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4 Translation of GIT committish: FILL-IN-HEAD-COMMITTISH
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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 @code{key}, and the @code{cdr} of each element
293 is called the @code{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
359 We have been using lists all along. A calculation, like @code{(+ 1 2)}
360 is also a list (containing the symbol @code{+} and the numbers 1
361 and@tie{}2). Normally lists are interpreted as calculations, and the
362 Scheme interpreter substitutes the outcome of the calculation. To enter a
363 list, we stop the evaluation. This is done by quoting the list with a
364 quote @code{'} symbol. So, for calculations do not use a quote.
366 Inside a quoted list or pair, there is no need to quote anymore. The
367 following is a pair of symbols, a list of symbols and a list of lists
372 #'(staff clef key-signature)
376 Scheme can be used to do calculations. It uses @emph{prefix}
377 syntax. Adding 1 and@tie{}2 is written as @code{(+ 1 2)} rather than the
378 traditional @math{1+2}.
385 The arrow @result{} shows that the result of evaluating @code{(+ 1 2)}
386 is@tie{}@code{3}. Calculations may be nested; the result of a function may
387 be used for another calculation.
395 These calculations are examples of evaluations; an expression like
396 @code{(* 3 4)} is replaced by its value @code{12}.
398 The same assignment can be done in completely in Scheme as well,
401 #(define twentyFour (* 2 twelve))
404 @c this next section is confusing -- need to rewrite
406 The @emph{name} of a variable is also an expression, similar to a
407 number or a string. It is entered as
414 @cindex quoting in Scheme
416 The quote mark @code{'} prevents the Scheme interpreter from substituting
417 @code{24} for the @code{twentyFour}. Instead, we get the name
420 @node Scheme procedures
421 @subsection Scheme procedures
423 @unnumberedsubsubsec Predicates
425 @unnumberedsubsubsec Return values
427 TODO -- write about scheme procedures
429 @node Scheme conditionals
430 @subsection Scheme conditionals
432 @unnumberedsubsubsec if
434 @unnumberedsubsubsec cond
437 @node Scheme in LilyPond
438 @section Scheme in LilyPond
442 * LilyPond Scheme syntax::
443 * LilyPond variables::
444 * Input variables and Scheme::
445 * Object properties::
446 * LilyPond compound variables::
447 * Internal music representation::
450 @node LilyPond Scheme syntax
451 @subsection LilyPond Scheme syntax
453 In a music file, snippets of Scheme code are introduced with the hash
454 mark @code{#}. So, the previous examples translated to LilyPond are
465 Note that LilyPond comments (@code{%} and @code{%@{ %@}}) cannot
466 be used within Scheme code. Comments in Guile Scheme are entered
470 ; this is a single-line comment
473 This a (non-nestable) Guile-style block comment
474 But these are rarely used by Schemers and never in
479 Multiple consecutive scheme expressions in a music file can be
480 combined using the @code{begin} operator. This permits the number
481 of hash marks to be reduced to one.
489 @c todo -- # introduces a scheme *expression*
490 @c need the concept of an expression
492 If @code{#} is followed by an opening parenthesis, @code{(}, as in
493 the example above, the parser will remain in Scheme mode until
494 a matching closing parenthesis, @code{)}, is found, so further
495 @code{#} symbols to introduce a Scheme section are not required.
497 For the rest of this section, we will assume that the data is entered
498 in a music file, so we add @code{#}s everywhere.
500 @node LilyPond variables
501 @subsection LilyPond variables
504 TODO -- make this read right
507 happens with variables. After defining a variable
514 variables can also be used in expressions, here
517 twentyFour = (* 2 twelve)
521 the number 24 is stored in the variable @code{twentyFour}.
523 @node Input variables and Scheme
524 @subsection Input variables and Scheme
526 The input format supports the notion of variables: in the following
527 example, a music expression is assigned to a variable with the name
531 traLaLa = @{ c'4 d'4 @}
536 There is also a form of scoping: in the following example, the
537 @code{\layout} block also contains a @code{traLaLa} variable, which is
538 independent of the outer @code{\traLaLa}.
540 traLaLa = @{ c'4 d'4 @}
541 \layout @{ traLaLa = 1.0 @}
544 In effect, each input file is a scope, and all @code{\header},
545 @code{\midi}, and @code{\layout} blocks are scopes nested inside that
548 Both variables and scoping are implemented in the GUILE module system.
549 An anonymous Scheme module is attached to each scope. An assignment of
552 traLaLa = @{ c'4 d'4 @}
556 is internally converted to a Scheme definition
558 (define traLaLa @var{Scheme value of `@code{... }'})
561 This means that input variables and Scheme variables may be freely
562 mixed. In the following example, a music fragment is stored in the
563 variable @code{traLaLa}, and duplicated using Scheme. The result is
564 imported in a @code{\score} block by means of a second variable
568 traLaLa = { c'4 d'4 }
570 %% dummy action to deal with parser lookahead
571 #(display "this needs to be here, sorry!")
573 #(define newLa (map ly:music-deep-copy
574 (list traLaLa traLaLa)))
576 (make-sequential-music newLa))
581 @c Due to parser lookahead
583 In this example, the assignment happens after the parser has
584 verified that nothing interesting happens after
585 @code{traLaLa = @{ ... @}}. Without the dummy statement in the
586 above example, the @code{newLa} definition is executed before
587 @code{traLaLa} is defined, leading to a syntax error.
589 The above example shows how to @q{export} music expressions from the
590 input to the Scheme interpreter. The opposite is also possible. By
591 wrapping a Scheme value in the function @code{ly:export}, a Scheme
592 value is interpreted as if it were entered in LilyPond syntax.
593 Instead of defining @code{\twice}, the example above could also have
598 @{ #(ly:export (make-sequential-music (list newLa))) @}
601 Scheme code is evaluated as soon as the parser encounters it. To
602 define some Scheme code in a macro (to be called later), use
603 @ref{Void functions}, or
607 (ly:set-option 'point-and-click #f))
616 Mixing Scheme and LilyPond variables is not possible with the
617 @code{--safe} option.
622 @node Object properties
623 @subsection Object properties
625 This syntax will be used very frequently, since many of the layout
626 tweaks involve assigning (Scheme) values to internal variables, for
630 \override Stem #'thickness = #2.6
633 This instruction adjusts the appearance of stems. The value @code{2.6}
634 is put into the @code{thickness} variable of a @code{Stem}
635 object. @code{thickness} is measured relative to the thickness of
636 staff lines, so these stem lines will be @code{2.6} times the
637 width of staff lines. This makes stems almost twice as thick as their
638 normal size. To distinguish between variables defined in input files (like
639 @code{twentyFour} in the example above) and variables of internal
640 objects, we will call the latter @q{properties} and the former
641 @q{variables.} So, the stem object has a @code{thickness} property,
642 while @code{twentyFour} is an variable.
644 @cindex properties vs. variables
645 @cindex variables vs. properties
647 @c todo -- here we're getting interesting. We're now introducing
648 @c LilyPond variable types. I think this deserves a section all
651 @node LilyPond compound variables
652 @subsection LilyPond compound variables
654 @unnumberedsubsubsec Offsets
656 Two-dimensional offsets (X and Y coordinates) as well as object sizes
657 (intervals with a left and right point) are entered as @code{pairs}. A
658 pair@footnote{In Scheme terminology, the pair is called @code{cons},
659 and its two elements are called @code{car} and @code{cdr} respectively.}
660 is entered as @code{(first . second)} and, like symbols, they must be quoted,
663 \override TextScript #'extra-offset = #'(1 . 2)
666 This assigns the pair (1, 2) to the @code{extra-offset} property of the
667 TextScript object. These numbers are measured in staff-spaces, so
668 this command moves the object 1 staff space to the right, and 2 spaces up.
670 @unnumberedsubsubsec Extents
672 todo -- write something about extents
674 @unnumberedsubsubsec Property alists
676 todo -- write something about property alists
678 @unnumberedsubsubsec Alist chains
680 todo -- write something about alist chains
682 @node Internal music representation
683 @subsection Internal music representation
685 When a music expression is parsed, it is converted into a set of
686 Scheme music objects. The defining property of a music object is that
687 it takes up time. Time is a rational number that measures the length
688 of a piece of music in whole notes.
690 A music object has three kinds of types:
693 music name: Each music expression has a name. For example, a note
694 leads to a @rinternals{NoteEvent}, and @code{\simultaneous} leads to
695 a @rinternals{SimultaneousMusic}. A list of all expressions
696 available is in the Internals Reference manual, under
697 @rinternals{Music expressions}.
700 @q{type} or interface: Each music name has several @q{types} or
701 interfaces, for example, a note is an @code{event}, but it is also a
702 @code{note-event}, a @code{rhythmic-event}, and a
703 @code{melodic-event}. All classes of music are listed in the
704 Internals Reference, under
705 @rinternals{Music classes}.
708 C++ object: Each music object is represented by an object of the C++
712 The actual information of a music expression is stored in properties.
713 For example, a @rinternals{NoteEvent} has @code{pitch} and
714 @code{duration} properties that store the pitch and duration of that
715 note. A list of all properties available can be found in the
716 Internals Reference, under @rinternals{Music properties}.
718 A compound music expression is a music object that contains other
719 music objects in its properties. A list of objects can be stored in
720 the @code{elements} property of a music object, or a single @q{child}
721 music object in the @code{element} property. For example,
722 @rinternals{SequentialMusic} has its children in @code{elements},
723 and @rinternals{GraceMusic} has its single argument in
724 @code{element}. The body of a repeat is stored in the @code{element}
725 property of @rinternals{RepeatedMusic}, and the alternatives in
728 @node Building complicated functions
729 @section Building complicated functions
731 This section explains how to gather the information necessary
732 to create complicated music functions.
735 * Displaying music expressions::
737 * Doubling a note with slurs (example)::
738 * Adding articulation to notes (example)::
742 @node Displaying music expressions
743 @subsection Displaying music expressions
745 @cindex internal storage
746 @cindex displaying music expressions
747 @cindex internal representation, displaying
749 @funindex \displayMusic
751 When writing a music function it is often instructive to inspect how
752 a music expression is stored internally. This can be done with the
753 music function @code{\displayMusic}
757 \displayMusic @{ c'4\f @}
774 (ly:make-duration 2 0 1 1)
776 (ly:make-pitch 0 0 0))
778 'AbsoluteDynamicEvent
783 By default, LilyPond will print these messages to the console along
784 with all the other messages. To split up these messages and save
785 the results of @code{\display@{STUFF@}}, redirect the output to
789 lilypond file.ly >display.txt
792 With a bit of reformatting, the above information is
796 (make-music 'SequentialMusic
797 'elements (list (make-music 'EventChord
798 'elements (list (make-music 'NoteEvent
799 'duration (ly:make-duration 2 0 1 1)
800 'pitch (ly:make-pitch 0 0 0))
801 (make-music 'AbsoluteDynamicEvent
805 A @code{@{ ... @}} music sequence has the name @code{SequentialMusic},
806 and its inner expressions are stored as a list in its @code{'elements}
807 property. A note is represented as an @code{EventChord} expression,
808 containing a @code{NoteEvent} object (storing the duration and
809 pitch properties) and any extra information (in this case, an
810 @code{AbsoluteDynamicEvent} with a @code{"f"} text property.
813 @node Music properties
814 @subsection Music properties
816 The @code{NoteEvent} object is the first object of the
817 @code{'elements} property of @code{someNote}.
821 \displayMusic \someNote
829 (ly:make-duration 2 0 1 1)
831 (ly:make-pitch 0 0 0))))
834 The @code{display-scheme-music} function is the function used by
835 @code{\displayMusic} to display the Scheme representation of a music
839 #(display-scheme-music (first (ly:music-property someNote 'elements)))
844 (ly:make-duration 2 0 1 1)
846 (ly:make-pitch 0 0 0))
849 Then the note pitch is accessed through the @code{'pitch} property
850 of the @code{NoteEvent} object,
853 #(display-scheme-music
854 (ly:music-property (first (ly:music-property someNote 'elements))
857 (ly:make-pitch 0 0 0)
860 The note pitch can be changed by setting this @code{'pitch} property,
862 @funindex \displayLilyMusic
865 #(set! (ly:music-property (first (ly:music-property someNote 'elements))
867 (ly:make-pitch 0 1 0)) ;; set the pitch to d'.
868 \displayLilyMusic \someNote
874 @node Doubling a note with slurs (example)
875 @subsection Doubling a note with slurs (example)
877 Suppose we want to create a function that translates input like
878 @code{a} into @code{a( a)}. We begin by examining the internal
879 representation of the desired result.
882 \displayMusic@{ a'( a') @}
893 (ly:make-duration 2 0 1 1)
895 (ly:make-pitch 0 5 0))
906 (ly:make-duration 2 0 1 1)
908 (ly:make-pitch 0 5 0))
915 The bad news is that the @code{SlurEvent} expressions
916 must be added @q{inside} the note (or more precisely,
917 inside the @code{EventChord} expression).
919 Now we examine the input,
931 (ly:make-duration 2 0 1 1)
933 (ly:make-pitch 0 5 0))))))
936 So in our function, we need to clone this expression (so that we
937 have two notes to build the sequence), add @code{SlurEvents} to the
938 @code{'elements} property of each one, and finally make a
939 @code{SequentialMusic} with the two @code{EventChords}.
942 doubleSlur = #(define-music-function (parser location note) (ly:music?)
943 "Return: @{ note ( note ) @}.
944 `note' is supposed to be an EventChord."
945 (let ((note2 (ly:music-deep-copy note)))
946 (set! (ly:music-property note 'elements)
947 (cons (make-music 'SlurEvent 'span-direction -1)
948 (ly:music-property note 'elements)))
949 (set! (ly:music-property note2 'elements)
950 (cons (make-music 'SlurEvent 'span-direction 1)
951 (ly:music-property note2 'elements)))
952 (make-music 'SequentialMusic 'elements (list note note2))))
956 @node Adding articulation to notes (example)
957 @subsection Adding articulation to notes (example)
959 The easy way to add articulation to notes is to merge two music
960 expressions into one context, as explained in
961 @ruser{Creating contexts}. However, suppose that we want to write
962 a music function that does this.
964 A @code{$variable} inside the @code{#@{...#@}} notation is like
965 a regular @code{\variable} in classical LilyPond notation. We
973 will not work in LilyPond. We could avoid this problem by attaching
974 the articulation to a fake note,
977 @{ << \music s1*0-.-> @}
981 but for the sake of this example, we will learn how to do this in
982 Scheme. We begin by examining our input and desired output,
994 (ly:make-duration 2 0 1 1)
996 (ly:make-pitch -1 0 0))))
1007 (ly:make-duration 2 0 1 1)
1009 (ly:make-pitch -1 0 0))
1016 We see that a note (@code{c4}) is represented as an @code{EventChord}
1017 expression, with a @code{NoteEvent} expression in its elements list. To
1018 add a marcato articulation, an @code{ArticulationEvent} expression must
1019 be added to the elements property of the @code{EventChord}
1022 To build this function, we begin with
1025 (define (add-marcato event-chord)
1026 "Add a marcato ArticulationEvent to the elements of `event-chord',
1027 which is supposed to be an EventChord expression."
1028 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1029 (set! (ly:music-property result-event-chord 'elements)
1030 (cons (make-music 'ArticulationEvent
1031 'articulation-type "marcato")
1032 (ly:music-property result-event-chord 'elements)))
1033 result-event-chord))
1036 The first line is the way to define a function in Scheme: the function
1037 name is @code{add-marcato}, and has one variable called
1038 @code{event-chord}. In Scheme, the type of variable is often clear
1039 from its name. (this is good practice in other programming languages,
1047 is a description of what the function does. This is not strictly
1048 necessary, but just like clear variable names, it is good practice.
1051 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1054 @code{let} is used to declare local variables. Here we use one local
1055 variable, named @code{result-event-chord}, to which we give the value
1056 @code{(ly:music-deep-copy event-chord)}. @code{ly:music-deep-copy} is
1057 a function specific to LilyPond, like all functions prefixed by
1058 @code{ly:}. It is use to make a copy of a music
1059 expression. Here we copy @code{event-chord} (the parameter of the
1060 function). Recall that our purpose is to add a marcato to an
1061 @code{EventChord} expression. It is better to not modify the
1062 @code{EventChord} which was given as an argument, because it may be
1065 Now we have a @code{result-event-chord}, which is a
1066 @code{NoteEventChord} expression and is a copy of
1067 @code{event-chord}. We add the marcato to its @code{'elements}
1071 (set! place new-value)
1074 Here, what we want to set (the @q{place}) is the @code{'elements}
1075 property of @code{result-event-chord} expression.
1078 (ly:music-property result-event-chord 'elements)
1081 @code{ly:music-property} is the function used to access music properties
1082 (the @code{'elements}, @code{'duration}, @code{'pitch}, etc, that we
1083 see in the @code{\displayMusic} output above). The new value is the
1084 former @code{'elements} property, with an extra item: the
1085 @code{ArticulationEvent} expression, which we copy from the
1086 @code{\displayMusic} output,
1089 (cons (make-music 'ArticulationEvent
1090 'articulation-type "marcato")
1091 (ly:music-property result-event-chord 'elements))
1094 @code{cons} is used to add an element to a list without modifying
1095 the original list. This is what we want: the same list as before,
1096 plus the new @code{ArticulationEvent} expression. The order
1097 inside the @code{'elements} property is not important here.
1099 Finally, once we have added the marcato articulation to its @code{elements}
1100 property, we can return @code{result-event-chord}, hence the last line of
1103 Now we transform the @code{add-marcato} function into a music
1107 addMarcato = #(define-music-function (parser location event-chord)
1109 "Add a marcato ArticulationEvent to the elements of `event-chord',
1110 which is supposed to be an EventChord expression."
1111 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1112 (set! (ly:music-property result-event-chord 'elements)
1113 (cons (make-music 'ArticulationEvent
1114 'articulation-type "marcato")
1115 (ly:music-property result-event-chord 'elements)))
1116 result-event-chord))
1119 We may verify that this music function works correctly,
1122 \displayMusic \addMarcato c4
1132 * Tweaking with Scheme::
1135 @c @node Tweaking with Scheme
1136 @c @section Tweaking with Scheme
1138 We have seen how LilyPond output can be heavily modified using
1140 @code{\override TextScript #'extra-offset = ( 1 . -1)}. But
1141 we have even more power if we use Scheme. For a full explanation
1142 of this, see the @ref{Scheme tutorial}, and
1143 @ref{Interfaces for programmers}.
1145 We can use Scheme to simply @code{\override} commands,
1147 TODO Find a simple example
1148 @c This isn't a valid example with skylining
1149 @c It works fine without padText -td
1153 @lilypond[quote,verbatim,ragged-right]
1154 padText = #(define-music-function (parser location padding) (number?)
1156 \once \override TextScript #'padding = #$padding
1160 c4^"piu mosso" b a b
1162 c4^"piu mosso" d e f
1164 c4^"piu mosso" fis a g
1170 We can use it to create new commands:
1172 @c Check this is a valid example with skylining
1173 @c It is - 'padding still works
1176 @lilypond[quote,verbatim,ragged-right]
1177 tempoPadded = #(define-music-function (parser location padding tempotext)
1180 \once \override Score.MetronomeMark #'padding = $padding
1181 \tempo \markup { \bold $tempotext }
1185 \tempo \markup { "Low tempo" }
1187 \tempoPadded #4.0 #"High tempo"
1193 Even music expressions can be passed in:
1195 @lilypond[quote,verbatim,ragged-right]
1196 pattern = #(define-music-function (parser location x y) (ly:music? ly:music?)
1203 \pattern {d16 dis} { ais16-> b\p }