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13 @chapter Scheme tutorial
18 @cindex Scheme, in-line code
19 @cindex accessing Scheme
20 @cindex evaluating Scheme
23 LilyPond uses the Scheme programming language, both as part of the
24 input syntax, and as internal mechanism to glue modules of the program
25 together. This section is a very brief overview of entering data in
26 Scheme. If you want to know more about Scheme, see
27 @uref{http://@/www@/.schemers@/.org}.
29 LilyPond uses the GNU Guile implementation of Scheme, which is
30 based on the Scheme @qq{R5RS} standard. If you are learning Scheme
31 to use with LilyPond, working with a different implementation (or
32 referring to a different standard) is not recommended. Information
33 on guile can be found at @uref{http://www.gnu.org/software/guile/}.
34 The @qq{R5RS} Scheme standard is located at
35 @uref{http://www.schemers.org/Documents/Standards/R5RS/}.
38 * Introduction to Scheme::
39 * Scheme in LilyPond::
40 * Building complicated functions::
43 @node Introduction to Scheme
44 @section Introduction to Scheme
46 We begin with an introduction to Scheme. For this brief introduction,
47 we will use the GUILE interpreter to explore how the language works.
48 Once we are familiar with Scheme, we will show how the language can
49 be integrated in LilyPond files.
55 * Scheme simple data types::
56 * Scheme compound data types::
57 * Calculations in Scheme::
59 * Scheme conditionals::
63 @subsection Scheme sandbox
65 The LilyPond installation includes the Guile implementation of
66 Scheme. On most systems you can experiment in a Scheme sandbox by
67 opening a terminal window and typing @q{guile}. On some systems,
68 notably Windows, you may need to set the environment variable
69 @code{GUILE_LOAD_PATH} to the directory @code{../usr/shr/guile/1.8}
70 in the LilyPond installation. For the full path to this directory
71 see @rlearning{Other sources of information}. Alternatively, Windows
72 users may simply choose @q{Run} from the Start menu and enter
75 Once the guile sandbox is running, you will received a guile prompt:
81 You can enter Scheme expressions at this prompt to experiment with Scheme.
83 @node Scheme variables
84 @subsection Scheme variables
86 Scheme variables can have any valid scheme value, including a Scheme
89 Scheme variables are created with @code{define}:
96 Scheme variables can be evaluated at the guile prompt simply by
97 typing the variable name:
105 Scheme variables can be printed on the display by use of the display function:
113 Note that the value @code{2} and the guile prompt @code{guile} both
114 showed up on the same line. This can be avoided by calling the newline
115 procedure or displaying a newline character.
118 guile> (display a)(newline)
120 guile> (display a)(display "\n")
125 Once a variable has been created, its value can be changed with @code{set!}:
128 guile> (set! a 12345)
134 @node Scheme simple data types
135 @subsection Scheme simple data types
137 The most basic concept in a language is data typing: numbers, character
138 strings, lists, etc. Here is a list of simple Scheme data types that are
139 often used with LilyPond.
143 Boolean values are True or False. The Scheme for True is @code{#t}
144 and False is @code{#f}.
149 Numbers are entered in the standard fashion,
150 @code{1} is the (integer) number one, while @code{-1.5} is a
151 floating point number (a non-integer number).
154 Strings are enclosed in double quotes,
160 Strings may span several lines:
169 and the newline characters at the end of each line will be included
172 Newline characters can also be added by including @code{\n} in the
176 "this\nis a\nmultiline string"
180 Quotation marks and backslashes are added to strings
181 by preceding them with a backslash.
182 The string @code{\a said "b"} is entered as
190 There are additional Scheme data types that are not discussed here.
191 For a complete listing see the Guile reference guide,
192 @uref{http://www.gnu.org/software/guile/manual/html_node/Simple-Data-Types.html}.
194 @node Scheme compound data types
195 @subsection Scheme compound data types
197 There are also compound data types in Scheme. The types commonly used in
198 LilyPond programming include pairs, lists, alists, and hash tables.
200 @unnumberedsubsubsec Pairs
202 The foundational compound data type of Scheme is the @code{pair}. As
203 might be expected from its name, a pair is two values glued together.
204 The operator used to form a pair is called @code{cons}.
212 Note that the pair is displayed as two items surrounded by
213 parentheses and separated by whitespace, a period (@code{.}), and
214 more whitespace. The period is @emph{not} a decimal point, but
215 rather an indicator of the pair.
217 Pairs can also be entered as literal values by preceding them with
218 a single quote character.
226 The two elements of a pair may be any valid Scheme value:
231 guile> '("blah-blah" . 3.1415926535)
232 ("blah-blah" . 3.1415926535)
236 The first and second elements of the pair can be accessed by the
237 Scheme procedures @code{car} and @code{cdr}, respectively.
240 guile> (define mypair (cons 123 "hello there")
251 Note: @code{cdr} is pronounced "could-er", according Sussman and
253 @uref{http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-14.html#footnote_Temp_133}
256 @unnumberedsubsubsec Lists
258 A very common Scheme data structure is the @emph{list}. Formally, a
259 list is defined as either the empty list (represented as @code{'()},
260 or a pair whose @code{cdr} is a list.
262 There are many ways of creating lists. Perhaps the most common is
263 with the @code{list} procedure:
266 guile> (list 1 2 3 "abc" 17.5)
270 As can be seen, a list is displayed in the form of individual elements
271 separated by whitespace and enclosed in parentheses. Unlike a pair,
272 there is no period between the elements.
274 A list can also be entered as a literal list by enclosing its
275 elements in parentheses, and adding a quote:
278 guile> '(17 23 "foo" "bar" "bazzle")
279 (17 23 "foo" "bar" "bazzle")
282 Lists are a central part of Scheme. In, fact, Scheme is considered
283 a dialect of lisp, where @q{lisp} is an abbreviation for
284 @q{List Processing}. Scheme expressions are all lists.
286 @unnumberedsubsubsec Association lists (alists)
288 A special type of list is an @emph{association list} or @emph{alist}.
289 An alist is used to store data for easy retrieval.
291 Alists are lists whose elements are pairs. The @code{car} of each
292 element is called the @emph{key}, and the @code{cdr} of each element
293 is called the @emph{value}. The Scheme procedure @code{assoc} is
294 used to retrieve an entry from the alist, and @code{cdr} is used to
298 guile> (define my-alist '((1 . "A") (2 . "B") (3 . "C")))
300 ((1 . "A") (2 . "B") (3 . "C"))
301 guile> (assoc 2 my-alist)
303 guile> (cdr (assoc 2 my-alist))
308 Alists are widely used in LilyPond to store properties and other data.
310 @unnumberedsubsubsec Hash tables
312 A data structure that is used occasionally in LilyPond. A hash table
313 is similar to an array, but the indexes to the array can be any type
314 of Scheme value, not just integers.
316 Hash tables are more efficient than alists if there is a lot of data
317 to store and the data changes very infrequently.
319 The syntax to create hash tables is a bit complex, but you
320 can see examples of it in the LilyPond source.
323 guile> (define h (make-hash-table 10))
326 guile> (hashq-set! h 'key1 "val1")
328 guile> (hashq-set! h 'key2 "val2")
330 guile> (hashq-set! h 3 "val3")
334 Values are retrieved from hash tables with @code{hashq-ref}.
337 guile> (hashq-ref h 3)
339 guile> (hashq-ref h 'key2)
344 Keys and values are retrieved as a pair with @code{hashq-get-handle}.
345 This is a preferred way, because it will return @code{#f} if a key is
349 guile> (hashq-get-handle h 'key1)
351 guile> (hashq-get-handle h 'frob)
356 @node Calculations in Scheme
357 @subsection Calculations in Scheme
360 We have been using lists all along. A calculation, like @code{(+ 1 2)}
361 is also a list (containing the symbol @code{+} and the numbers 1
362 and@tie{}2). Normally lists are interpreted as calculations, and the
363 Scheme interpreter substitutes the outcome of the calculation. To enter a
364 list, we stop the evaluation. This is done by quoting the list with a
365 quote @code{'} symbol. So, for calculations do not use a quote.
367 Inside a quoted list or pair, there is no need to quote anymore. The
368 following is a pair of symbols, a list of symbols and a list of lists
373 #'(staff clef key-signature)
378 Scheme can be used to do calculations. It uses @emph{prefix}
379 syntax. Adding 1 and@tie{}2 is written as @code{(+ 1 2)} rather than the
380 traditional @math{1+2}.
387 Calculations may be nested; the result of a function may
388 be used for another calculation.
396 These calculations are examples of evaluations; an expression like
397 @code{(* 3 4)} is replaced by its value @code{12}.
399 When the scheme parser encounters an expression that is a list, the
400 first element of the list is treated as a procedure to be evaluated
401 with the arguments of the remainder of the list. Therefore, all operators
402 in Scheme are prefix operators.
404 If the first element of a Scheme expression that is a list passed to the parser
405 is @emph{not} an operator or procedure, an error will occur:
414 <unnamed port>:52:1: In expression (1 2 3):
415 <unnamed port>:52:1: Wrong type to apply: 1
420 Here you can see that the interpreter was trying to treat 1 as an operator
421 or procedure, and it couldn't. Hence the error is "Wrong type to apply: 1".
423 To create a list, then , we need to use the list operator, or we need to
424 quote the list so that the interpreter will not try to evaluate it.
434 This is an error that can appear as you are working with Scheme in LilyPond.
437 The same assignment can be done in completely in Scheme as well,
440 #(define twentyFour (* 2 twelve))
443 @c this next section is confusing -- need to rewrite
445 The @emph{name} of a variable is also an expression, similar to a
446 number or a string. It is entered as
453 @cindex quoting in Scheme
455 The quote mark @code{'} prevents the Scheme interpreter from substituting
456 @code{24} for the @code{twentyFour}. Instead, we get the name
461 @node Scheme procedures
462 @subsection Scheme procedures
464 @unnumberedsubsubsec Predicates
466 @unnumberedsubsubsec Return values
468 TODO -- write about scheme procedures
470 @node Scheme conditionals
471 @subsection Scheme conditionals
473 @unnumberedsubsubsec if
475 @unnumberedsubsubsec cond
478 @node Scheme in LilyPond
479 @section Scheme in LilyPond
483 * LilyPond Scheme syntax::
484 * LilyPond variables::
485 * Input variables and Scheme::
486 * Object properties::
487 * LilyPond compound variables::
488 * Internal music representation::
491 @node LilyPond Scheme syntax
492 @subsection LilyPond Scheme syntax
494 In a music file, snippets of Scheme code are introduced with the hash
495 mark @code{#}. So, the previous examples translated to LilyPond are
506 Note that LilyPond comments (@code{%} and @code{%@{ %@}}) cannot
507 be used within Scheme code. Comments in Guile Scheme are entered
511 ; this is a single-line comment
514 This a (non-nestable) Guile-style block comment
515 But these are rarely used by Schemers and never in
520 Multiple consecutive scheme expressions in a music file can be
521 combined using the @code{begin} operator. This permits the number
522 of hash marks to be reduced to one.
530 @c todo -- # introduces a scheme *expression*
531 @c need the concept of an expression
533 If @code{#} is followed by an opening parenthesis, @code{(}, as in
534 the example above, the parser will remain in Scheme mode until
535 a matching closing parenthesis, @code{)}, is found, so further
536 @code{#} symbols to introduce a Scheme section are not required.
538 For the rest of this section, we will assume that the data is entered
539 in a music file, so we add @code{#}s everywhere.
541 @node LilyPond variables
542 @subsection LilyPond variables
545 TODO -- make this read right
548 happens with variables. After defining a variable
555 variables can also be used in expressions, here
558 twentyFour = (* 2 twelve)
562 the number 24 is stored in the variable @code{twentyFour}.
564 @node Input variables and Scheme
565 @subsection Input variables and Scheme
567 The input format supports the notion of variables: in the following
568 example, a music expression is assigned to a variable with the name
572 traLaLa = @{ c'4 d'4 @}
577 There is also a form of scoping: in the following example, the
578 @code{\layout} block also contains a @code{traLaLa} variable, which is
579 independent of the outer @code{\traLaLa}.
581 traLaLa = @{ c'4 d'4 @}
582 \layout @{ traLaLa = 1.0 @}
585 In effect, each input file is a scope, and all @code{\header},
586 @code{\midi}, and @code{\layout} blocks are scopes nested inside that
589 Both variables and scoping are implemented in the GUILE module system.
590 An anonymous Scheme module is attached to each scope. An assignment of
593 traLaLa = @{ c'4 d'4 @}
597 is internally converted to a Scheme definition
599 (define traLaLa @var{Scheme value of `@code{... }'})
602 This means that input variables and Scheme variables may be freely
603 mixed. In the following example, a music fragment is stored in the
604 variable @code{traLaLa}, and duplicated using Scheme. The result is
605 imported in a @code{\score} block by means of a second variable
609 traLaLa = { c'4 d'4 }
611 %% dummy action to deal with parser lookahead
612 #(display "this needs to be here, sorry!")
614 #(define newLa (map ly:music-deep-copy
615 (list traLaLa traLaLa)))
617 (make-sequential-music newLa))
622 @c Due to parser lookahead
624 In this example, the assignment happens after the parser has
625 verified that nothing interesting happens after
626 @code{traLaLa = @{ ... @}}. Without the dummy statement in the
627 above example, the @code{newLa} definition is executed before
628 @code{traLaLa} is defined, leading to a syntax error.
630 The above example shows how to @q{export} music expressions from the
631 input to the Scheme interpreter. The opposite is also possible. By
632 wrapping a Scheme value in the function @code{ly:export}, a Scheme
633 value is interpreted as if it were entered in LilyPond syntax.
634 Instead of defining @code{\twice}, the example above could also have
639 @{ #(ly:export (make-sequential-music (list newLa))) @}
642 Scheme code is evaluated as soon as the parser encounters it. To
643 define some Scheme code in a macro (to be called later), use
644 @ref{Void functions}, or
648 (ly:set-option 'point-and-click #f))
657 Mixing Scheme and LilyPond variables is not possible with the
658 @code{--safe} option.
663 @node Object properties
664 @subsection Object properties
666 This syntax will be used very frequently, since many of the layout
667 tweaks involve assigning (Scheme) values to internal variables, for
671 \override Stem #'thickness = #2.6
674 This instruction adjusts the appearance of stems. The value @code{2.6}
675 is put into the @code{thickness} variable of a @code{Stem}
676 object. @code{thickness} is measured relative to the thickness of
677 staff lines, so these stem lines will be @code{2.6} times the
678 width of staff lines. This makes stems almost twice as thick as their
679 normal size. To distinguish between variables defined in input files (like
680 @code{twentyFour} in the example above) and variables of internal
681 objects, we will call the latter @q{properties} and the former
682 @q{variables.} So, the stem object has a @code{thickness} property,
683 while @code{twentyFour} is an variable.
685 @cindex properties vs. variables
686 @cindex variables vs. properties
688 @c todo -- here we're getting interesting. We're now introducing
689 @c LilyPond variable types. I think this deserves a section all
692 @node LilyPond compound variables
693 @subsection LilyPond compound variables
695 @unnumberedsubsubsec Offsets
697 Two-dimensional offsets (X and Y coordinates) as well as object sizes
698 (intervals with a left and right point) are entered as @code{pairs}. A
699 pair@footnote{In Scheme terminology, the pair is called @code{cons},
700 and its two elements are called @code{car} and @code{cdr} respectively.}
701 is entered as @code{(first . second)} and, like symbols, they must be quoted,
704 \override TextScript #'extra-offset = #'(1 . 2)
707 This assigns the pair (1, 2) to the @code{extra-offset} property of the
708 TextScript object. These numbers are measured in staff-spaces, so
709 this command moves the object 1 staff space to the right, and 2 spaces up.
711 @unnumberedsubsubsec Extents
713 todo -- write something about extents
715 @unnumberedsubsubsec Property alists
717 todo -- write something about property alists
719 @unnumberedsubsubsec Alist chains
721 todo -- write something about alist chains
723 @node Internal music representation
724 @subsection Internal music representation
726 When a music expression is parsed, it is converted into a set of
727 Scheme music objects. The defining property of a music object is that
728 it takes up time. Time is a rational number that measures the length
729 of a piece of music in whole notes.
731 A music object has three kinds of types:
734 music name: Each music expression has a name. For example, a note
735 leads to a @rinternals{NoteEvent}, and @code{\simultaneous} leads to
736 a @rinternals{SimultaneousMusic}. A list of all expressions
737 available is in the Internals Reference manual, under
738 @rinternals{Music expressions}.
741 @q{type} or interface: Each music name has several @q{types} or
742 interfaces, for example, a note is an @code{event}, but it is also a
743 @code{note-event}, a @code{rhythmic-event}, and a
744 @code{melodic-event}. All classes of music are listed in the
745 Internals Reference, under
746 @rinternals{Music classes}.
749 C++ object: Each music object is represented by an object of the C++
753 The actual information of a music expression is stored in properties.
754 For example, a @rinternals{NoteEvent} has @code{pitch} and
755 @code{duration} properties that store the pitch and duration of that
756 note. A list of all properties available can be found in the
757 Internals Reference, under @rinternals{Music properties}.
759 A compound music expression is a music object that contains other
760 music objects in its properties. A list of objects can be stored in
761 the @code{elements} property of a music object, or a single @q{child}
762 music object in the @code{element} property. For example,
763 @rinternals{SequentialMusic} has its children in @code{elements},
764 and @rinternals{GraceMusic} has its single argument in
765 @code{element}. The body of a repeat is stored in the @code{element}
766 property of @rinternals{RepeatedMusic}, and the alternatives in
769 @node Building complicated functions
770 @section Building complicated functions
772 This section explains how to gather the information necessary
773 to create complicated music functions.
776 * Displaying music expressions::
778 * Doubling a note with slurs (example)::
779 * Adding articulation to notes (example)::
783 @node Displaying music expressions
784 @subsection Displaying music expressions
786 @cindex internal storage
787 @cindex displaying music expressions
788 @cindex internal representation, displaying
790 @funindex \displayMusic
792 When writing a music function it is often instructive to inspect how
793 a music expression is stored internally. This can be done with the
794 music function @code{\displayMusic}
798 \displayMusic @{ c'4\f @}
815 (ly:make-duration 2 0 1 1)
817 (ly:make-pitch 0 0 0))
819 'AbsoluteDynamicEvent
824 By default, LilyPond will print these messages to the console along
825 with all the other messages. To split up these messages and save
826 the results of @code{\display@{STUFF@}}, redirect the output to
830 lilypond file.ly >display.txt
833 With a bit of reformatting, the above information is
837 (make-music 'SequentialMusic
838 'elements (list (make-music 'EventChord
839 'elements (list (make-music 'NoteEvent
840 'duration (ly:make-duration 2 0 1 1)
841 'pitch (ly:make-pitch 0 0 0))
842 (make-music 'AbsoluteDynamicEvent
846 A @code{@{ ... @}} music sequence has the name @code{SequentialMusic},
847 and its inner expressions are stored as a list in its @code{'elements}
848 property. A note is represented as an @code{EventChord} expression,
849 containing a @code{NoteEvent} object (storing the duration and
850 pitch properties) and any extra information (in this case, an
851 @code{AbsoluteDynamicEvent} with a @code{"f"} text property.
854 @node Music properties
855 @subsection Music properties
857 The @code{NoteEvent} object is the first object of the
858 @code{'elements} property of @code{someNote}.
862 \displayMusic \someNote
870 (ly:make-duration 2 0 1 1)
872 (ly:make-pitch 0 0 0))))
875 The @code{display-scheme-music} function is the function used by
876 @code{\displayMusic} to display the Scheme representation of a music
880 #(display-scheme-music (first (ly:music-property someNote 'elements)))
885 (ly:make-duration 2 0 1 1)
887 (ly:make-pitch 0 0 0))
890 Then the note pitch is accessed through the @code{'pitch} property
891 of the @code{NoteEvent} object,
894 #(display-scheme-music
895 (ly:music-property (first (ly:music-property someNote 'elements))
898 (ly:make-pitch 0 0 0)
901 The note pitch can be changed by setting this @code{'pitch} property,
903 @funindex \displayLilyMusic
906 #(set! (ly:music-property (first (ly:music-property someNote 'elements))
908 (ly:make-pitch 0 1 0)) ;; set the pitch to d'.
909 \displayLilyMusic \someNote
915 @node Doubling a note with slurs (example)
916 @subsection Doubling a note with slurs (example)
918 Suppose we want to create a function that translates input like
919 @code{a} into @code{a( a)}. We begin by examining the internal
920 representation of the desired result.
923 \displayMusic@{ a'( a') @}
934 (ly:make-duration 2 0 1 1)
936 (ly:make-pitch 0 5 0))
947 (ly:make-duration 2 0 1 1)
949 (ly:make-pitch 0 5 0))
956 The bad news is that the @code{SlurEvent} expressions
957 must be added @q{inside} the note (or more precisely,
958 inside the @code{EventChord} expression).
960 Now we examine the input,
972 (ly:make-duration 2 0 1 1)
974 (ly:make-pitch 0 5 0))))))
977 So in our function, we need to clone this expression (so that we
978 have two notes to build the sequence), add @code{SlurEvents} to the
979 @code{'elements} property of each one, and finally make a
980 @code{SequentialMusic} with the two @code{EventChords}.
983 doubleSlur = #(define-music-function (parser location note) (ly:music?)
984 "Return: @{ note ( note ) @}.
985 `note' is supposed to be an EventChord."
986 (let ((note2 (ly:music-deep-copy note)))
987 (set! (ly:music-property note 'elements)
988 (cons (make-music 'SlurEvent 'span-direction -1)
989 (ly:music-property note 'elements)))
990 (set! (ly:music-property note2 'elements)
991 (cons (make-music 'SlurEvent 'span-direction 1)
992 (ly:music-property note2 'elements)))
993 (make-music 'SequentialMusic 'elements (list note note2))))
997 @node Adding articulation to notes (example)
998 @subsection Adding articulation to notes (example)
1000 The easy way to add articulation to notes is to merge two music
1001 expressions into one context, as explained in
1002 @ruser{Creating contexts}. However, suppose that we want to write
1003 a music function that does this.
1005 A @code{$variable} inside the @code{#@{...#@}} notation is like
1006 a regular @code{\variable} in classical LilyPond notation. We
1014 will not work in LilyPond. We could avoid this problem by attaching
1015 the articulation to a fake note,
1018 @{ << \music s1*0-.-> @}
1022 but for the sake of this example, we will learn how to do this in
1023 Scheme. We begin by examining our input and desired output,
1035 (ly:make-duration 2 0 1 1)
1037 (ly:make-pitch -1 0 0))))
1048 (ly:make-duration 2 0 1 1)
1050 (ly:make-pitch -1 0 0))
1057 We see that a note (@code{c4}) is represented as an @code{EventChord}
1058 expression, with a @code{NoteEvent} expression in its elements list. To
1059 add a marcato articulation, an @code{ArticulationEvent} expression must
1060 be added to the elements property of the @code{EventChord}
1063 To build this function, we begin with
1066 (define (add-marcato event-chord)
1067 "Add a marcato ArticulationEvent to the elements of `event-chord',
1068 which is supposed to be an EventChord expression."
1069 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1070 (set! (ly:music-property result-event-chord 'elements)
1071 (cons (make-music 'ArticulationEvent
1072 'articulation-type "marcato")
1073 (ly:music-property result-event-chord 'elements)))
1074 result-event-chord))
1077 The first line is the way to define a function in Scheme: the function
1078 name is @code{add-marcato}, and has one variable called
1079 @code{event-chord}. In Scheme, the type of variable is often clear
1080 from its name. (this is good practice in other programming languages,
1088 is a description of what the function does. This is not strictly
1089 necessary, but just like clear variable names, it is good practice.
1092 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1095 @code{let} is used to declare local variables. Here we use one local
1096 variable, named @code{result-event-chord}, to which we give the value
1097 @code{(ly:music-deep-copy event-chord)}. @code{ly:music-deep-copy} is
1098 a function specific to LilyPond, like all functions prefixed by
1099 @code{ly:}. It is use to make a copy of a music
1100 expression. Here we copy @code{event-chord} (the parameter of the
1101 function). Recall that our purpose is to add a marcato to an
1102 @code{EventChord} expression. It is better to not modify the
1103 @code{EventChord} which was given as an argument, because it may be
1106 Now we have a @code{result-event-chord}, which is a
1107 @code{NoteEventChord} expression and is a copy of
1108 @code{event-chord}. We add the marcato to its @code{'elements}
1112 (set! place new-value)
1115 Here, what we want to set (the @q{place}) is the @code{'elements}
1116 property of @code{result-event-chord} expression.
1119 (ly:music-property result-event-chord 'elements)
1122 @code{ly:music-property} is the function used to access music properties
1123 (the @code{'elements}, @code{'duration}, @code{'pitch}, etc, that we
1124 see in the @code{\displayMusic} output above). The new value is the
1125 former @code{'elements} property, with an extra item: the
1126 @code{ArticulationEvent} expression, which we copy from the
1127 @code{\displayMusic} output,
1130 (cons (make-music 'ArticulationEvent
1131 'articulation-type "marcato")
1132 (ly:music-property result-event-chord 'elements))
1135 @code{cons} is used to add an element to a list without modifying
1136 the original list. This is what we want: the same list as before,
1137 plus the new @code{ArticulationEvent} expression. The order
1138 inside the @code{'elements} property is not important here.
1140 Finally, once we have added the marcato articulation to its @code{elements}
1141 property, we can return @code{result-event-chord}, hence the last line of
1144 Now we transform the @code{add-marcato} function into a music
1148 addMarcato = #(define-music-function (parser location event-chord)
1150 "Add a marcato ArticulationEvent to the elements of `event-chord',
1151 which is supposed to be an EventChord expression."
1152 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1153 (set! (ly:music-property result-event-chord 'elements)
1154 (cons (make-music 'ArticulationEvent
1155 'articulation-type "marcato")
1156 (ly:music-property result-event-chord 'elements)))
1157 result-event-chord))
1160 We may verify that this music function works correctly,
1163 \displayMusic \addMarcato c4
1173 * Tweaking with Scheme::
1176 @c @node Tweaking with Scheme
1177 @c @section Tweaking with Scheme
1179 We have seen how LilyPond output can be heavily modified using
1181 @code{\override TextScript #'extra-offset = ( 1 . -1)}. But
1182 we have even more power if we use Scheme. For a full explanation
1183 of this, see the @ref{Scheme tutorial}, and
1184 @ref{Interfaces for programmers}.
1186 We can use Scheme to simply @code{\override} commands,
1188 TODO Find a simple example
1189 @c This isn't a valid example with skylining
1190 @c It works fine without padText -td
1194 @lilypond[quote,verbatim,ragged-right]
1195 padText = #(define-music-function (parser location padding) (number?)
1197 \once \override TextScript #'padding = #$padding
1201 c4^"piu mosso" b a b
1203 c4^"piu mosso" d e f
1205 c4^"piu mosso" fis a g
1211 We can use it to create new commands:
1213 @c Check this is a valid example with skylining
1214 @c It is - 'padding still works
1217 @lilypond[quote,verbatim,ragged-right]
1218 tempoPadded = #(define-music-function (parser location padding tempotext)
1221 \once \override Score.MetronomeMark #'padding = $padding
1222 \tempo \markup { \bold $tempotext }
1226 \tempo \markup { "Low tempo" }
1228 \tempoPadded #4.0 #"High tempo"
1234 Even music expressions can be passed in:
1236 @lilypond[quote,verbatim,ragged-right]
1237 pattern = #(define-music-function (parser location x y) (ly:music? ly:music?)
1244 \pattern {d16 dis} { ais16-> b\p }