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14 @chapter Scheme tutorial
19 @cindex Scheme, in-line code
20 @cindex accessing Scheme
21 @cindex evaluating Scheme
24 LilyPond uses the Scheme programming language, both as part of the
25 input syntax, and as internal mechanism to glue modules of the program
26 together. This section is a very brief overview of entering data in
27 Scheme. If you want to know more about Scheme, see
28 @uref{http://@/www@/.schemers@/.org}.
30 LilyPond uses the GNU Guile implementation of Scheme, which is
31 based on the Scheme @qq{R5RS} standard. If you are learning Scheme
32 to use with LilyPond, working with a different implementation (or
33 referring to a different standard) is not recommended. Information
34 on guile can be found at @uref{http://www.gnu.org/software/guile/}.
35 The @qq{R5RS} Scheme standard is located at
36 @uref{http://www.schemers.org/Documents/Standards/R5RS/}.
39 * Introduction to Scheme::
40 * Scheme in LilyPond::
41 * Building complicated functions::
44 @node Introduction to Scheme
45 @section Introduction to Scheme
47 We begin with an introduction to Scheme. For this brief introduction,
48 we will use the GUILE interpreter to explore how the language works.
49 Once we are familiar with Scheme, we will show how the language can
50 be integrated in LilyPond files.
56 * Scheme simple data types::
57 * Scheme compound data types::
58 * Calculations in Scheme::
60 * Scheme conditionals::
64 @subsection Scheme sandbox
66 The LilyPond installation includes the Guile implementation of
67 Scheme. On most systems you can experiment in a Scheme sandbox by
68 opening a terminal window and typing @q{guile}. On some systems,
69 notably Windows, you may need to set the environment variable
70 @code{GUILE_LOAD_PATH} to the directory @code{../usr/shr/guile/1.8}
71 in the LilyPond installation. For the full path to this directory
72 see @rlearning{Other sources of information}. Alternatively, Windows
73 users may simply choose @q{Run} from the Start menu and enter
76 Once the guile sandbox is running, you will receive a guile prompt:
82 You can enter Scheme expressions at this prompt to experiment with Scheme.
84 @node Scheme variables
85 @subsection Scheme variables
87 Scheme variables can have any valid scheme value, including a Scheme
90 Scheme variables are created with @code{define}:
97 Scheme variables can be evaluated at the guile prompt simply by
98 typing the variable name:
106 Scheme variables can be printed on the display by using the display function:
114 Note that both the value @code{2} and the guile prompt @code{guile}
115 showed up on the same line. This can be avoided by calling the
116 newline procedure or displaying a newline character.
119 guile> (display a)(newline)
121 guile> (display a)(display "\n")
126 Once a variable has been created, its value can be changed with @code{set!}:
129 guile> (set! a 12345)
135 @node Scheme simple data types
136 @subsection Scheme simple data types
138 The most basic concept in a language is data typing: numbers, character
139 strings, lists, etc. Here is a list of simple Scheme data types that are
140 often used with LilyPond.
144 Boolean values are True or False. The Scheme for True is @code{#t}
145 and False is @code{#f}.
150 Numbers are entered in the standard fashion,
151 @code{1} is the (integer) number one, while @code{-1.5} is a
152 floating point number (a non-integer number).
155 Strings are enclosed in double quotes:
161 Strings may span several lines:
170 and the newline characters at the end of each line will be included
173 Newline characters can also be added by including @code{\n} in the
177 "this\nis a\nmultiline string"
181 Quotation marks and backslashes are added to strings
182 by preceding them with a backslash.
183 The string @code{\a said "b"} is entered as
191 There are additional Scheme data types that are not discussed here.
192 For a complete listing see the Guile reference guide,
193 @uref{http://www.gnu.org/software/guile/manual/html_node/Simple-Data-Types.html}.
195 @node Scheme compound data types
196 @subsection Scheme compound data types
198 There are also compound data types in Scheme. The types commonly used in
199 LilyPond programming include pairs, lists, alists, and hash tables.
201 @unnumberedsubsubsec Pairs
203 The foundational compound data type of Scheme is the @code{pair}. As
204 might be expected from its name, a pair is two values glued together.
205 The operator used to form a pair is called @code{cons}.
213 Note that the pair is displayed as two items surrounded by
214 parentheses and separated by whitespace, a period (@code{.}), and
215 more whitespace. The period is @emph{not} a decimal point, but
216 rather an indicator of the pair.
218 Pairs can also be entered as literal values by preceding them with
219 a single quote character.
227 The two elements of a pair may be any valid Scheme value:
232 guile> '("blah-blah" . 3.1415926535)
233 ("blah-blah" . 3.1415926535)
237 The first and second elements of the pair can be accessed by the
238 Scheme procedures @code{car} and @code{cdr}, respectively.
241 guile> (define mypair (cons 123 "hello there")
252 Note: @code{cdr} is pronounced "could-er", according Sussman and
254 @uref{http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-14.html#footnote_Temp_133}
257 @unnumberedsubsubsec Lists
259 A very common Scheme data structure is the @emph{list}. Formally, a
260 list is defined as either the empty list (represented as @code{'()},
261 or a pair whose @code{cdr} is a list.
263 There are many ways of creating lists. Perhaps the most common is
264 with the @code{list} procedure:
267 guile> (list 1 2 3 "abc" 17.5)
271 As can be seen, a list is displayed in the form of individual elements
272 separated by whitespace and enclosed in parentheses. Unlike a pair,
273 there is no period between the elements.
275 A list can also be entered as a literal list by enclosing its
276 elements in parentheses, and adding a quote:
279 guile> '(17 23 "foo" "bar" "bazzle")
280 (17 23 "foo" "bar" "bazzle")
283 Lists are a central part of Scheme. In, fact, Scheme is considered
284 a dialect of lisp, where @q{lisp} is an abbreviation for
285 @q{List Processing}. Scheme expressions are all lists.
287 @unnumberedsubsubsec Association lists (alists)
289 A special type of list is an @emph{association list} or @emph{alist}.
290 An alist is used to store data for easy retrieval.
292 Alists are lists whose elements are pairs. The @code{car} of each
293 element is called the @emph{key}, and the @code{cdr} of each element
294 is called the @emph{value}. The Scheme procedure @code{assoc} is
295 used to retrieve an entry from the alist, and @code{cdr} is used to
299 guile> (define my-alist '((1 . "A") (2 . "B") (3 . "C")))
301 ((1 . "A") (2 . "B") (3 . "C"))
302 guile> (assoc 2 my-alist)
304 guile> (cdr (assoc 2 my-alist))
309 Alists are widely used in LilyPond to store properties and other data.
311 @unnumberedsubsubsec Hash tables
313 A data structure that is used occasionally in LilyPond. A hash table
314 is similar to an array, but the indexes to the array can be any type
315 of Scheme value, not just integers.
317 Hash tables are more efficient than alists if there is a lot of data
318 to store and the data changes very infrequently.
320 The syntax to create hash tables is a bit complex, but you
321 can see examples of it in the LilyPond source.
324 guile> (define h (make-hash-table 10))
327 guile> (hashq-set! h 'key1 "val1")
329 guile> (hashq-set! h 'key2 "val2")
331 guile> (hashq-set! h 3 "val3")
335 Values are retrieved from hash tables with @code{hashq-ref}.
338 guile> (hashq-ref h 3)
340 guile> (hashq-ref h 'key2)
345 Keys and values are retrieved as a pair with @code{hashq-get-handle}.
346 This is a preferred way, because it will return @code{#f} if a key is
350 guile> (hashq-get-handle h 'key1)
352 guile> (hashq-get-handle h 'frob)
357 @node Calculations in Scheme
358 @subsection Calculations in Scheme
361 We have been using lists all along. A calculation, like @code{(+ 1 2)}
362 is also a list (containing the symbol @code{+} and the numbers 1
363 and@tie{}2). Normally lists are interpreted as calculations, and the
364 Scheme interpreter substitutes the outcome of the calculation. To enter a
365 list, we stop the evaluation. This is done by quoting the list with a
366 quote @code{'} symbol. So, for calculations do not use a quote.
368 Inside a quoted list or pair, there is no need to quote anymore. The
369 following is a pair of symbols, a list of symbols and a list of lists
374 #'(staff clef key-signature)
379 Scheme can be used to do calculations. It uses @emph{prefix}
380 syntax. Adding 1 and@tie{}2 is written as @code{(+ 1 2)} rather than the
381 traditional @math{1+2}.
388 Calculations may be nested; the result of a function may
389 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 Scheme calculations are sensitive to the differences between integers
400 and non-integers. Integer calculations are exact, while non-integers
401 are calculated to the appropriate limits of precision:
410 When the scheme interpreter encounters an expression that is a list,
411 the first element of the list is treated as a procedure to be
412 evaluated with the arguments of the remainder of the list. Therefore,
413 all operators in Scheme are prefix operators.
415 If the first element of a Scheme expression that is a list passed to
416 the interpreter is @emph{not} an operator or procedure, an error will
426 <unnamed port>:52:1: In expression (1 2 3):
427 <unnamed port>:52:1: Wrong type to apply: 1
432 Here you can see that the interpreter was trying to treat 1 as an
433 operator or procedure, and it couldn't. Hence the error is "Wrong
436 Therefore, to create a list we need to use the list operator, or to
437 quote the list so that the interpreter will not try to evaluate it.
447 This is an error that can appear as you are working with Scheme in LilyPond.
450 The same assignment can be done in completely in Scheme as well,
453 #(define twentyFour (* 2 twelve))
456 @c this next section is confusing -- need to rewrite
458 The @emph{name} of a variable is also an expression, similar to a
459 number or a string. It is entered as
466 @cindex quoting in Scheme
468 The quote mark @code{'} prevents the Scheme interpreter from substituting
469 @code{24} for the @code{twentyFour}. Instead, we get the name
474 @node Scheme procedures
475 @subsection Scheme procedures
477 Scheme procedures are executable scheme expressions that return a
478 value resulting from their execution. They can also manipulate
479 variables defined outside of the procedure.
481 @unnumberedsubsubsec Defining procedures
483 Procedures are defined in Scheme with define
486 (define (function-name arg1 arg2 ... argn)
487 scheme-expression-that-gives-a-return-value)
490 For example, we could define a procedure to calculate the average:
493 guile> (define (average x y) (/ (+ x y) 2))
495 #<procedure average (x y)>
498 Once a procedure is defined, it is called by putting the procedure
499 name and the arguments in a list. For example, we can calculate
500 the average of 3 and 12:
503 guile> (average 3 12)
507 @unnumberedsubsubsec Predicates
509 Scheme procedures that return boolean values are often called
510 @emph{predicates}. By convention (but not necessity), predicate names
511 typically end in a question mark:
514 guile> (define (less-than-ten? x) (< x 10))
515 guile> (less-than-ten? 9)
517 guile> (less-than-ten? 15)
521 @unnumberedsubsubsec Return values
523 Sometimes the user would like to have multiple Scheme expressions in
524 a procedure. There are two ways that multiple expressions can be
525 combined. The first is the @code{begin} procedure, which allows
526 multiple expressions to be evaluated, and returns the value of
530 guile> (begin (+ 1 2) (- 5 8) (* 2 2))
534 The second way to combine multiple expressions is in a @code{let} block.
535 In a let block, a series of bindings are created, and then a sequence
536 of expressions that can include those bindings is evaluated. The
537 return value of the let block is the return value of the last
538 statement in the let block:
541 guile> (let ((x 2) (y 3) (z 4)) (display (+ x y)) (display (- z 4))
542 ... (+ (* x y) (/ z x)))
546 @node Scheme conditionals
547 @subsection Scheme conditionals
549 @unnumberedsubsubsec if
551 Scheme has an @code{if} procedure:
554 (if test-expression true-expression false-expression)
557 @var{test-expression} is an expression that returns a boolean
558 value. If @var{test-expression} returns @code{#t}, the if
559 procedure returns the value of @var{true-expression}, otherwise
560 it returns the value of @var{false-expression}.
565 guile> (if (> a b) "a is greater than b" "a is not greater than b")
566 "a is not greater than b"
569 @unnumberedsubsubsec cond
571 Another conditional procedure in scheme is @code{cond}:
574 (cond (test-expression-1 result-expression-sequence-1)
575 (test-expression-2 result-expression-sequence-2)
577 (test-expression-n result-expression-sequence-n))
585 guile> (cond ((< a b) "a is less than b")
586 ... ((= a b) "a equals b")
587 ... ((> a b) "a is greater than b"))
591 @node Scheme in LilyPond
592 @section Scheme in LilyPond
596 * LilyPond Scheme syntax::
597 * LilyPond variables::
598 * Input variables and Scheme::
599 * Object properties::
600 * LilyPond compound variables::
601 * Internal music representation::
604 @node LilyPond Scheme syntax
605 @subsection LilyPond Scheme syntax
607 In a music file, snippets of Scheme code are introduced with the hash
608 mark @code{#}. So, the previous examples translated to LilyPond are
619 Note that LilyPond comments (@code{%} and @code{%@{ %@}}) cannot
620 be used within Scheme code. Comments in Guile Scheme are entered
624 ; this is a single-line comment
627 This a (non-nestable) Guile-style block comment
628 But these are rarely used by Schemers and never in
633 Multiple consecutive scheme expressions in a music file can be
634 combined using the @code{begin} operator. This permits the number
635 of hash marks to be reduced to one.
643 @c todo -- # introduces a scheme *expression*
644 @c need the concept of an expression
646 If @code{#} is followed by an opening parenthesis, @code{(}, as in
647 the example above, the parser will remain in Scheme mode until
648 a matching closing parenthesis, @code{)}, is found, so further
649 @code{#} symbols to introduce a Scheme section are not required.
651 For the rest of this section, we will assume that the data is entered
652 in a music file, so we add @code{#}s everywhere.
654 @node LilyPond variables
655 @subsection LilyPond variables
658 TODO -- make this read right
660 A similar thing happens with variables. After defining a variable
667 variables can also be used in expressions, here
670 twentyFour = (* 2 twelve)
674 the number 24 is stored in the variable @code{twentyFour}.
676 @node Input variables and Scheme
677 @subsection Input variables and Scheme
679 The input format supports the notion of variables: in the following
680 example, a music expression is assigned to a variable with the name
684 traLaLa = @{ c'4 d'4 @}
689 There is also a form of scoping: in the following example, the
690 @code{\layout} block also contains a @code{traLaLa} variable, which is
691 independent of the outer @code{\traLaLa}.
693 traLaLa = @{ c'4 d'4 @}
694 \layout @{ traLaLa = 1.0 @}
697 In effect, each input file is a scope, and all @code{\header},
698 @code{\midi}, and @code{\layout} blocks are scopes nested inside that
701 Both variables and scoping are implemented in the GUILE module system.
702 An anonymous Scheme module is attached to each scope. An assignment of
705 traLaLa = @{ c'4 d'4 @}
709 is internally converted to a Scheme definition
711 (define traLaLa @var{Scheme value of `@code{... }'})
714 This means that input variables and Scheme variables may be freely
715 mixed. In the following example, a music fragment is stored in the
716 variable @code{traLaLa}, and duplicated using Scheme. The result is
717 imported in a @code{\score} block by means of a second variable
721 traLaLa = { c'4 d'4 }
723 %% dummy action to deal with parser lookahead
724 #(display "this needs to be here, sorry!")
726 #(define newLa (map ly:music-deep-copy
727 (list traLaLa traLaLa)))
729 (make-sequential-music newLa))
734 @c Due to parser lookahead
736 In this example, the assignment happens after the parser has
737 verified that nothing interesting happens after
738 @code{traLaLa = @{ ... @}}. Without the dummy statement in the
739 above example, the @code{newLa} definition is executed before
740 @code{traLaLa} is defined, leading to a syntax error.
742 The above example shows how to @q{export} music expressions from the
743 input to the Scheme interpreter. The opposite is also possible. By
744 wrapping a Scheme value in the function @code{ly:export}, a Scheme
745 value is interpreted as if it were entered in LilyPond syntax.
746 Instead of defining @code{\twice}, the example above could also have
751 @{ #(ly:export (make-sequential-music (list newLa))) @}
754 Scheme code is evaluated as soon as the parser encounters it. To
755 define some Scheme code in a macro (to be called later), use
756 @ref{Void functions}, or
760 (ly:set-option 'point-and-click #f))
769 Mixing Scheme and LilyPond variables is not possible with the
770 @code{--safe} option.
775 @node Object properties
776 @subsection Object properties
778 This syntax will be used very frequently, since many of the layout
779 tweaks involve assigning (Scheme) values to internal variables, for
783 \override Stem #'thickness = #2.6
786 This instruction adjusts the appearance of stems. The value @code{2.6}
787 is put into the @code{thickness} variable of a @code{Stem}
788 object. @code{thickness} is measured relative to the thickness of
789 staff lines, so these stem lines will be @code{2.6} times the
790 width of staff lines. This makes stems almost twice as thick as their
791 normal size. To distinguish between variables defined in input files (like
792 @code{twentyFour} in the example above) and variables of internal
793 objects, we will call the latter @q{properties} and the former
794 @q{variables.} So, the stem object has a @code{thickness} property,
795 while @code{twentyFour} is an variable.
797 @cindex properties vs. variables
798 @cindex variables vs. properties
800 @c todo -- here we're getting interesting. We're now introducing
801 @c LilyPond variable types. I think this deserves a section all
804 @node LilyPond compound variables
805 @subsection LilyPond compound variables
807 @unnumberedsubsubsec Offsets
809 Two-dimensional offsets (X and Y coordinates) as well as object sizes
810 (intervals with a left and right point) are entered as @code{pairs}. A
811 pair@footnote{In Scheme terminology, the pair is called @code{cons},
812 and its two elements are called @code{car} and @code{cdr} respectively.}
813 is entered as @code{(first . second)} and, like symbols, they must be quoted,
816 \override TextScript #'extra-offset = #'(1 . 2)
819 This assigns the pair (1, 2) to the @code{extra-offset} property of the
820 TextScript object. These numbers are measured in staff-spaces, so
821 this command moves the object 1 staff space to the right, and 2 spaces up.
823 @unnumberedsubsubsec Extents
825 todo -- write something about extents
827 @unnumberedsubsubsec Property alists
829 todo -- write something about property alists
831 @unnumberedsubsubsec Alist chains
833 todo -- write something about alist chains
835 @node Internal music representation
836 @subsection Internal music representation
838 When a music expression is parsed, it is converted into a set of
839 Scheme music objects. The defining property of a music object is that
840 it takes up time. Time is a rational number that measures the length
841 of a piece of music in whole notes.
843 A music object has three kinds of types:
846 music name: Each music expression has a name. For example, a note
847 leads to a @rinternals{NoteEvent}, and @code{\simultaneous} leads to
848 a @rinternals{SimultaneousMusic}. A list of all expressions
849 available is in the Internals Reference manual, under
850 @rinternals{Music expressions}.
853 @q{type} or interface: Each music name has several @q{types} or
854 interfaces, for example, a note is an @code{event}, but it is also a
855 @code{note-event}, a @code{rhythmic-event}, and a
856 @code{melodic-event}. All classes of music are listed in the
857 Internals Reference, under
858 @rinternals{Music classes}.
861 C++ object: Each music object is represented by an object of the C++
865 The actual information of a music expression is stored in properties.
866 For example, a @rinternals{NoteEvent} has @code{pitch} and
867 @code{duration} properties that store the pitch and duration of that
868 note. A list of all properties available can be found in the
869 Internals Reference, under @rinternals{Music properties}.
871 A compound music expression is a music object that contains other
872 music objects in its properties. A list of objects can be stored in
873 the @code{elements} property of a music object, or a single @q{child}
874 music object in the @code{element} property. For example,
875 @rinternals{SequentialMusic} has its children in @code{elements},
876 and @rinternals{GraceMusic} has its single argument in
877 @code{element}. The body of a repeat is stored in the @code{element}
878 property of @rinternals{RepeatedMusic}, and the alternatives in
881 @node Building complicated functions
882 @section Building complicated functions
884 This section explains how to gather the information necessary
885 to create complicated music functions.
888 * Displaying music expressions::
890 * Doubling a note with slurs (example)::
891 * Adding articulation to notes (example)::
895 @node Displaying music expressions
896 @subsection Displaying music expressions
898 @cindex internal storage
899 @cindex displaying music expressions
900 @cindex internal representation, displaying
902 @funindex \displayMusic
904 When writing a music function it is often instructive to inspect how
905 a music expression is stored internally. This can be done with the
906 music function @code{\displayMusic}
910 \displayMusic @{ c'4\f @}
927 (ly:make-duration 2 0 1 1)
929 (ly:make-pitch 0 0 0))
931 'AbsoluteDynamicEvent
936 By default, LilyPond will print these messages to the console along
937 with all the other messages. To split up these messages and save
938 the results of @code{\display@{STUFF@}}, redirect the output to
942 lilypond file.ly >display.txt
945 With a bit of reformatting, the above information is easier to read,
948 (make-music 'SequentialMusic
949 'elements (list (make-music 'EventChord
950 'elements (list (make-music 'NoteEvent
951 'duration (ly:make-duration 2 0 1 1)
952 'pitch (ly:make-pitch 0 0 0))
953 (make-music 'AbsoluteDynamicEvent
957 A @code{@{ ... @}} music sequence has the name @code{SequentialMusic},
958 and its inner expressions are stored as a list in its @code{'elements}
959 property. A note is represented as an @code{EventChord} expression,
960 containing a @code{NoteEvent} object (storing the duration and
961 pitch properties) and any extra information (in this case, an
962 @code{AbsoluteDynamicEvent} with a @code{"f"} text property.
965 @node Music properties
966 @subsection Music properties
968 The @code{NoteEvent} object is the first object of the
969 @code{'elements} property of @code{someNote}.
973 \displayMusic \someNote
981 (ly:make-duration 2 0 1 1)
983 (ly:make-pitch 0 0 0))))
986 The @code{display-scheme-music} function is the function used by
987 @code{\displayMusic} to display the Scheme representation of a music
991 #(display-scheme-music (first (ly:music-property someNote 'elements)))
996 (ly:make-duration 2 0 1 1)
998 (ly:make-pitch 0 0 0))
1001 Then the note pitch is accessed through the @code{'pitch} property
1002 of the @code{NoteEvent} object,
1005 #(display-scheme-music
1006 (ly:music-property (first (ly:music-property someNote 'elements))
1009 (ly:make-pitch 0 0 0)
1012 The note pitch can be changed by setting this @code{'pitch} property,
1014 @funindex \displayLilyMusic
1017 #(set! (ly:music-property (first (ly:music-property someNote 'elements))
1019 (ly:make-pitch 0 1 0)) ;; set the pitch to d'.
1020 \displayLilyMusic \someNote
1026 @node Doubling a note with slurs (example)
1027 @subsection Doubling a note with slurs (example)
1029 Suppose we want to create a function that translates input like
1030 @code{a} into @code{a( a)}. We begin by examining the internal
1031 representation of the desired result.
1034 \displayMusic@{ a'( a') @}
1045 (ly:make-duration 2 0 1 1)
1047 (ly:make-pitch 0 5 0))
1058 (ly:make-duration 2 0 1 1)
1060 (ly:make-pitch 0 5 0))
1067 The bad news is that the @code{SlurEvent} expressions
1068 must be added @q{inside} the note (or more precisely,
1069 inside the @code{EventChord} expression).
1071 Now we examine the input,
1083 (ly:make-duration 2 0 1 1)
1085 (ly:make-pitch 0 5 0))))))
1088 So in our function, we need to clone this expression (so that we
1089 have two notes to build the sequence), add @code{SlurEvents} to the
1090 @code{'elements} property of each one, and finally make a
1091 @code{SequentialMusic} with the two @code{EventChords}.
1094 doubleSlur = #(define-music-function (parser location note) (ly:music?)
1095 "Return: @{ note ( note ) @}.
1096 `note' is supposed to be an EventChord."
1097 (let ((note2 (ly:music-deep-copy note)))
1098 (set! (ly:music-property note 'elements)
1099 (cons (make-music 'SlurEvent 'span-direction -1)
1100 (ly:music-property note 'elements)))
1101 (set! (ly:music-property note2 'elements)
1102 (cons (make-music 'SlurEvent 'span-direction 1)
1103 (ly:music-property note2 'elements)))
1104 (make-music 'SequentialMusic 'elements (list note note2))))
1108 @node Adding articulation to notes (example)
1109 @subsection Adding articulation to notes (example)
1111 The easy way to add articulation to notes is to merge two music
1112 expressions into one context, as explained in
1113 @ruser{Creating contexts}. However, suppose that we want to write
1114 a music function that does this.
1116 A @code{$variable} inside the @code{#@{...#@}} notation is like
1117 a regular @code{\variable} in classical LilyPond notation. We
1125 will not work in LilyPond. We could avoid this problem by attaching
1126 the articulation to a fake note,
1129 @{ << \music s1*0-.-> @}
1133 but for the sake of this example, we will learn how to do this in
1134 Scheme. We begin by examining our input and desired output,
1146 (ly:make-duration 2 0 1 1)
1148 (ly:make-pitch -1 0 0))))
1159 (ly:make-duration 2 0 1 1)
1161 (ly:make-pitch -1 0 0))
1168 We see that a note (@code{c4}) is represented as an @code{EventChord}
1169 expression, with a @code{NoteEvent} expression in its elements list. To
1170 add a marcato articulation, an @code{ArticulationEvent} expression must
1171 be added to the elements property of the @code{EventChord}
1174 To build this function, we begin with
1177 (define (add-marcato event-chord)
1178 "Add a marcato ArticulationEvent to the elements of `event-chord',
1179 which is supposed to be an EventChord expression."
1180 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1181 (set! (ly:music-property result-event-chord 'elements)
1182 (cons (make-music 'ArticulationEvent
1183 'articulation-type "marcato")
1184 (ly:music-property result-event-chord 'elements)))
1185 result-event-chord))
1188 The first line is the way to define a function in Scheme: the function
1189 name is @code{add-marcato}, and has one variable called
1190 @code{event-chord}. In Scheme, the type of variable is often clear
1191 from its name. (this is good practice in other programming languages,
1199 is a description of what the function does. This is not strictly
1200 necessary, but just like clear variable names, it is good practice.
1203 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1206 @code{let} is used to declare local variables. Here we use one local
1207 variable, named @code{result-event-chord}, to which we give the value
1208 @code{(ly:music-deep-copy event-chord)}. @code{ly:music-deep-copy} is
1209 a function specific to LilyPond, like all functions prefixed by
1210 @code{ly:}. It is use to make a copy of a music
1211 expression. Here we copy @code{event-chord} (the parameter of the
1212 function). Recall that our purpose is to add a marcato to an
1213 @code{EventChord} expression. It is better to not modify the
1214 @code{EventChord} which was given as an argument, because it may be
1217 Now we have a @code{result-event-chord}, which is a
1218 @code{NoteEventChord} expression and is a copy of
1219 @code{event-chord}. We add the marcato to its @code{'elements}
1223 (set! place new-value)
1226 Here, what we want to set (the @q{place}) is the @code{'elements}
1227 property of @code{result-event-chord} expression.
1230 (ly:music-property result-event-chord 'elements)
1233 @code{ly:music-property} is the function used to access music properties
1234 (the @code{'elements}, @code{'duration}, @code{'pitch}, etc, that we
1235 see in the @code{\displayMusic} output above). The new value is the
1236 former @code{'elements} property, with an extra item: the
1237 @code{ArticulationEvent} expression, which we copy from the
1238 @code{\displayMusic} output,
1241 (cons (make-music 'ArticulationEvent
1242 'articulation-type "marcato")
1243 (ly:music-property result-event-chord 'elements))
1246 @code{cons} is used to add an element to a list without modifying
1247 the original list. This is what we want: the same list as before,
1248 plus the new @code{ArticulationEvent} expression. The order
1249 inside the @code{'elements} property is not important here.
1251 Finally, once we have added the marcato articulation to its @code{elements}
1252 property, we can return @code{result-event-chord}, hence the last line of
1255 Now we transform the @code{add-marcato} function into a music
1259 addMarcato = #(define-music-function (parser location event-chord)
1261 "Add a marcato ArticulationEvent to the elements of `event-chord',
1262 which is supposed to be an EventChord expression."
1263 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1264 (set! (ly:music-property result-event-chord 'elements)
1265 (cons (make-music 'ArticulationEvent
1266 'articulation-type "marcato")
1267 (ly:music-property result-event-chord 'elements)))
1268 result-event-chord))
1271 We may verify that this music function works correctly,
1274 \displayMusic \addMarcato c4
1284 * Tweaking with Scheme::
1287 @c @node Tweaking with Scheme
1288 @c @section Tweaking with Scheme
1290 We have seen how LilyPond output can be heavily modified using
1292 @code{\override TextScript #'extra-offset = ( 1 . -1)}. But
1293 we have even more power if we use Scheme. For a full explanation
1294 of this, see the @ref{Scheme tutorial}, and
1295 @ref{Interfaces for programmers}.
1297 We can use Scheme to simply @code{\override} commands,
1299 TODO Find a simple example
1300 @c This isn't a valid example with skylining
1301 @c It works fine without padText -td
1305 @lilypond[quote,verbatim,ragged-right]
1306 padText = #(define-music-function (parser location padding) (number?)
1308 \once \override TextScript #'padding = #$padding
1312 c4^"piu mosso" b a b
1314 c4^"piu mosso" d e f
1316 c4^"piu mosso" fis a g
1322 We can use it to create new commands:
1324 @c Check this is a valid example with skylining
1325 @c It is - 'padding still works
1328 @lilypond[quote,verbatim,ragged-right]
1329 tempoPadded = #(define-music-function (parser location padding tempotext)
1332 \once \override Score.MetronomeMark #'padding = $padding
1333 \tempo \markup { \bold $tempotext }
1337 \tempo \markup { "Low tempo" }
1339 \tempoPadded #4.0 #"High tempo"
1345 Even music expressions can be passed in:
1347 @lilypond[quote,verbatim,ragged-right]
1348 pattern = #(define-music-function (parser location x y) (ly:music? ly:music?)
1355 \pattern {d16 dis} { ais16-> b\p }