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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 received 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 use of the display function:
114 Note that the value @code{2} and the guile prompt @code{guile} both
115 showed up on the same line. This can be avoided by calling the newline
116 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.
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}
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 @subheading 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 @subheading 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.
395 These calculations are examples of evaluations; an expression like
396 @code{(* 3 4)} is replaced by its value @code{12}.
398 Scheme calculations are sensitive to the differences between integers
399 and non-integers. Integer calculations are exact, while non-integers
400 are calculated to the appropriate limits of precision:
409 When the scheme interpreter encounters an expression that is a list, the
410 first element of the list is treated as a procedure to be evaluated
411 with the arguments of the remainder of the list. Therefore, all operators
412 in Scheme are prefix operators.
414 If the first element of a Scheme expression that is a list passed to the
415 interpreter`is @emph{not} an operator or procedure, an error will occur:
424 <unnamed port>:52:1: In expression (1 2 3):
425 <unnamed port>:52:1: Wrong type to apply: 1
430 Here you can see that the interpreter was trying to treat 1 as an operator
431 or procedure, and it couldn't. Hence the error is "Wrong type to apply: 1".
433 To create a list, then , we need to use the list operator, or we need to
434 quote the list so that the interpreter will not try to evaluate it.
444 This is an error that can appear as you are working with Scheme in LilyPond.
447 The same assignment can be done in completely in Scheme as well,
450 #(define twentyFour (* 2 twelve))
453 @c this next section is confusing -- need to rewrite
455 The @emph{name} of a variable is also an expression, similar to a
456 number or a string. It is entered as
463 @cindex quoting in Scheme
465 The quote mark @code{'} prevents the Scheme interpreter from substituting
466 @code{24} for the @code{twentyFour}. Instead, we get the name
471 @node Scheme procedures
472 @subsection Scheme procedures
474 Scheme procedures are executable scheme expressions that return
475 a value resulting from their execution., They can also manipulate
476 variables defined outside of the procedure.
478 @subheading Defining procedures
480 Procedures are defined in Scheme with define
483 (define (function-name arg1 arg2 ... argn)
484 scheme-expression-that-gives-a-return-value)
487 For example, we could define a procedure to calculate the average:
490 guile> (define (average x y) (/ (+ x y) 2))
492 #<procedure average (x y)>
495 Once a procedure is defined, it is called by putting the procedure
496 name and the arguments in a list. For example, we can calculate
497 the average of 3 and 12:
500 guile> (average 3 12)
504 @subheading Predicates
506 Scheme procedures that return boolean values are often called
507 @emph{predicates}. By convention (but not necessity), predicate names
508 typically end in a question mark:
511 guile> (define (less-than-ten? x) (< x 10))
512 guile> (less-than-ten? 9)
514 guile> (less-than-ten? 15)
518 @subheading Return values
520 Sometimes the user would like to have multiple Scheme expressions in
521 a procedure. There are two ways that multiple expressions can be
522 combined. The first is the @code{begin} procedure, which allows
523 multiple expressions to be evaluated, and returns the value of
527 guile> (begin (+ 1 2) (- 5 8) (* 2 2))
531 The second way to combine multiple expressions is in a @code{let} block.
532 In a let block, a series of bindings are created, and then a sequence
533 of expressions that can include those bindings is evaluated. The
534 return value of the let block is the return value of the last
535 statement in the let block:
538 guile> (let ((x 2) (y 3) (z 4)) (display (+ x y)) (display (- z 4))
539 ... (+ (* x y) (/ z x)))
543 @node Scheme conditionals
544 @subsection Scheme conditionals
548 Scheme has an @code{if} procedure:
551 (if test-expression true-expression false-expression)
554 @var{test-expression} is an expression that returns a boolean
555 value. If @var{test-expression} returns @code{#t}, the if
556 procedure returns the value of @var{true-expression}, otherwise
557 it returns the value of @var{false-expression}.
562 guile> (if (> a b) "a is greater than b" "a is not greater than b")
563 "a is not greater than b"
568 Another conditional procedure in scheme is @code{cond}:
571 (cond (test-expression-1 result-expression-sequence-1)
572 (test-expression-2 result-expression-sequence-2)
574 (test-expression-n result-expression-sequence-n))
582 guile> (cond ((< a b) "a is less than b")
583 ... ((= a b) "a equals b")
584 ... ((> a b) "a is greater than b"))
588 @node Scheme in LilyPond
589 @section Scheme in LilyPond
593 * LilyPond Scheme syntax::
594 * LilyPond variables::
595 * Input variables and Scheme::
596 * Object properties::
597 * LilyPond compound variables::
598 * Internal music representation::
601 @node LilyPond Scheme syntax
602 @subsection LilyPond Scheme syntax
604 In a music file, snippets of Scheme code are introduced with the hash
605 mark @code{#}. So, the previous examples translated to LilyPond are
616 Note that LilyPond comments (@code{%} and @code{%@{ %@}}) cannot
617 be used within Scheme code. Comments in Guile Scheme are entered
621 ; this is a single-line comment
624 This a (non-nestable) Guile-style block comment
625 But these are rarely used by Schemers and never in
630 Multiple consecutive scheme expressions in a music file can be
631 combined using the @code{begin} operator. This permits the number
632 of hash marks to be reduced to one.
640 @c todo -- # introduces a scheme *expression*
641 @c need the concept of an expression
643 If @code{#} is followed by an opening parenthesis, @code{(}, as in
644 the example above, the parser will remain in Scheme mode until
645 a matching closing parenthesis, @code{)}, is found, so further
646 @code{#} symbols to introduce a Scheme section are not required.
648 For the rest of this section, we will assume that the data is entered
649 in a music file, so we add @code{#}s everywhere.
651 @node LilyPond variables
652 @subsection LilyPond variables
655 TODO -- make this read right
658 happens with variables. After defining a variable
665 variables can also be used in expressions, here
668 twentyFour = (* 2 twelve)
672 the number 24 is stored in the variable @code{twentyFour}.
674 @node Input variables and Scheme
675 @subsection Input variables and Scheme
677 The input format supports the notion of variables: in the following
678 example, a music expression is assigned to a variable with the name
682 traLaLa = @{ c'4 d'4 @}
687 There is also a form of scoping: in the following example, the
688 @code{\layout} block also contains a @code{traLaLa} variable, which is
689 independent of the outer @code{\traLaLa}.
691 traLaLa = @{ c'4 d'4 @}
692 \layout @{ traLaLa = 1.0 @}
695 In effect, each input file is a scope, and all @code{\header},
696 @code{\midi}, and @code{\layout} blocks are scopes nested inside that
699 Both variables and scoping are implemented in the GUILE module system.
700 An anonymous Scheme module is attached to each scope. An assignment of
703 traLaLa = @{ c'4 d'4 @}
707 is internally converted to a Scheme definition
709 (define traLaLa @var{Scheme value of `@code{... }'})
712 This means that input variables and Scheme variables may be freely
713 mixed. In the following example, a music fragment is stored in the
714 variable @code{traLaLa}, and duplicated using Scheme. The result is
715 imported in a @code{\score} block by means of a second variable
719 traLaLa = { c'4 d'4 }
721 %% dummy action to deal with parser lookahead
722 #(display "this needs to be here, sorry!")
724 #(define newLa (map ly:music-deep-copy
725 (list traLaLa traLaLa)))
727 (make-sequential-music newLa))
732 @c Due to parser lookahead
734 In this example, the assignment happens after the parser has
735 verified that nothing interesting happens after
736 @code{traLaLa = @{ ... @}}. Without the dummy statement in the
737 above example, the @code{newLa} definition is executed before
738 @code{traLaLa} is defined, leading to a syntax error.
740 The above example shows how to @q{export} music expressions from the
741 input to the Scheme interpreter. The opposite is also possible. By
742 wrapping a Scheme value in the function @code{ly:export}, a Scheme
743 value is interpreted as if it were entered in LilyPond syntax.
744 Instead of defining @code{\twice}, the example above could also have
749 @{ #(ly:export (make-sequential-music (list newLa))) @}
752 Scheme code is evaluated as soon as the parser encounters it. To
753 define some Scheme code in a macro (to be called later), use
754 @ref{Void functions}, or
758 (ly:set-option 'point-and-click #f))
767 Mixing Scheme and LilyPond variables is not possible with the
768 @code{--safe} option.
773 @node Object properties
774 @subsection Object properties
776 This syntax will be used very frequently, since many of the layout
777 tweaks involve assigning (Scheme) values to internal variables, for
781 \override Stem #'thickness = #2.6
784 This instruction adjusts the appearance of stems. The value @code{2.6}
785 is put into the @code{thickness} variable of a @code{Stem}
786 object. @code{thickness} is measured relative to the thickness of
787 staff lines, so these stem lines will be @code{2.6} times the
788 width of staff lines. This makes stems almost twice as thick as their
789 normal size. To distinguish between variables defined in input files (like
790 @code{twentyFour} in the example above) and variables of internal
791 objects, we will call the latter @q{properties} and the former
792 @q{variables.} So, the stem object has a @code{thickness} property,
793 while @code{twentyFour} is an variable.
795 @cindex properties vs. variables
796 @cindex variables vs. properties
798 @c todo -- here we're getting interesting. We're now introducing
799 @c LilyPond variable types. I think this deserves a section all
802 @node LilyPond compound variables
803 @subsection LilyPond compound variables
807 Two-dimensional offsets (X and Y coordinates) as well as object sizes
808 (intervals with a left and right point) are entered as @code{pairs}. A
809 pair@footnote{In Scheme terminology, the pair is called @code{cons},
810 and its two elements are called @code{car} and @code{cdr} respectively.}
811 is entered as @code{(first . second)} and, like symbols, they must be quoted,
814 \override TextScript #'extra-offset = #'(1 . 2)
817 This assigns the pair (1, 2) to the @code{extra-offset} property of the
818 TextScript object. These numbers are measured in staff-spaces, so
819 this command moves the object 1 staff space to the right, and 2 spaces up.
823 todo -- write something about extents
825 @subheading Property alists
827 todo -- write something about property alists
829 @subheading Alist chains
831 todo -- write something about alist chains
833 @node Internal music representation
834 @subsection Internal music representation
836 When a music expression is parsed, it is converted into a set of
837 Scheme music objects. The defining property of a music object is that
838 it takes up time. Time is a rational number that measures the length
839 of a piece of music in whole notes.
841 A music object has three kinds of types:
844 music name: Each music expression has a name. For example, a note
845 leads to a @rinternals{NoteEvent}, and @code{\simultaneous} leads to
846 a @rinternals{SimultaneousMusic}. A list of all expressions
847 available is in the Internals Reference manual, under
848 @rinternals{Music expressions}.
851 @q{type} or interface: Each music name has several @q{types} or
852 interfaces, for example, a note is an @code{event}, but it is also a
853 @code{note-event}, a @code{rhythmic-event}, and a
854 @code{melodic-event}. All classes of music are listed in the
855 Internals Reference, under
856 @rinternals{Music classes}.
859 C++ object: Each music object is represented by an object of the C++
863 The actual information of a music expression is stored in properties.
864 For example, a @rinternals{NoteEvent} has @code{pitch} and
865 @code{duration} properties that store the pitch and duration of that
866 note. A list of all properties available can be found in the
867 Internals Reference, under @rinternals{Music properties}.
869 A compound music expression is a music object that contains other
870 music objects in its properties. A list of objects can be stored in
871 the @code{elements} property of a music object, or a single @q{child}
872 music object in the @code{element} property. For example,
873 @rinternals{SequentialMusic} has its children in @code{elements},
874 and @rinternals{GraceMusic} has its single argument in
875 @code{element}. The body of a repeat is stored in the @code{element}
876 property of @rinternals{RepeatedMusic}, and the alternatives in
879 @node Building complicated functions
880 @section Building complicated functions
882 This section explains how to gather the information necessary
883 to create complicated music functions.
886 * Displaying music expressions::
888 * Doubling a note with slurs (example)::
889 * Adding articulation to notes (example)::
892 @node Displaying music expressions
893 @subsection Displaying music expressions
895 @cindex internal storage
896 @cindex displaying music expressions
897 @cindex internal representation, displaying
899 @funindex \displayMusic
901 When writing a music function it is often instructive to inspect how
902 a music expression is stored internally. This can be done with the
903 music function @code{\displayMusic}
907 \displayMusic @{ c'4\f @}
924 (ly:make-duration 2 0 1 1)
926 (ly:make-pitch 0 0 0))
928 'AbsoluteDynamicEvent
933 By default, LilyPond will print these messages to the console along
934 with all the other messages. To split up these messages and save
935 the results of @code{\display@{STUFF@}}, redirect the output to
939 lilypond file.ly >display.txt
942 With a bit of reformatting, the above information is
946 (make-music 'SequentialMusic
947 'elements (list (make-music 'EventChord
948 'elements (list (make-music 'NoteEvent
949 'duration (ly:make-duration 2 0 1 1)
950 'pitch (ly:make-pitch 0 0 0))
951 (make-music 'AbsoluteDynamicEvent
955 A @code{@{ ... @}} music sequence has the name @code{SequentialMusic},
956 and its inner expressions are stored as a list in its @code{'elements}
957 property. A note is represented as an @code{EventChord} expression,
958 containing a @code{NoteEvent} object (storing the duration and
959 pitch properties) and any extra information (in this case, an
960 @code{AbsoluteDynamicEvent} with a @code{"f"} text property.
963 @node Music properties
964 @subsection Music properties
966 The @code{NoteEvent} object is the first object of the
967 @code{'elements} property of @code{someNote}.
971 \displayMusic \someNote
979 (ly:make-duration 2 0 1 1)
981 (ly:make-pitch 0 0 0))))
984 The @code{display-scheme-music} function is the function used by
985 @code{\displayMusic} to display the Scheme representation of a music
989 #(display-scheme-music (first (ly:music-property someNote 'elements)))
994 (ly:make-duration 2 0 1 1)
996 (ly:make-pitch 0 0 0))
999 Then the note pitch is accessed through the @code{'pitch} property
1000 of the @code{NoteEvent} object,
1003 #(display-scheme-music
1004 (ly:music-property (first (ly:music-property someNote 'elements))
1007 (ly:make-pitch 0 0 0)
1010 The note pitch can be changed by setting this @code{'pitch} property,
1012 @funindex \displayLilyMusic
1015 #(set! (ly:music-property (first (ly:music-property someNote 'elements))
1017 (ly:make-pitch 0 1 0)) ;; set the pitch to d'.
1018 \displayLilyMusic \someNote
1024 @node Doubling a note with slurs (example)
1025 @subsection Doubling a note with slurs (example)
1027 Suppose we want to create a function that translates input like
1028 @code{a} into @code{a( a)}. We begin by examining the internal
1029 representation of the desired result.
1032 \displayMusic@{ a'( a') @}
1043 (ly:make-duration 2 0 1 1)
1045 (ly:make-pitch 0 5 0))
1056 (ly:make-duration 2 0 1 1)
1058 (ly:make-pitch 0 5 0))
1065 The bad news is that the @code{SlurEvent} expressions
1066 must be added @q{inside} the note (or more precisely,
1067 inside the @code{EventChord} expression).
1069 Now we examine the input,
1081 (ly:make-duration 2 0 1 1)
1083 (ly:make-pitch 0 5 0))))))
1086 So in our function, we need to clone this expression (so that we
1087 have two notes to build the sequence), add @code{SlurEvents} to the
1088 @code{'elements} property of each one, and finally make a
1089 @code{SequentialMusic} with the two @code{EventChords}.
1092 doubleSlur = #(define-music-function (parser location note) (ly:music?)
1093 "Return: @{ note ( note ) @}.
1094 `note' is supposed to be an EventChord."
1095 (let ((note2 (ly:music-deep-copy note)))
1096 (set! (ly:music-property note 'elements)
1097 (cons (make-music 'SlurEvent 'span-direction -1)
1098 (ly:music-property note 'elements)))
1099 (set! (ly:music-property note2 'elements)
1100 (cons (make-music 'SlurEvent 'span-direction 1)
1101 (ly:music-property note2 'elements)))
1102 (make-music 'SequentialMusic 'elements (list note note2))))
1106 @node Adding articulation to notes (example)
1107 @subsection Adding articulation to notes (example)
1109 The easy way to add articulation to notes is to merge two music
1110 expressions into one context, as explained in
1111 @ruser{Creating contexts}. However, suppose that we want to write
1112 a music function that does this.
1114 A @code{$variable} inside the @code{#@{...#@}} notation is like
1115 a regular @code{\variable} in classical LilyPond notation. We
1123 will not work in LilyPond. We could avoid this problem by attaching
1124 the articulation to a fake note,
1127 @{ << \music s1*0-.-> @}
1131 but for the sake of this example, we will learn how to do this in
1132 Scheme. We begin by examining our input and desired output,
1144 (ly:make-duration 2 0 1 1)
1146 (ly:make-pitch -1 0 0))))
1157 (ly:make-duration 2 0 1 1)
1159 (ly:make-pitch -1 0 0))
1166 We see that a note (@code{c4}) is represented as an @code{EventChord}
1167 expression, with a @code{NoteEvent} expression in its elements list. To
1168 add a marcato articulation, an @code{ArticulationEvent} expression must
1169 be added to the elements property of the @code{EventChord}
1172 To build this function, we begin with
1175 (define (add-marcato event-chord)
1176 "Add a marcato ArticulationEvent to the elements of `event-chord',
1177 which is supposed to be an EventChord expression."
1178 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1179 (set! (ly:music-property result-event-chord 'elements)
1180 (cons (make-music 'ArticulationEvent
1181 'articulation-type "marcato")
1182 (ly:music-property result-event-chord 'elements)))
1183 result-event-chord))
1186 The first line is the way to define a function in Scheme: the function
1187 name is @code{add-marcato}, and has one variable called
1188 @code{event-chord}. In Scheme, the type of variable is often clear
1189 from its name. (this is good practice in other programming languages,
1197 is a description of what the function does. This is not strictly
1198 necessary, but just like clear variable names, it is good practice.
1201 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1204 @code{let} is used to declare local variables. Here we use one local
1205 variable, named @code{result-event-chord}, to which we give the value
1206 @code{(ly:music-deep-copy event-chord)}. @code{ly:music-deep-copy} is
1207 a function specific to LilyPond, like all functions prefixed by
1208 @code{ly:}. It is use to make a copy of a music
1209 expression. Here we copy @code{event-chord} (the parameter of the
1210 function). Recall that our purpose is to add a marcato to an
1211 @code{EventChord} expression. It is better to not modify the
1212 @code{EventChord} which was given as an argument, because it may be
1215 Now we have a @code{result-event-chord}, which is a
1216 @code{NoteEventChord} expression and is a copy of
1217 @code{event-chord}. We add the marcato to its @code{'elements}
1221 (set! place new-value)
1224 Here, what we want to set (the @q{place}) is the @code{'elements}
1225 property of @code{result-event-chord} expression.
1228 (ly:music-property result-event-chord 'elements)
1231 @code{ly:music-property} is the function used to access music properties
1232 (the @code{'elements}, @code{'duration}, @code{'pitch}, etc, that we
1233 see in the @code{\displayMusic} output above). The new value is the
1234 former @code{'elements} property, with an extra item: the
1235 @code{ArticulationEvent} expression, which we copy from the
1236 @code{\displayMusic} output,
1239 (cons (make-music 'ArticulationEvent
1240 'articulation-type "marcato")
1241 (ly:music-property result-event-chord 'elements))
1244 @code{cons} is used to add an element to a list without modifying
1245 the original list. This is what we want: the same list as before,
1246 plus the new @code{ArticulationEvent} expression. The order
1247 inside the @code{'elements} property is not important here.
1249 Finally, once we have added the marcato articulation to its @code{elements}
1250 property, we can return @code{result-event-chord}, hence the last line of
1253 Now we transform the @code{add-marcato} function into a music
1257 addMarcato = #(define-music-function (parser location event-chord)
1259 "Add a marcato ArticulationEvent to the elements of `event-chord',
1260 which is supposed to be an EventChord expression."
1261 (let ((result-event-chord (ly:music-deep-copy event-chord)))
1262 (set! (ly:music-property result-event-chord 'elements)
1263 (cons (make-music 'ArticulationEvent
1264 'articulation-type "marcato")
1265 (ly:music-property result-event-chord 'elements)))
1266 result-event-chord))
1269 We may verify that this music function works correctly,
1272 \displayMusic \addMarcato c4
1282 * Tweaking with Scheme::
1285 @c @node Tweaking with Scheme
1286 @c @section Tweaking with Scheme
1288 We have seen how LilyPond output can be heavily modified using
1290 @code{\override TextScript #'extra-offset = ( 1 . -1)}. But
1291 we have even more power if we use Scheme. For a full explanation
1292 of this, see the @ref{Scheme tutorial}, and
1293 @ref{Interfaces for programmers}.
1295 We can use Scheme to simply @code{\override} commands,
1297 TODO Find a simple example
1298 @c This isn't a valid example with skylining
1299 @c It works fine without padText -td
1303 @lilypond[quote,verbatim,ragged-right]
1304 padText = #(define-music-function (parser location padding) (number?)
1306 \once \override TextScript #'padding = #$padding
1310 c4^"piu mosso" b a b
1312 c4^"piu mosso" d e f
1314 c4^"piu mosso" fis a g
1320 We can use it to create new commands:
1322 @c Check this is a valid example with skylining
1323 @c It is - 'padding still works
1326 @lilypond[quote,verbatim,ragged-right]
1327 tempoPadded = #(define-music-function (parser location padding tempotext)
1330 \once \override Score.MetronomeMark #'padding = $padding
1331 \tempo \markup { \bold $tempotext }
1335 \tempo \markup { "Low tempo" }
1337 \tempoPadded #4.0 #"High tempo"
1343 Even music expressions can be passed in:
1345 @lilypond[quote,verbatim,ragged-right]
1346 pattern = #(define-music-function (parser location x y) (ly:music? ly:music?)
1353 \pattern {d16 dis} { ais16-> b\p }