (ly:music-property m 'elements)))
     m)
  \score { \notes \apply #rev-music-1 { c4 d4 } }
@end lilypond

The use of such a function is very limited. The effect of this
function is void when applied to an argument which is does not have
multiple children.  The following function application has no effect:

@example
  \apply #rev-music-1 \grace @{ c4 d4 @}
@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{\apply} 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.
@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))
@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

@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 @} >>
@end example

Other applications of @code{\apply} 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  (@inputfileref{input/test,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 Markup programmer interface
@appendixsec Markup programmer interface


@menu
* Markup construction in scheme::  
* Markup command definition::   
@end menu

@node Markup construction in scheme
@appendixsubsec 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 < @{ \bold \italic "hello" \raise #0.4 "world" @}
                  \bigger @{ foo bar baz @} >
@end example

@noindent
This example exposes the main translation rules between regular
LilyPond markup syntax and scheme markup syntax, which are summed up
is this table:
@multitable @columnfractions .5 .5
@item @b{LilyPond} @tab @b{Scheme}
@item @code{\command} @tab @code{#:command}
@item @code{\variable} @tab @code{variable}
@item @code{@{ ... @}} @tab @code{#:line ( ... )}
@item @code{\center-align < ... >} @tab @code{#:center ( ... )}
@item @code{string} @tab @code{"string"}
@item @code{#scheme-arg} @tab @code{scheme-arg}
@end multitable

Besides, the whole scheme language is accessible inside the
@code{markup} macro: thus, one may use function calls inside
@code{markup} in order to manipulate character strings for
instance. This proves useful when defining new markup commands (see
@ref{Markup command definition}).

@refbugs

One can not feed the @code{#:line} (resp @code{#:center},
@code{#:column}) command with a variable or the result of a function
call. E.g.:
@lisp
(markup #:line (fun-that-returns-markups))
@end lisp
is illegal. One should use the @code{make-line-markup} (resp
@code{make-center-markup}, @code{make-column-markup}) function
instead:
@lisp
(markup (make-line-markup (fun-that-returns-markups)))
@end lisp

@node Markup command definition
@appendixsubsec Markup command definition

New markup commands can be defined
with  the @code{def-markup-command} scheme macro.
@lisp
(def-markup-command (@var{command-name} @var{paper} @var{props} @var{arg1} @var{arg2} ...)
            (@var{arg1-type?} @var{arg2-type?} ...)
  ..command body..)
@end lisp

The arguments signify

@table @var
@item argi
@var{i}th command argument
@item argi-type?
a type predicate for the i@var{th} argument
@item paper
the `paper' 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 @TeX{}'s small caps font.  Normally, we could select the
small caps font as follows:

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

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 have to
define a function using @code{def-markup-command}. The command should
take a single argument, of markup type. Therefore, the start of the
definition should read
@example
  (def-markup-command (smallcaps paper 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 paper  @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
consing 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 translated a
bit to the left and top.  We will define a @code{\character} command
that takes into account the needed translation, and uses the newly
defined @code{\smallcaps} command:

@verbatim
#(def-markup-command (character paper props name) (string?)
   "Print the character name in small caps, translated to the left and
   top. Syntax: \\character #\"name\""
   (interpret-markup paper props 
    (markup "" #:translate (cons -4 2) #:smallcaps name)))
@end verbatim

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{""}) before the
translated text.  Now the @code{""} 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:
@verbatim
\score {
    \notes { \fatText
        c''^\markup \character #"Cleopatra"
        e'^\markup \character #"Giulio Cesare"
    }
}
@end verbatim

@lilypond[raggedright]
#(def-markup-command (smallcaps paper props str) (string?)
   "Print the string argument in small caps. Syntax: \\smallcaps #\"string\""
   (interpret-markup paper 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)))))

#(def-markup-command (character paper props name) (string?)
   "Print the character name in small caps, translated to the left and
   top. Syntax: \\character #\"name\""
   (interpret-markup paper props 
    (markup "" #:translate (cons -4 0) #:smallcaps name)))

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

We have used the @code{caps} font shape, but suppose that our font
that 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
#(def-markup-command (smallcaps paper props str) (string?)
   "Print the string argument in small caps."
   (interpret-markup paper 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{paper} and
@code{props} arguments.



@node Contexts for programmers
@appendixsec Contexts for programmers


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

@node Context evaluation
@appendixsubsec Context evaluation

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
@appendixsubsec Running a function on all layout objects

The most versatile way of tuning an object is @code{\applyoutput}. Its
syntax is
@example
\applyoutput @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
@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