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14 @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/share/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 However, a hands-on Scheme sandbox with all of Lilypond loaded is
76 available with this command line:
78 lilypond scheme-sandbox
82 Once the sandbox is running, you will receive a guile prompt:
88 You can enter Scheme expressions at this prompt to experiment with
89 Scheme. If you want to be able to use the GNU readline library for
90 nicer editing of the Scheme command line, check the file
91 @file{ly/scheme-sandbox.ly} for more information. If you already have
92 enabled the readline library for your interactive Guile sessions outside
93 of LilyPond, this should work in the sandbox as well.
95 @node Scheme variables
96 @subsection Scheme variables
98 Scheme variables can have any valid scheme value, including a Scheme
101 Scheme variables are created with @code{define}:
108 Scheme variables can be evaluated at the guile prompt simply by
109 typing the variable name:
117 Scheme variables can be printed on the display by using the display function:
125 Note that both the value @code{2} and the guile prompt @code{guile}
126 showed up on the same line. This can be avoided by calling the
127 newline procedure or displaying a newline character.
130 guile> (display a)(newline)
132 guile> (display a)(display "\n")
137 Once a variable has been created, its value can be changed with @code{set!}:
140 guile> (set! a 12345)
146 @node Scheme simple data types
147 @subsection Scheme simple data types
149 The most basic concept in a language is data typing: numbers, character
150 strings, lists, etc. Here is a list of simple Scheme data types that are
151 often used with LilyPond.
155 Boolean values are True or False. The Scheme for True is @code{#t}
156 and False is @code{#f}.
161 Numbers are entered in the standard fashion,
162 @code{1} is the (integer) number one, while @w{@code{-1.5}} is a
163 floating point number (a non-integer number).
166 Strings are enclosed in double quotes:
172 Strings may span several lines:
181 and the newline characters at the end of each line will be included
184 Newline characters can also be added by including @code{\n} in the
188 "this\nis a\nmultiline string"
192 Quotation marks and backslashes are added to strings
193 by preceding them with a backslash.
194 The string @code{\a said "b"} is entered as
202 There are additional Scheme data types that are not discussed here.
203 For a complete listing see the Guile reference guide,
204 @uref{http://www.gnu.org/software/guile/manual/html_node/Simple-Data-Types.html}.
206 @node Scheme compound data types
207 @subsection Scheme compound data types
209 There are also compound data types in Scheme. The types commonly used in
210 LilyPond programming include pairs, lists, alists, and hash tables.
215 * Association lists (alists)::
220 @unnumberedsubsubsec Pairs
222 The foundational compound data type of Scheme is the @code{pair}. As
223 might be expected from its name, a pair is two values glued together.
224 The operator used to form a pair is called @code{cons}.
232 Note that the pair is displayed as two items surrounded by
233 parentheses and separated by whitespace, a period (@code{.}), and
234 more whitespace. The period is @emph{not} a decimal point, but
235 rather an indicator of the pair.
237 Pairs can also be entered as literal values by preceding them with
238 a single quote character.
246 The two elements of a pair may be any valid Scheme value:
251 guile> '("blah-blah" . 3.1415926535)
252 ("blah-blah" . 3.1415926535)
256 The first and second elements of the pair can be accessed by the
257 Scheme procedures @code{car} and @code{cdr}, respectively.
260 guile> (define mypair (cons 123 "hello there")
271 Note: @code{cdr} is pronounced "could-er", according Sussman and
273 @uref{http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-14.html#footnote_Temp_133}
276 @unnumberedsubsubsec Lists
278 A very common Scheme data structure is the @emph{list}. Formally, a
279 list is defined as either the empty list (represented as @code{'()},
280 or a pair whose @code{cdr} is a list.
282 There are many ways of creating lists. Perhaps the most common is
283 with the @code{list} procedure:
286 guile> (list 1 2 3 "abc" 17.5)
290 As can be seen, a list is displayed in the form of individual elements
291 separated by whitespace and enclosed in parentheses. Unlike a pair,
292 there is no period between the elements.
294 A list can also be entered as a literal list by enclosing its
295 elements in parentheses, and adding a quote:
298 guile> '(17 23 "foo" "bar" "bazzle")
299 (17 23 "foo" "bar" "bazzle")
302 Lists are a central part of Scheme. In, fact, Scheme is considered
303 a dialect of lisp, where @q{lisp} is an abbreviation for
304 @q{List Processing}. Scheme expressions are all lists.
306 @node Association lists (alists)
307 @unnumberedsubsubsec Association lists (alists)
309 A special type of list is an @emph{association list} or @emph{alist}.
310 An alist is used to store data for easy retrieval.
312 Alists are lists whose elements are pairs. The @code{car} of each
313 element is called the @emph{key}, and the @code{cdr} of each element
314 is called the @emph{value}. The Scheme procedure @code{assoc} is
315 used to retrieve an entry from the alist, and @code{cdr} is used to
319 guile> (define my-alist '((1 . "A") (2 . "B") (3 . "C")))
321 ((1 . "A") (2 . "B") (3 . "C"))
322 guile> (assoc 2 my-alist)
324 guile> (cdr (assoc 2 my-alist))
329 Alists are widely used in LilyPond to store properties and other data.
332 @unnumberedsubsubsec Hash tables
334 A data structure that is used occasionally in LilyPond. A hash table
335 is similar to an array, but the indexes to the array can be any type
336 of Scheme value, not just integers.
338 Hash tables are more efficient than alists if there is a lot of data
339 to store and the data changes very infrequently.
341 The syntax to create hash tables is a bit complex, but you
342 can see examples of it in the LilyPond source.
345 guile> (define h (make-hash-table 10))
348 guile> (hashq-set! h 'key1 "val1")
350 guile> (hashq-set! h 'key2 "val2")
352 guile> (hashq-set! h 3 "val3")
356 Values are retrieved from hash tables with @code{hashq-ref}.
359 guile> (hashq-ref h 3)
361 guile> (hashq-ref h 'key2)
366 Keys and values are retrieved as a pair with @code{hashq-get-handle}.
367 This is a preferred way, because it will return @code{#f} if a key is
371 guile> (hashq-get-handle h 'key1)
373 guile> (hashq-get-handle h 'frob)
378 @node Calculations in Scheme
379 @subsection Calculations in Scheme
382 We have been using lists all along. A calculation, like @code{(+ 1 2)}
383 is also a list (containing the symbol @code{+} and the numbers 1
384 and@tie{}2). Normally lists are interpreted as calculations, and the
385 Scheme interpreter substitutes the outcome of the calculation. To enter a
386 list, we stop the evaluation. This is done by quoting the list with a
387 quote @code{'} symbol. So, for calculations do not use a quote.
389 Inside a quoted list or pair, there is no need to quote anymore. The
390 following is a pair of symbols, a list of symbols and a list of lists
395 #'(staff clef key-signature)
400 Scheme can be used to do calculations. It uses @emph{prefix}
401 syntax. Adding 1 and@tie{}2 is written as @code{(+ 1 2)} rather than the
402 traditional @math{1+2}.
409 Calculations may be nested; the result of a function may
410 be used for another calculation.
417 These calculations are examples of evaluations; an expression like
418 @code{(* 3 4)} is replaced by its value @code{12}.
420 Scheme calculations are sensitive to the differences between integers
421 and non-integers. Integer calculations are exact, while non-integers
422 are calculated to the appropriate limits of precision:
431 When the scheme interpreter encounters an expression that is a list,
432 the first element of the list is treated as a procedure to be
433 evaluated with the arguments of the remainder of the list. Therefore,
434 all operators in Scheme are prefix operators.
436 If the first element of a Scheme expression that is a list passed to
437 the interpreter is @emph{not} an operator or procedure, an error will
447 <unnamed port>:52:1: In expression (1 2 3):
448 <unnamed port>:52:1: Wrong type to apply: 1
453 Here you can see that the interpreter was trying to treat 1 as an
454 operator or procedure, and it couldn't. Hence the error is "Wrong
457 Therefore, to create a list we need to use the list operator, or to
458 quote the list so that the interpreter will not try to evaluate it.
468 This is an error that can appear as you are working with Scheme in LilyPond.
471 The same assignment can be done in completely in Scheme as well,
474 #(define twentyFour (* 2 twelve))
477 @c this next section is confusing -- need to rewrite
479 The @emph{name} of a variable is also an expression, similar to a
480 number or a string. It is entered as
487 @cindex quoting in Scheme
489 The quote mark @code{'} prevents the Scheme interpreter from substituting
490 @code{24} for the @code{twentyFour}. Instead, we get the name
495 @node Scheme procedures
496 @subsection Scheme procedures
498 Scheme procedures are executable scheme expressions that return a
499 value resulting from their execution. They can also manipulate
500 variables defined outside of the procedure.
503 * Defining procedures::
508 @node Defining procedures
509 @unnumberedsubsubsec Defining procedures
511 Procedures are defined in Scheme with define
514 (define (function-name arg1 arg2 ... argn)
515 scheme-expression-that-gives-a-return-value)
518 For example, we could define a procedure to calculate the average:
521 guile> (define (average x y) (/ (+ x y) 2))
523 #<procedure average (x y)>
526 Once a procedure is defined, it is called by putting the procedure
527 name and the arguments in a list. For example, we can calculate
528 the average of 3 and 12:
531 guile> (average 3 12)
536 @unnumberedsubsubsec Predicates
538 Scheme procedures that return boolean values are often called
539 @emph{predicates}. By convention (but not necessity), predicate names
540 typically end in a question mark:
543 guile> (define (less-than-ten? x) (< x 10))
544 guile> (less-than-ten? 9)
546 guile> (less-than-ten? 15)
551 @unnumberedsubsubsec Return values
553 Scheme procedures always return a return value, which is the value
554 of the last expression executed in the procedure. The return
555 value can be any valid Scheme value, including a complex data
556 structure or a procedure.
558 Sometimes the user would like to have multiple Scheme expressions in
559 a procedure. There are two ways that multiple expressions can be
560 combined. The first is the @code{begin} procedure, which allows
561 multiple expressions to be evaluated, and returns the value of
565 guile> (begin (+ 1 2) (- 5 8) (* 2 2))
569 The second way to combine multiple expressions is in a @code{let} block.
570 In a let block, a series of bindings are created, and then a sequence
571 of expressions that can include those bindings is evaluated. The
572 return value of the let block is the return value of the last
573 statement in the let block:
576 guile> (let ((x 2) (y 3) (z 4)) (display (+ x y)) (display (- z 4))
577 @dots{} (+ (* x y) (/ z x)))
581 @node Scheme conditionals
582 @subsection Scheme conditionals
590 @unnumberedsubsubsec if
592 Scheme has an @code{if} procedure:
595 (if test-expression true-expression false-expression)
598 @var{test-expression} is an expression that returns a boolean
599 value. If @var{test-expression} returns @code{#t}, the if
600 procedure returns the value of @var{true-expression}, otherwise
601 it returns the value of @var{false-expression}.
606 guile> (if (> a b) "a is greater than b" "a is not greater than b")
607 "a is not greater than b"
611 @unnumberedsubsubsec cond
613 Another conditional procedure in scheme is @code{cond}:
616 (cond (test-expression-1 result-expression-sequence-1)
617 (test-expression-2 result-expression-sequence-2)
619 (test-expression-n result-expression-sequence-n))
627 guile> (cond ((< a b) "a is less than b")
628 ... ((= a b) "a equals b")
629 ... ((> a b) "a is greater than b"))
633 @node Scheme in LilyPond
634 @section Scheme in LilyPond
638 * LilyPond Scheme syntax::
639 * LilyPond variables::
640 * Input variables and Scheme::
641 * Importing Scheme in LilyPond::
642 * Object properties::
643 * LilyPond compound variables::
644 * Internal music representation::
647 @node LilyPond Scheme syntax
648 @subsection LilyPond Scheme syntax
652 The Guile interpreter is part of LilyPond, which means that
653 Scheme can be included in LilyPond input files. There are several
654 methods for including Scheme in LilyPond.
656 The simplest way is to use a hash mark@tie{}@code{#} before a Scheme
659 Now LilyPond's input is structured into tokens and expressions, much
660 like human language is structured into words and sentences. LilyPond
661 has a lexer that recognizes tokens (literal numbers, strings, Scheme
662 elements, pitches and so on), and a parser that understands the syntax,
663 @rcontrib{LilyPond grammar}. Once it knows that a particular syntax rule
664 applies, it executes actions associated with it.
666 The hash mark@tie{}@code{#} method of embedding Scheme is a natural fit
667 for this system. Once the lexer sees a hash mark, it calls the Scheme
668 reader to read one full Scheme expression (this can be an identifier, an
669 expression enclosed in parentheses, or several other things). After the
670 Scheme expression is read, it is stored away as the value for an
671 @code{SCM_TOKEN} in the grammar. Once the parser knows how to make use
672 of this token, it calls Guile for evaluating the Scheme expression.
673 Since the parser usually requires a bit of lookahead from the lexer to
674 make its parsing decisions, this separation of reading and evaluation
675 between lexer and parser is exactly what is needed to keep the execution
676 of LilyPond and Scheme expressions in sync. For this reason, you should
677 use the hash mark@tie{}@code{#} for calling Scheme whenever this is
680 Another way to call the Scheme interpreter from LilyPond is the use of
681 dollar@tie{}@code{$} instead of a hash mark for introducing Scheme
682 expressions. In this case, Lilypond evaluates the code right after the
683 lexer has read it. It checks the resulting type of the Scheme
684 expression and then picks a token type (one of several
685 @code{xxx_IDENTIFIER} in the syntax) for it. It creates a @emph{copy}
686 of the value and uses that for the value of the token. If the value of
687 the expression is void (Guile's value of @code{*unspecified*}), nothing
688 at all is passed to the parser.
690 This is, in fact, exactly the same mechanism that Lilypond employs when
691 you call any variable or music function by name, as @code{\name}, with
692 the only difference that the name is determined by the Lilypond lexer
693 without consulting the Scheme reader, and thus only variable names
694 consistent with the current Lilypond mode are accepted.
696 The immediate action of @code{$} can lead to surprises, @ref{Input
697 variables and Scheme}. Using @code{#} where the parser supports it
698 is usually preferable. Inside of music expressions, expressions
699 created using @code{#} @emph{are} interpreted as music. However,
700 they are @emph{not} copied before use. If they are part of some
701 structure that might still get used, you may need to use
702 @code{ly:music-deep-copy} explicitly.
706 There are also @q{list splicing} operators @code{$@@} and @code{#@@}
707 that insert all elements of a list in the surrounding context.
709 Now let's take a look at some actual Scheme code. Scheme procedures can
710 be defined in LilyPond input files:
713 #(define (average a b c) (/ (+ a b c) 3))
716 Note that LilyPond comments (@code{%} and @code{%@{ %@}}) cannot
717 be used within Scheme code, even in a LilyPond input file, because
718 the Guile interpreter, not the LilyPond lexer, is reading
719 the Scheme expression. Comments in Guile Scheme are entered
723 ; this is a single-line comment
726 This a (non-nestable) Guile-style block comment
727 But these are rarely used by Schemers and never in
732 For the rest of this section, we will assume that the data is entered
733 in a music file, so we add@tie{}@code{#}s at the beginning of each Scheme
736 All of the top-level Scheme expressions in a LilyPond input file can
737 be combined into a single Scheme expression by the use of the
738 @code{begin} statement:
747 @node LilyPond variables
748 @subsection LilyPond variables
750 LilyPond variables are stored internally in the form of Scheme
764 This means that LilyPond variables are available
765 for use in Scheme expressions. For example, we could use
768 twentyFour = #(* 2 twelve)
772 which would result in the number 24 being stored in the
773 LilyPond (and Scheme) variable @code{twentyFour}.
775 The usual way to refer to Lilypond variables, @ref{LilyPond Scheme
776 syntax}, is to call them using a backslash, i.e., @code{\twentyFour}.
777 Since this creates a copy of the value for most of LilyPond's internal
778 types, in particular music expressions, music functions don't usually
779 create copies of material they change. For this reason, music
780 expressions given with @code{#} should usually not contain material that
781 is not either created from scratch or explicitly copied rather than
784 @node Input variables and Scheme
785 @subsection Input variables and Scheme
787 The input format supports the notion of variables: in the following
788 example, a music expression is assigned to a variable with the name
792 traLaLa = @{ c'4 d'4 @}
797 There is also a form of scoping: in the following example, the
798 @code{\layout} block also contains a @code{traLaLa} variable, which is
799 independent of the outer @code{\traLaLa}.
802 traLaLa = @{ c'4 d'4 @}
803 \layout @{ traLaLa = 1.0 @}
807 In effect, each input file is a scope, and all @code{\header},
808 @code{\midi}, and @code{\layout} blocks are scopes nested inside that
811 Both variables and scoping are implemented in the GUILE module system.
812 An anonymous Scheme module is attached to each scope. An assignment of
816 traLaLa = @{ c'4 d'4 @}
820 is internally converted to a Scheme definition:
823 (define traLaLa @var{Scheme value of `@code{@dots{}}'})
826 This means that LilyPond variables and Scheme variables may be freely
827 mixed. In the following example, a music fragment is stored in the
828 variable @code{traLaLa}, and duplicated using Scheme. The result is
829 imported in a @code{\score} block by means of a second variable
833 traLaLa = { c'4 d'4 }
835 #(define newLa (map ly:music-deep-copy
836 (list traLaLa traLaLa)))
838 (make-sequential-music newLa))
843 @c Due to parser lookahead
845 This is actually a rather interesting example. The assignment will only
846 take place after the parser has ascertained that nothing akin to
847 @code{\addlyrics} follows, so it needs to check what comes next. It
848 reads @code{#} and the following Scheme expression @emph{without}
849 evaluating it, so it can go ahead with the assignment, and
850 @emph{afterwards} execute the Scheme code without problem.
852 @node Importing Scheme in LilyPond
853 @subsection Importing Scheme in LilyPond
857 The above example shows how to @q{export} music expressions from the
858 input to the Scheme interpreter. The opposite is also possible. By
859 placing it after @code{$}, a Scheme
860 value is interpreted as if it were entered in LilyPond syntax.
861 Instead of defining @code{\twice}, the example above could also have
866 $(make-sequential-music newLa)
869 You can use @code{$} with a Scheme expression anywhere you could use
870 @code{\@var{name}} after having assigned the Scheme expression to a
871 variable @var{name}. This replacement happens in the @q{lexer}, so
872 Lilypond is not even aware of the difference.
874 One drawback, however, is that of timing. If we had been using @code{$}
875 instead of @code{#} for defining @code{newLa} in the above example, the
876 following Scheme definition would have failed because @code{traLaLa}
877 would not yet have been defined. For an explanation of this timing
878 problem, @ref{LilyPond Scheme syntax}.
882 A further convenience can be the @q{list splicing} operators @code{$@@}
883 and @code{#@@} for inserting the elements of a list in the surrounding
884 context. Using those, the last part of the example could have been
892 Here, every element of the list stored in @code{newLa} is taken in
893 sequence and inserted into the list, as if we had written
896 @{ #(first newLa) #(second newLa) @}
899 Now in all of these forms, the Scheme code is evaluated while the
900 input is still being consumed, either in the lexer or in the parser.
901 If you need it to be executed at a later point of time, check out
902 @ref{Void scheme functions}, or store it in a procedure:
906 (ly:set-option 'point-and-click #f))
915 Mixing Scheme and LilyPond variables is not possible with the
916 @option{--safe} option.
919 @node Object properties
920 @subsection Object properties
922 Object properties are stored in LilyPond in the form of alist-chains,
923 which are lists of alists. Properties are set by adding values at
924 the beginning of the property list. Properties are read by retrieving
925 values from the alists.
927 Setting a new value for a property requires assigning a value to
928 the alist with both a key and a value. The LilyPond syntax for doing
932 \override Stem.thickness = #2.6
935 This instruction adjusts the appearance of stems. An alist entry
936 @code{'(thickness . 2.6)} is added to the property list of the
938 object. @code{thickness} is measured relative to the thickness of
939 staff lines, so these stem lines will be @code{2.6} times the
940 width of staff lines. This makes stems almost twice as thick as their
941 normal size. To distinguish between variables defined in input files (like
942 @code{twentyFour} in the example above) and variables of internal
943 objects, we will call the latter @q{properties} and the former
944 @q{variables.} So, the stem object has a @code{thickness} property,
945 while @code{twentyFour} is a variable.
947 @cindex properties vs. variables
948 @cindex variables vs. properties
950 @c todo -- here we're getting interesting. We're now introducing
951 @c LilyPond variable types. I think this deserves a section all
954 @node LilyPond compound variables
955 @subsection LilyPond compound variables
966 @unnumberedsubsubsec Offsets
968 Two-dimensional offsets (X and Y coordinates) are stored as @emph{pairs}.
969 The @code{car} of the offset is the X coordinate, and the @code{cdr} is
973 \override TextScript.extra-offset = #'(1 . 2)
976 This assigns the pair @code{(1 . 2)} to the @code{extra-offset}
978 TextScript object. These numbers are measured in staff-spaces, so
979 this command moves the object 1 staff space to the right, and 2 spaces up.
981 Procedures for working with offsets are found in @file{scm/lily-library.scm}.
984 @unnumberedsubsubsec Fractions
986 Fractions as used by LilyPond are again stored as @emph{pairs}, this
987 time of unsigned integers. While Scheme can represent rational numbers
988 as a native type, musically @samp{2/4} and @samp{1/2} are not the same,
989 and we need to be able to distinguish between them. Similarly there are
990 no negative @q{fractions} in LilyPond's mind. So @code{2/4} in LilyPond
991 means @code{(2 . 4)} in Scheme, and @code{#2/4} in LilyPond means
992 @code{1/2} in Scheme.
995 @unnumberedsubsubsec Extents
997 Pairs are also used to store intervals, which represent a range of numbers
998 from the minimum (the @code{car}) to the maximum (the @code{cdr}).
999 Intervals are used to store the X- and Y- extents of printable objects.
1000 For X extents, the @code{car} is the left hand X coordinate, and the
1001 @code{cdr} is the right hand X coordinate. For Y extents, the @code{car}
1002 is the bottom coordinate, and the @code{cdr} is the top coordinate.
1004 Procedures for working with intervals are found in
1005 @file{scm/lily-library.scm}. These procedures should be used when possible
1006 to ensure consistency of code.
1008 @node Property alists
1009 @unnumberedsubsubsec Property alists
1011 A property alist is a LilyPond data structure that is an alist whose
1012 keys are properties and whose values are Scheme expressions that give
1013 the desired value for the property.
1015 LilyPond properties are Scheme symbols, such as @code{'thickness}.
1018 @unnumberedsubsubsec Alist chains
1020 An alist chain is a list containing property alists.
1022 The set of all properties that will apply to a grob is typically
1023 stored as an alist chain. In order to find the value for a particular
1024 property that a grob should have, each alist in the chain is searched in
1025 order, looking for an entry containing the property key. The first alist
1026 entry found is returned, and the value is the property value.
1028 The Scheme procedure @code{chain-assoc-get} is normally used to get
1029 grob property values.
1031 @node Internal music representation
1032 @subsection Internal music representation
1034 Internally, music is represented as a Scheme list. The list contains
1035 various elements that affect the printed output. Parsing is the process
1036 of converting music from the LilyPond input representation to the
1037 internal Scheme representation.
1039 When a music expression is parsed, it is converted into a set of
1040 Scheme music objects. The defining property of a music object is that
1041 it takes up time. The time it takes up is called its @emph{duration}.
1042 Durations are expressed as a rational number that measures the length
1043 of the music object in whole notes.
1045 A music object has three kinds of types:
1048 music name: Each music expression has a name. For example, a note
1049 leads to a @rinternals{NoteEvent}, and @code{\simultaneous} leads to
1050 a @rinternals{SimultaneousMusic}. A list of all expressions
1051 available is in the Internals Reference manual, under
1052 @rinternals{Music expressions}.
1055 @q{type} or interface: Each music name has several @q{types} or
1056 interfaces, for example, a note is an @code{event}, but it is also a
1057 @code{note-event}, a @code{rhythmic-event}, and a
1058 @code{melodic-event}. All classes of music are listed in the
1059 Internals Reference, under
1060 @rinternals{Music classes}.
1063 C++ object: Each music object is represented by an object of the C++
1067 The actual information of a music expression is stored in properties.
1068 For example, a @rinternals{NoteEvent} has @code{pitch} and
1069 @code{duration} properties that store the pitch and duration of that
1070 note. A list of all properties available can be found in the
1071 Internals Reference, under @rinternals{Music properties}.
1073 A compound music expression is a music object that contains other
1074 music objects in its properties. A list of objects can be stored in
1075 the @code{elements} property of a music object, or a single @q{child}
1076 music object in the @code{element} property. For example,
1077 @rinternals{SequentialMusic} has its children in @code{elements},
1078 and @rinternals{GraceMusic} has its single argument in
1079 @code{element}. The body of a repeat is stored in the @code{element}
1080 property of @rinternals{RepeatedMusic}, and the alternatives in
1083 @node Building complicated functions
1084 @section Building complicated functions
1086 This section explains how to gather the information necessary
1087 to create complicated music functions.
1090 * Displaying music expressions::
1091 * Music properties::
1092 * Doubling a note with slurs (example)::
1093 * Adding articulation to notes (example)::
1096 @node Displaying music expressions
1097 @subsection Displaying music expressions
1099 @cindex internal storage
1100 @cindex displaying music expressions
1101 @cindex internal representation, displaying
1102 @cindex displayMusic
1103 @funindex \displayMusic
1105 When writing a music function it is often instructive to inspect how
1106 a music expression is stored internally. This can be done with the
1107 music function @code{\displayMusic}
1111 \displayMusic @{ c'4\f @}
1126 'AbsoluteDynamicEvent
1130 (ly:make-duration 2 0 1/1)
1132 (ly:make-pitch 0 0 0))))
1135 By default, LilyPond will print these messages to the console along
1136 with all the other messages. To split up these messages and save
1137 the results of @code{\display@{STUFF@}}, you can specify an optional
1142 \displayMusic #(open-output-file "display.txt") @{ c'4\f @}
1146 This will overwrite a previous output file whenever it is called; if you
1147 need to write more than one expression, you would use a variable for
1148 your port and reuse it:
1151 port = #(open-output-file "display.txt")
1152 \displayMusic \port @{ c'4\f @}
1153 \displayMusic \port @{ d'4 @}
1154 #(close-output-port port)
1158 Guile's manual describes ports in detail. Closing the port is actually
1159 only necessary if you need to read the file before Lilypond finishes; in
1160 the first example, we did not bother to do so.
1162 A bit of reformatting makes the above information easier to read:
1165 (make-music 'SequentialMusic
1167 (make-music 'NoteEvent
1168 'articulations (list
1169 (make-music 'AbsoluteDynamicEvent
1172 'duration (ly:make-duration 2 0 1/1)
1173 'pitch (ly:make-pitch 0 0 0))))
1176 A @code{@{ @dots{} @}} music sequence has the name
1177 @code{SequentialMusic}, and its inner expressions are stored as a list
1178 in its @code{'elements} property. A note is represented as a
1179 @code{NoteEvent} object (storing the duration and pitch properties) with
1180 attached information (in this case, an @code{AbsoluteDynamicEvent} with
1181 a @code{"f"} text property) stored in its @code{articulations} property.
1184 @code{\displayMusic} returns the music it displays, so it will get
1185 interpreted as well as displayed. To avoid interpretation, write
1186 @code{\void} before @code{\displayMusic}.
1188 @node Music properties
1189 @subsection Music properties
1191 TODO -- make sure we delineate between @emph{music} properties,
1192 @emph{context} properties, and @emph{layout} properties. These
1193 are potentially confusing.
1195 Let's look at an example:
1199 \displayMusic \someNote
1204 (ly:make-duration 2 0 1/1)
1206 (ly:make-pitch 0 0 0))
1209 The @code{NoteEvent} object is the representation of @code{someNote}.
1210 Straightforward. How about putting c' in a chord?
1214 \displayMusic \someNote
1222 (ly:make-duration 2 0 1/1)
1224 (ly:make-pitch 0 0 0))))
1227 Now the @code{NoteEvent} object is the first object of the
1228 @code{'elements} property of @code{someNote}.
1230 The @code{display-scheme-music} function is the function used by
1231 @code{\displayMusic} to display the Scheme representation of a music
1235 #(display-scheme-music (first (ly:music-property someNote 'elements)))
1240 (ly:make-duration 2 0 1/1)
1242 (ly:make-pitch 0 0 0))
1245 Then the note pitch is accessed through the @code{'pitch} property
1246 of the @code{NoteEvent} object,
1249 #(display-scheme-music
1250 (ly:music-property (first (ly:music-property someNote 'elements))
1253 (ly:make-pitch 0 0 0)
1256 The note pitch can be changed by setting this @code{'pitch} property,
1258 @funindex \displayLilyMusic
1261 #(set! (ly:music-property (first (ly:music-property someNote 'elements))
1263 (ly:make-pitch 0 1 0)) ;; set the pitch to d'.
1264 \displayLilyMusic \someNote
1270 @node Doubling a note with slurs (example)
1271 @subsection Doubling a note with slurs (example)
1273 Suppose we want to create a function that translates input like
1274 @code{a} into @code{@{ a( a) @}}. We begin by examining the internal
1275 representation of the desired result.
1278 \displayMusic@{ a'( a') @}
1291 (ly:make-duration 2 0 1/1)
1293 (ly:make-pitch 0 5 0))
1302 (ly:make-duration 2 0 1/1)
1304 (ly:make-pitch 0 5 0))))
1307 The bad news is that the @code{SlurEvent} expressions
1308 must be added @q{inside} the note (in its @code{articulations}
1311 Now we examine the input,
1319 (ly:make-duration 2 0 1/1)
1321 (ly:make-pitch 0 5 0))))
1324 So in our function, we need to clone this expression (so that we have
1325 two notes to build the sequence), add a @code{SlurEvent} to the
1326 @code{'articulations} property of each one, and finally make a
1327 @code{SequentialMusic} with the two @code{NoteEvent} elements. For adding to a
1328 property, it is useful to know that an unset property is read out as
1329 @code{'()}, the empty list, so no special checks are required before we
1330 put another element at the front of the @code{articulations} property.
1333 doubleSlur = #(define-music-function (parser location note) (ly:music?)
1334 "Return: @{ note ( note ) @}.
1335 `note' is supposed to be a single note."
1336 (let ((note2 (ly:music-deep-copy note)))
1337 (set! (ly:music-property note 'articulations)
1338 (cons (make-music 'SlurEvent 'span-direction -1)
1339 (ly:music-property note 'articulations)))
1340 (set! (ly:music-property note2 'articulations)
1341 (cons (make-music 'SlurEvent 'span-direction 1)
1342 (ly:music-property note2 'articulations)))
1343 (make-music 'SequentialMusic 'elements (list note note2))))
1347 @node Adding articulation to notes (example)
1348 @subsection Adding articulation to notes (example)
1350 The easy way to add articulation to notes is to merge two music
1351 expressions into one context.
1352 However, suppose that we want to write a music function that does this.
1353 This will have the additional advantage that we can use that music
1354 function to add an articulation (like a fingering instruction) to a
1355 single note inside of a chord which is not possible if we just merge
1358 A @code{$variable} inside the @code{#@{@dots{}#@}} notation is like
1359 a regular @code{\variable} in classical LilyPond notation. We
1367 will not work in LilyPond. We could avoid this problem by attaching
1368 the articulation to an empty chord,
1371 @{ << \music <> -. -> >> @}
1375 but for the sake of this example, we will learn how to do this in
1376 Scheme. We begin by examining our input and desired output,
1385 (ly:make-duration 2 0 1/1)
1387 (ly:make-pitch -1 0 0))))
1400 (ly:make-duration 2 0 1/1)
1402 (ly:make-pitch -1 0 0))
1405 We see that a note (@code{c4}) is represented as an @code{NoteEvent}
1406 expression. To add an accent articulation, an @code{ArticulationEvent}
1407 expression must be added to the @code{articulations} property of the
1408 @code{NoteEvent} expression.
1410 To build this function, we begin with
1413 (define (add-accent note-event)
1414 "Add an accent ArticulationEvent to the articulations of `note-event',
1415 which is supposed to be a NoteEvent expression."
1416 (set! (ly:music-property note-event 'articulations)
1417 (cons (make-music 'ArticulationEvent
1418 'articulation-type "accent")
1419 (ly:music-property note-event 'articulations)))
1423 The first line is the way to define a function in Scheme: the function
1424 name is @code{add-accent}, and has one variable called
1425 @code{note-event}. In Scheme, the type of variable is often clear
1426 from its name. (this is good practice in other programming languages,
1430 "Add an accent@dots{}"
1434 is a description of what the function does. This is not strictly
1435 necessary, but just like clear variable names, it is good practice.
1437 You may wonder why we modify the note event directly instead of working
1438 on a copy (@code{ly:music-deep-copy} can be used for that). The reason
1439 is a silent contract: music functions are allowed to modify their
1440 arguments: they are either generated from scratch (like user input) or
1441 are already copied (referencing a music variable with @samp{\name} or
1442 music from immediate Scheme expressions @samp{$(@dots{})} provides a
1443 copy). Since it would be inefficient to create unnecessary copies, the
1444 return value from a music function is @emph{not} copied. So to heed
1445 that contract, you must not use any arguments more than once, and
1446 returning it counts as one use.
1448 In an earlier example, we constructed music by repeating a given music
1449 argument. In that case, at least one repetition had to be a copy of its
1450 own. If it weren't, strange things may happen. For example, if you use
1451 @code{\relative} or @code{\transpose} on the resulting music containing
1452 the same elements multiple times, those will be subjected to
1453 relativation or transposition multiple times. If you assign them to a
1454 music variable, the curse is broken since referencing @samp{\name} will
1455 again create a copy which does not retain the identity of the repeated
1458 Now while the above function is not a music function, it will normally
1459 be used within music functions. So it makes sense to heed the same
1460 contract we use for music functions: the input may be modified for
1461 producing the output, and the caller is responsible for creating copies
1462 if it still needs the unchanged argument itself. If you take a look at
1463 LilyPond's own functions like @code{music-map}, you'll find that they
1464 stick with the same principles.
1466 Where were we? Now we have a @code{note-event} we may modify, not
1467 because of using @code{ly:music-deep-copy} but because of a long-winded
1468 explanation. We add the accent to its @code{'articulations} list
1472 (set! place new-value)
1475 Here, what we want to set (the @q{place}) is the @code{'articulations}
1476 property of @code{note-event} expression.
1479 (ly:music-property note-event 'articulations)
1482 @code{ly:music-property} is the function used to access music properties
1483 (the @code{'articulations}, @code{'duration}, @code{'pitch}, etc, that we
1484 see in the @code{\displayMusic} output above). The new value is the
1485 former @code{'articulations} property, with an extra item: the
1486 @code{ArticulationEvent} expression, which we copy from the
1487 @code{\displayMusic} output,
1490 (cons (make-music 'ArticulationEvent
1491 'articulation-type "accent")
1492 (ly:music-property result-event-chord 'articulations))
1495 @code{cons} is used to add an element to the front of a list without
1496 modifying the original list. This is what we want: the same list as
1497 before, plus the new @code{ArticulationEvent} expression. The order
1498 inside the @code{'articulations} property is not important here.
1500 Finally, once we have added the accent articulation to its
1501 @code{articulations} property, we can return @code{note-event}, hence
1502 the last line of the function.
1504 Now we transform the @code{add-accent} function into a music
1505 function (a matter of some syntactic sugar and a declaration of the type
1506 of its sole @q{real} argument).
1509 addAccent = #(define-music-function (parser location note-event)
1511 "Add an accent ArticulationEvent to the articulations of `note-event',
1512 which is supposed to be a NoteEvent expression."
1513 (set! (ly:music-property note-event 'articulations)
1514 (cons (make-music 'ArticulationEvent
1515 'articulation-type "accent")
1516 (ly:music-property note-event 'articulations)))
1520 We may verify that this music function works correctly,
1523 \displayMusic \addAccent c4
1532 * Tweaking with Scheme::
1535 @c @nod e Tweaking with Scheme
1536 @c @sectio n Tweaking with Scheme
1538 We have seen how LilyPond output can be heavily modified using
1540 @code{\override TextScript.extra-offset = ( 1 . -1)}. But
1541 we have even more power if we use Scheme. For a full explanation
1542 of this, see the @ref{Scheme tutorial}, and
1543 @ref{Interfaces for programmers}.
1545 We can use Scheme to simply @code{\override} commands,
1547 TODO Find a simple example
1548 @c This isn't a valid example with skylining
1549 @c It works fine without padText -td
1553 @lilypond[quote,verbatim,ragged-right]
1554 padText = #(define-music-function (parser location padding) (number?)
1556 \once \override TextScript.padding = #padding
1560 c4^"piu mosso" b a b
1562 c4^"piu mosso" d e f
1564 c4^"piu mosso" fis a g
1570 We can use it to create new commands:
1572 @c Check this is a valid example with skylining
1573 @c It is - 'padding still works
1576 @lilypond[quote,verbatim,ragged-right]
1577 tempoPadded = #(define-music-function (parser location padding tempotext)
1580 \once \override Score.MetronomeMark.padding = #padding
1581 \tempo \markup { \bold #tempotext }
1585 \tempo \markup { "Low tempo" }
1587 \tempoPadded #4.0 "High tempo"
1593 Even music expressions can be passed in:
1595 @lilypond[quote,verbatim,ragged-right]
1596 pattern = #(define-music-function (parser location x y) (ly:music? ly:music?)
1603 \pattern {d16 dis} { ais16-> b\p }