Guide, node Updating translation committishes..
@end ignore
-@c \version "2.14.0"
+@c \version "2.16.0"
@node Interfaces for programmers
@chapter Interfaces for programmers
@ref{Scheme tutorial}.
@menu
+* Lilypond code blocks::
+* Scheme functions::
* Music functions::
+* Event functions::
* Markup functions::
* Contexts for programmers::
* Callback functions::
* Difficult tweaks::
@end menu
+@node Lilypond code blocks
+@section Lilypond code blocks
+
+Lilypond code blocks look like
+@example
+ #@{ @var{Lilypond code} #@}
+@end example
+They can be used anywhere where you can write Scheme code: the Scheme
+reader actually is changed for accommodating LilyPond code blocks and
+can deal with embedded Scheme expressions starting with @code{$}
+and@w{ }@code{#}.
+
+It extracts the Lilypond code block and generates a call to the
+LilyPond @code{parser} which is executed at runtime to interpret the
+LilyPond code block. Any embedded Scheme expression is executed in
+the lexical environment of the Lilypond code block, so you have access
+to local variables and function parameters at the point the Lilypond
+code block is written.
+
+A LilyPond code block may contain anything that you can use on the right
+side of an assignment. In addition, an empty LilyPond block corresponds
+to a void music expression, and a LilyPond block containing multiple
+music events gets turned into a sequential music expression.
+
+@node Scheme functions
+@section Scheme functions
+@cindex Scheme functions (LilyPond syntax)
+
+@emph{Scheme functions} are Scheme procedures that can create Scheme
+expressions from input written in LilyPond syntax. They can be called
+in pretty much all places where using @code{#} for specifying a value in
+Scheme syntax is allowed. While Scheme has functions of its own, this
+chapter is concerned with @emph{syntactic} functions, functions that
+receive arguments specified in LilyPond syntax.
+
+@menu
+* Scheme function definitions::
+* Scheme function usage::
+* Void scheme functions::
+@end menu
+
+@node Scheme function definitions
+@subsection Scheme function definitions
+@funindex define-scheme-function
+
+The general form for defining scheme functions is:
+
+@example
+function =
+#(define-scheme-function
+ (parser location @var{arg1} @var{arg2} @dots{})
+ (@var{type1?} @var{type2?} @dots{})
+ @var{body})
+@end example
+
+@noindent
+where
+
+@multitable @columnfractions .33 .66
+@item @code{parser}
+@tab needs to be literally @code{parser} in order to give LilyPond code
+blocks (@code{#@{}@dots{}@code{#@}}) access to the parser.
+
+@item @code{@var{argN}}
+@tab @var{n}th argument
+
+@item @code{@var{typeN?}}
+@tab a Scheme @emph{type predicate} for which @code{@var{argN}}
+must return @code{#t}. Some of these predicates are specially
+recognized by the parser, see below. There is also a special form
+@code{(@emph{predicate?} @emph{default})} for specifying optional
+arguments. If the actual argument is missing when the function is being
+called, the default value is substituted instead. Default values are
+evaluated at definition time (including LilyPond code blocks!), so if
+you need a default calculated at runtime, instead write a special value
+you can easily recognize. If you write the predicate in parentheses but
+don't follow it with a default value, @code{#f} is used as the default.
+Default values are not verified with @emph{predicate?} at either
+definition or run time: it is your responsibility to deal with the
+values you specify. Default values that happen to be music expressions
+are copied while setting @code{origin} to the @code{location} parameter.
+
+@item @code{@var{body}}
+@tab A sequence of Scheme forms evaluated in order, the last one being
+used as the return value of the scheme function. It may contain
+LilyPond code blocks enclosed in hashed braces
+(@tie{}@w{@code{#@{@dots{}#@}}}@tie{}), like described in @ref{Lilypond
+code blocks}. Within LilyPond code blocks, use @code{#} to reference
+function arguments (eg., @samp{#arg1}) or to start an inline Scheme
+expression containing function arguments (eg., @w{@samp{#(cons arg1
+arg2)}}). Where normal Scheme expressions using @code{#} don't do the
+trick, you might need to revert to immediate Scheme expressions using
+@code{$}, for example as @samp{$music}.
+
+If your function returns a music expression, it is given a useful value
+of @code{origin}.
+@end multitable
+
+@noindent
+Some type predicates are specially handled by the parser since it
+can't recognize the arguments reliably otherwise. Currently these are
+@code{ly:pitch?} and @code{ly:duration?}.
+
+Suitability of arguments for all other predicates is determined by
+actually calling the predicate after Lilypond has already converted them
+into a Scheme expression. As a consequence, the argument can be
+specified in Scheme syntax if desired (introduced with @code{#} or as
+the result of calling a scheme function), but Lilypond will also convert
+a number of Lilypond constructs into Scheme before actually checking the
+predicate on them. Currently, those include music, postevents, simple
+strings (with or without quotes), numbers, full markups and markup
+lists, score, book, bookpart, context definition and output definition
+blocks.
+
+For some kinds of expression (like most music not enclosed in braces)
+Lilypond needs to look further than the expression itself in order to
+determine its end. If such an expression were considered for an
+optional argument by evaluating its predicate, Lilypond would not be
+able to ``backup'' when it decides the expression does not fit the
+parameter. So some forms of music might need to be enclosed in braces
+to make them acceptable to Lilypond. There are also some other
+ambiguities that Lilypond sorts out by checking with predicate
+functions: is @samp{-3} a fingering postevent or a negative number? Is
+@code{"a" 4} in lyric mode a string followed by a number, or a lyric
+event of duration @code{4}? Lilypond decides by asking the predicates.
+That means that you should avoid permissive predicates like
+@code{scheme?} if you have a particular use in mind instead of a general
+purpose function.
+
+For a list of available predefined type predicates, see
+@ruser{Predefined type predicates}.
+
+@seealso
+
+Notation Reference:
+@ruser{Predefined type predicates}.
+
+Installed Files:
+@file{lily/music-scheme.cc},
+@file{scm/c++.scm},
+@file{scm/lily.scm}.
+
+@node Scheme function usage
+@subsection Scheme function usage
+
+Scheme functions can be called pretty much anywhere where a Scheme
+expression starting with @code{#} can be written. You call a scheme
+function by writing its name preceded by @code{\}, followed by its
+arguments. Once an optional argument predicate does not match an
+argument, Lilypond skips this and all following optional arguments,
+replacing them with their specified default, and @q{backs up} the
+argument that did not match to the place of the next mandatory argument.
+Since the backed up argument needs to go somewhere, optional arguments
+are not actually considered optional unless followed by a mandatory
+argument.
+
+There is one exception: if you write @code{\default} in the place of an
+optional argument, this and all following optional arguments are skipped
+and replaced by their default. This works even when no mandatory
+argument follows since @code{\default} does not need to get backed up.
+The @code{mark} and @code{key} commands make use of that trick to
+provide their default behavior when just followed by @code{\default}.
+
+Apart from places where a Scheme value is required, there are a few
+places where @code{#} expressions are currently accepted and evaluated
+for their side effects but otherwise ignored. Mostly those are the
+places where an assignment would be acceptable as well.
+
+Since it is a bad idea to return values that can be misinterpreted in
+some context, you should use normal scheme functions only for those
+cases where you always return a useful value, and use void scheme
+functions (@pxref{Void scheme functions}) otherwise.
+
+@node Void scheme functions
+@subsection Void scheme functions
+@funindex define-void-function
+@funindex \void
+
+Sometimes a procedure is executed in order to perform an action rather
+than return a value. Some programming languages (like C and Scheme) use
+functions for either concept and just discard the returned value
+(usually by allowing any expression to act as statement, ignoring the
+result). This is clever but error-prone: most C compilers nowadays
+offer warnings for various non-``void'' expressions being discarded.
+For many functions executing an action, the Scheme standards declare the
+return value to be unspecified. Lilypond's Scheme interpreter Guile has
+a unique value @code{*unspecified*} that it usually (such when using
+@code{set!} directly on a variable) but unfortunately not consistently
+returns in such cases.
+
+Defining a Lilypond function with @code{define-void-function} makes
+sure that this special value (the only value satisfying the predicate
+@code{void?}) will be returned.
+
+@example
+noPointAndClick =
+#(define-void-function
+ (parser location)
+ ()
+ (ly:set-option 'point-and-click #f))
+...
+\noPointAndClick % disable point and click
+@end example
+
+If you want to evaluate an expression only for its side-effect and
+don't want any value it may return interpreted, you can do so by
+prefixing it with @code{\void}:
+
+@example
+\void #(hashq-set! some-table some-key some-value)
+@end example
+
+That way, you can be sure that Lilypond will not assign meaning to the
+returned value regardless of where it encounters it. This will also
+work for music functions such as @code{\displayMusic}.
@node Music functions
@section Music functions
-@emph{Music functions} are scheme procedures that can create music
+@cindex music functions
+
+@emph{Music functions} are Scheme procedures that can create music
expressions automatically, and can be used to greatly simplify the
input file.
@menu
-* Music function syntax::
+* Music function definitions::
+* Music function usage::
* Simple substitution functions::
* Intermediate substitution functions::
* Mathematics in functions::
* Functions without arguments::
-* Void functions::
+* Void music functions::
@end menu
-@node Music function syntax
-@subsection Music function syntax
+@node Music function definitions
+@subsection Music function definitions
+@cindex defining music functions
+@funindex define-music-function
-The general form for music functions is:
+The general form for defining music functions is:
@example
function =
#(define-music-function
(parser location @var{arg1} @var{arg2} @dots{})
(@var{type1?} @var{type2?} @dots{})
- @var{music})
+ @var{body})
@end example
@noindent
-where
-
-@multitable @columnfractions .33 .66
-@item @code{@var{argN}}
-@tab @var{n}th argument
-
-@item @code{@var{typeN?}}
-@tab a scheme @emph{type predicate} for which @code{@var{argN}}
-must return @code{#t}.
+quite in analogy to @ref{Scheme function definitions}. More often than
+not, @var{body} will be a @ref{Lilypond code blocks, Lilypond code block}.
-@item @code{@var{music}}
-@tab A music expression, optionally written in scheme, with any
-LilyPond code enclosed in hashed braces
-(@tie{}@w{@code{#@{@dots{}#@}}}@tie{}). Within LilyPond code
-blocks, use @code{$} to reference function arguments (eg.,
-@samp{$arg1}) or to start an inline scheme expression containing
-function arguments (eg., @w{@samp{$(cons arg1 arg2)}}).
-
-@end multitable
-
-@noindent
For a list of available type predicates, see
-@ruser{Predefined type predicates}. User-defined type predicates
-are also allowed.
-
+@ruser{Predefined type predicates}.
@seealso
@file{scm/lily.scm}.
+@node Music function usage
+@subsection Music function usage
+Music functions may currently be used in several places. Depending on
+where they are used, restrictions apply in order to be able to parse
+them unambiguously. The result a music function returns must be
+compatible with the context in which it is called.
+
+@itemize
+@item
+At top level in a music expression. No restriction apply here.
+
+@item
+As a post-event, explicitly started with a direction indicator (one of
+@code{-}, @code{^}, @w{and @code{_}}). Note that returning a post-event
+will be acceptable for music functions called as normal music, leading
+to a result roughly equivalent to
+@example
+s 1*0-\fun
+@end example
+
+In this case, you can't use an @emph{open} music expression as the last
+argument, one that would end with a music expression able to accept
+additional postevents.
+
+@item
+As a chord constituent. The returned expression must be of
+@code{rhythmic-event} type, most likely a @code{NoteEvent}.
+@end itemize
+
+@noindent
+The special rules for trailing arguments make it possible to write
+polymorphic functions like @code{\tweak} that can be applied to
+different constructs.
+
@node Simple substitution functions
@subsection Simple substitution functions
(parser location beg-end)
(pair?)
#@{
- \once \override Beam #'positions = $beg-end
+ \once \override Beam #'positions = #beg-end
#@})
\relative c' @{
(parser location beg end)
(number? number?)
#{
- \once \override Beam #'positions = $(cons beg end)
+ \once \override Beam #'positions = #(cons beg end)
#})
\relative c' {
(parser location mag)
(number?)
#{
- \override Stem #'length = $(* 7.0 mag)
+ \override Stem #'length = #(* 7.0 mag)
\override NoteHead #'font-size =
- $(inexact->exact (* (/ 6.0 (log 2.0)) (log mag)))
+ #(inexact->exact (* (/ 6.0 (log 2.0)) (log mag)))
#})
AltOff = {
(parser location mag music)
(number? ly:music?)
#{
- \override Stem #'length = $(* 7.0 mag)
+ \override Stem #'length = #(* 7.0 mag)
\override NoteHead #'font-size =
- $(inexact->exact (* (/ 6.0 (log 2.0)) (log mag)))
- $music
+ #(inexact->exact (* (/ 6.0 (log 2.0)) (log mag)))
+ #music
\revert Stem #'length
\revert NoteHead #'font-size
#})
@end example
-@node Void functions
-@subsection Void functions
+@node Void music functions
+@subsection Void music functions
-A music function must return a music expression, but sometimes we
-may want to have a function that does not involve music (such as
-turning off Point and Click). To do this, we return a @code{void}
-music expression.
+A music function must return a music expression. If you want to
+execute a function only for its side effect, you should use
+@code{define-void-function}. But there may be cases where you
+sometimes want to produce a music expression, and sometimes not (like
+in the previous example). Returning a @code{void} music expression
+via @code{#@{ #@}} will achieve that.
-That is why the form that is returned is the
-@w{@code{(make-music @dots{})}}. With the @code{'void} property
-set to @code{#t}, the parser is told to actually disregard this
-returned music expression. Thus the important part of the void
-music function is the processing done by the function, not the
-music expression that is returned.
+@node Event functions
+@section Event functions
+@funindex define-event-function
+@cindex event functions
-@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
+To use a music function in the place of an event, you need to write a
+direction indicator before it. But sometimes, this does not quite match
+the syntax of constructs you want to replace. For example, if you want
+to write dynamics commands, those are usually attached without direction
+indicator, like @code{c'\pp}. Here is a way to write arbitrary
+dynamics:
+@lilypond[quote,verbatim,ragged-right]
+dyn=#(define-event-function (parser location arg) (markup?)
+ (make-dynamic-script arg))
+\relative c' { c\dyn pfsss }
+@end lilypond
+You could do the same using a music function, but then you always would
+have to write a direction indicator before calling it, like
+@code{@w{c-\dyn pfsss}}.
@node Markup functions
@noindent
is equivalent to:
@example
-\markup \column @{ \line @{ \bold \italic "hello" \raise #0.4 "world" @}
- \larger \line @{ foo bar baz @} @}
+#@{ \markup \column @{ \line @{ \bold \italic "hello" \raise #0.4 "world" @}
+ \larger \line @{ foo bar baz @} @} #@}
@end example
@noindent
This example demonstrates the main translation rules between regular
-LilyPond markup syntax and Scheme markup syntax.
+LilyPond markup syntax and Scheme markup syntax. Using @code{#@{
+@dots{} #@}} for entering in LilyPond syntax will often be most
+convenient, but we explain how to use the @code{markup} macro to get a
+Scheme-only solution.
@quotation
@multitable @columnfractions .3 .3
override it too:
@lilypond[quote,verbatim,ragged-right]
-\markup \override #'(box-padding . 0.4) \box \override #'(box-padding . 0.6) \box A
+\markup \override #'(box-padding . 0.4) \box
+ \override #'(box-padding . 0.6) \box A
@end lilypond
Repeating this lengthy markup would be painful. This is where a markup
taking one argument (the text). This draws the two boxes, with some
padding.
+@lisp
+#(define-markup-command (double-box layout props text) (markup?)
+ "Draw a double box around text."
+ (interpret-markup layout props
+ #@{\markup \override #'(box-padding . 0.4) \box
+ \override #'(box-padding . 0.6) \box @{ #text @}#@}))
+@end lisp
+
+or, equivalently
+
@lisp
#(define-markup-command (double-box layout props text) (markup?)
"Draw a double box around text."
@code{text} is the name of the command argument, and @code{markup?} its
type: it identifies it as a markup. The @code{interpret-markup}
function is used in most of markup commands: it builds a stencil, using
-@code{layout}, @code{props}, and a markup. Here, this markup is built
-using the @code{markup} scheme macro, see @ref{Markup construction in Scheme}.
-The transformation from @code{\markup} expression to scheme
-markup expression is straight-forward.
+@code{layout}, @code{props}, and a markup. In the second case, this
+markup is built using the @code{markup} scheme macro, see @ref{Markup
+construction in Scheme}. The transformation from @code{\markup}
+expression to scheme markup expression is straight-forward.
The new command can be used as follow:
@code{box-padding} will be used for the inner padding. The new code is
now as follows:
+@lisp
+#(define-markup-command (double-box layout props text) (markup?)
+ #:properties ((inter-box-padding 0.4)
+ (box-padding 0.6))
+ "Draw a double box around text."
+ (interpret-markup layout props
+ #@{\markup \override #`(box-padding . ,inter-box-padding) \box
+ \override #`(box-padding . ,box-padding) \box
+ @{ #text @} #@}))
+@end lisp
+
+Again, the equivalent version using the markup macro would be:
+
@lisp
#(define-markup-command (double-box layout props text) (markup?)
#:properties ((inter-box-padding 0.4)
(box-padding 0.6))
"Draw a double box around text."
(interpret-markup layout props
- (markup #:override `(box-padding . ,inter-box-padding) #:box
- #:override `(box-padding . ,box-padding) #:box text)))
+ #{\markup \override #`(box-padding . ,inter-box-padding) \box
+ \override #`(box-padding . ,box-padding) \box
+ { #text } #}))
\markup \double-box A
\markup \override #'(inter-box-padding . 0.8) \double-box A
In the following example, a @code{\paragraph} markup list command is
defined, which returns a list of justified lines, the first one being
indented. The indent width is taken from the @code{props} argument.
+
+@example
+#(define-markup-list-command (paragraph layout props args) (markup-list?)
+ #:properties ((par-indent 2))
+ (interpret-markup-list layout props
+ #@{\markuplist \justified-lines @{ \hspace #par-indent #args @} #@}))
+@end example
+
+
+The version using just Scheme is more complex:
@example
#(define-markup-list-command (paragraph layout props args) (markup-list?)
#:properties ((par-indent 2))
First, the function gets the indent width, a property here named
@code{par-indent}, from the property list @code{props}. If the
property is not found, the default value is @code{2}. Then, a
-list of justified lines is made using the
-@code{make-justified-lines-markup-list} function, which is related
-to the @code{\justified-lines} built-in markup list command. A
-horizontal space is added at the beginning using the
-@code{make-hspace-markup} function. Finally, the markup list is
+list of justified lines is made using the built-in markup list command
+@code{\justified-lines}, which is related to the
+@code{make-justified-lines-markup-list} function. A
+horizontal space is added at the beginning using @code{\hspace} (or the
+@code{make-hspace-markup} function). Finally, the markup list is
interpreted using the @code{interpret-markup-list} function.
This new markup list command can be used as follows:
@example
-\markuplines @{
+\markuplist @{
\paragraph @{
The art of music typography is called \italic @{(plate) engraving.@}
The term derives from the traditional process of music printing.
@node Inline Scheme code
@section Inline Scheme code
+TODO: the example for this section is ill-chosen since
+@example
+F = -\tweak #'font-size #-3 -\flageolet
+@end example
+(note the @samp{-} marking it as a post event) will actually work fine
+for the stated purpose. Until this section gets a rewrite, let's
+pretend we don't know.
+
The main disadvantage of @code{\tweak} is its syntactical
inflexibility. For example, the following produces a syntax error.
@end example
@noindent
-In other words, @code{\tweak} doesn't behave like an articulation
-regarding the syntax; in particular, it can't be attached with
-@code{^} and @code{_}.
-
Using Scheme, this problem can be avoided. The route to the
result is given in @ref{Adding articulation to notes (example)},
especially how to use @code{\displayMusic} as a helping guide.
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