X-Git-Url: https://git.donarmstrong.com/?a=blobdiff_plain;f=Documentation%2Fextending%2Fscheme-tutorial.itely;h=5984c4f038c926d962627ef67aafb5b6cf1e886f;hb=3025442087de8dd3f88736290887ced86e360c25;hp=7b38a76ef9634833ce2230e349d56444610ffc1d;hpb=2940cdd12c135ef94e4c95ac815e76014eef9c79;p=lilypond.git diff --git a/Documentation/extending/scheme-tutorial.itely b/Documentation/extending/scheme-tutorial.itely index 7b38a76ef9..5984c4f038 100644 --- a/Documentation/extending/scheme-tutorial.itely +++ b/Documentation/extending/scheme-tutorial.itely @@ -8,12 +8,11 @@ Guide, node Updating translation committishes.. @end ignore -@c \version "2.14.0" +@c \version "2.19.22" @node Scheme tutorial @chapter Scheme tutorial -@funindex # @cindex Scheme @cindex GUILE @cindex Scheme, in-line code @@ -67,19 +66,31 @@ The LilyPond installation includes the Guile implementation of Scheme. On most systems you can experiment in a Scheme sandbox by opening a terminal window and typing @q{guile}. On some systems, notably Windows, you may need to set the environment variable -@code{GUILE_LOAD_PATH} to the directory @code{../usr/shr/guile/1.8} +@code{GUILE_LOAD_PATH} to the directory @code{../usr/share/guile/1.8} in the LilyPond installation. For the full path to this directory see @rlearning{Other sources of information}. Alternatively, Windows users may simply choose @q{Run} from the Start menu and enter @q{guile}. -Once the guile sandbox is running, you will receive a guile prompt: +However, a hands-on Scheme sandbox with all of LilyPond loaded is +available with this command line: +@example +lilypond scheme-sandbox +@end example + +@noindent +Once the sandbox is running, you will receive a guile prompt: @lisp guile> @end lisp -You can enter Scheme expressions at this prompt to experiment with Scheme. +You can enter Scheme expressions at this prompt to experiment with +Scheme. If you want to be able to use the GNU readline library for +nicer editing of the Scheme command line, check the file +@file{ly/scheme-sandbox.ly} for more information. If you already have +enabled the readline library for your interactive Guile sessions outside +of LilyPond, this should work in the sandbox as well. @node Scheme variables @subsection Scheme variables @@ -148,7 +159,7 @@ and False is @code{#f}. @item Numbers Numbers are entered in the standard fashion, -@code{1} is the (integer) number one, while @code{-1.5} is a +@code{1} is the (integer) number one, while @w{@code{-1.5}} is a floating point number (a non-integer number). @item Strings @@ -198,7 +209,15 @@ For a complete listing see the Guile reference guide, There are also compound data types in Scheme. The types commonly used in LilyPond programming include pairs, lists, alists, and hash tables. -@subheading Pairs +@menu +* Pairs:: +* Lists:: +* Association lists (alists):: +* Hash tables:: +@end menu + +@node Pairs +@unnumberedsubsubsec Pairs The foundational compound data type of Scheme is the @code{pair}. As might be expected from its name, a pair is two values glued together. @@ -239,7 +258,7 @@ Scheme procedures @code{car} and @code{cdr}, respectively. @lisp guile> (define mypair (cons 123 "hello there") -... ) +@dots{} ) guile> (car mypair) 123 guile> (cdr mypair) @@ -253,11 +272,13 @@ Note: @code{cdr} is pronounced "could-er", according Sussman and Abelson, see @uref{http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-14.html#footnote_Temp_133} -@subheading Lists +@node Lists +@unnumberedsubsubsec Lists -A very common Scheme data structure is the @emph{list}. Formally, a -list is defined as either the empty list (represented as @code{'()}, -or a pair whose @code{cdr} is a list. +A very common Scheme data structure is the @emph{list}. Formally, +a @q{proper} list is defined to be either the empty list with its +input form @code{'()} and length@tie{}0, or a pair whose +@code{cdr} in turn is a shorter list. There are many ways of creating lists. Perhaps the most common is with the @code{list} procedure: @@ -267,12 +288,19 @@ guile> (list 1 2 3 "abc" 17.5) (1 2 3 "abc" 17.5) @end lisp -As can be seen, a list is displayed in the form of individual elements -separated by whitespace and enclosed in parentheses. Unlike a pair, -there is no period between the elements. +Representing a list as individual +elements separated by whitespace and enclosed in parentheses +is actually a compacted rendition of the actual dotted pairs +constituting the list, where the dot and an immediately following +starting paren are removed along with the matching closing paren. +Without this compaction, the output would have been +@lisp +(1 . (2 . (3 . ("abc" . (17.5 . ()))))) +@end lisp -A list can also be entered as a literal list by enclosing its -elements in parentheses, and adding a quote: +As with the output, a list can also be entered (after adding a +quote to avoid interpretation as a function call) as a literal +list by enclosing its elements in parentheses: @lisp guile> '(17 23 "foo" "bar" "bazzle") @@ -283,7 +311,8 @@ Lists are a central part of Scheme. In, fact, Scheme is considered a dialect of lisp, where @q{lisp} is an abbreviation for @q{List Processing}. Scheme expressions are all lists. -@subheading Association lists (alists) +@node Association lists (alists) +@unnumberedsubsubsec Association lists (alists) A special type of list is an @emph{association list} or @emph{alist}. An alist is used to store data for easy retrieval. @@ -307,7 +336,8 @@ guile> Alists are widely used in LilyPond to store properties and other data. -@subheading Hash tables +@node Hash tables +@unnumberedsubsubsec Hash tables A data structure that is used occasionally in LilyPond. A hash table is similar to an array, but the indexes to the array can be any type @@ -477,7 +507,14 @@ Scheme procedures are executable scheme expressions that return a value resulting from their execution. They can also manipulate variables defined outside of the procedure. -@subheading Defining procedures +@menu +* Defining procedures:: +* Predicates:: +* Return values:: +@end menu + +@node Defining procedures +@unnumberedsubsubsec Defining procedures Procedures are defined in Scheme with define @@ -503,7 +540,8 @@ guile> (average 3 12) 15/2 @end lisp -@subheading Predicates +@node Predicates +@unnumberedsubsubsec Predicates Scheme procedures that return boolean values are often called @emph{predicates}. By convention (but not necessity), predicate names @@ -517,7 +555,8 @@ guile> (less-than-ten? 15) #f @end lisp -@subheading Return values +@node Return values +@unnumberedsubsubsec Return values Scheme procedures always return a return value, which is the value of the last expression executed in the procedure. The return @@ -543,14 +582,20 @@ statement in the let block: @lisp guile> (let ((x 2) (y 3) (z 4)) (display (+ x y)) (display (- z 4)) -... (+ (* x y) (/ z x))) +@dots{} (+ (* x y) (/ z x))) 508 @end lisp @node Scheme conditionals @subsection Scheme conditionals -@subheading if +@menu +* if:: +* cond:: +@end menu + +@node if +@unnumberedsubsubsec if Scheme has an @code{if} procedure: @@ -570,14 +615,15 @@ guile> (if (> a b) "a is greater than b" "a is not greater than b") "a is not greater than b" @end lisp -@subheading cond +@node cond +@unnumberedsubsubsec cond Another conditional procedure in scheme is @code{cond}: @example (cond (test-expression-1 result-expression-sequence-1) (test-expression-2 result-expression-sequence-2) - ... + @dots{} (test-expression-n result-expression-sequence-n)) @end example @@ -600,6 +646,7 @@ guile> (cond ((< a b) "a is less than b") * LilyPond Scheme syntax:: * LilyPond variables:: * Input variables and Scheme:: +* Importing Scheme in LilyPond:: * Object properties:: * LilyPond compound variables:: * Internal music representation:: @@ -607,17 +654,68 @@ guile> (cond ((< a b) "a is less than b") @node LilyPond Scheme syntax @subsection LilyPond Scheme syntax +@funindex $ +@funindex # The Guile interpreter is part of LilyPond, which means that -Scheme can be included in LilyPond input files. The hash mark @code{#} -is used to tell the LilyPond parser that the next value is a Scheme -value. +Scheme can be included in LilyPond input files. There are several +methods for including Scheme in LilyPond. -Once the parser sees a hash mark, input is passed to the Guile -interpreter to evaluate the Scheme expression. The interpreter continues -to process input until the end of a Scheme expression is seen. +The simplest way is to use a hash mark@tie{}@code{#} before a Scheme +expression. -Scheme procedures can be defined in LilyPond input files: +Now LilyPond's input is structured into tokens and expressions, much +like human language is structured into words and sentences. LilyPond +has a lexer that recognizes tokens (literal numbers, strings, Scheme +elements, pitches and so on), and a parser that understands the syntax, +@rcontrib{LilyPond grammar}. Once it knows that a particular syntax rule +applies, it executes actions associated with it. + +The hash mark@tie{}@code{#} method of embedding Scheme is a natural fit +for this system. Once the lexer sees a hash mark, it calls the Scheme +reader to read one full Scheme expression (this can be an identifier, an +expression enclosed in parentheses, or several other things). After the +Scheme expression is read, it is stored away as the value for an +@code{SCM_TOKEN} in the grammar. Once the parser knows how to make use +of this token, it calls Guile for evaluating the Scheme expression. +Since the parser usually requires a bit of lookahead from the lexer to +make its parsing decisions, this separation of reading and evaluation +between lexer and parser is exactly what is needed to keep the execution +of LilyPond and Scheme expressions in sync. For this reason, you should +use the hash mark@tie{}@code{#} for calling Scheme whenever this is +feasible. + +Another way to call the Scheme interpreter from LilyPond is the use of +dollar@tie{}@code{$} instead of a hash mark for introducing Scheme +expressions. In this case, LilyPond evaluates the code right after the +lexer has read it. It checks the resulting type of the Scheme +expression and then picks a token type (one of several +@code{xxx_IDENTIFIER} in the syntax) for it. It creates a @emph{copy} +of the value and uses that for the value of the token. If the value of +the expression is void (Guile's value of @code{*unspecified*}), nothing +at all is passed to the parser. + +This is, in fact, exactly the same mechanism that LilyPond employs when +you call any variable or music function by name, as @code{\name}, with +the only difference that the name is determined by the LilyPond lexer +without consulting the Scheme reader, and thus only variable names +consistent with the current LilyPond mode are accepted. + +The immediate action of @code{$} can lead to surprises, see +@ref{Importing Scheme in LilyPond}. Using @code{#} where the +parser supports it is usually preferable. Inside of music expressions, +expressions created using @code{#} @emph{are} interpreted as +music. However, they are @emph{not} copied before use. If they are +part of some structure that might still get used, you may need to use +@code{ly:music-deep-copy} explicitly. + +@funindex $@@ +@funindex #@@ +There are also @q{list splicing} operators @code{$@@} and @code{#@@} +that insert all elements of a list in the surrounding context. + +Now let's take a look at some actual Scheme code. Scheme procedures can +be defined in LilyPond input files: @example #(define (average a b c) (/ (+ a b c) 3)) @@ -625,7 +723,7 @@ Scheme procedures can be defined in LilyPond input files: Note that LilyPond comments (@code{%} and @code{%@{ %@}}) cannot be used within Scheme code, even in a LilyPond input file, because -the Guile interpreter, not the LilyPond parser, is interpreting +the Guile interpreter, not the LilyPond lexer, is reading the Scheme expression. Comments in Guile Scheme are entered as follows: @@ -640,11 +738,11 @@ as follows: @end example For the rest of this section, we will assume that the data is entered -in a music file, so we add @code{#}s at the beginning of each Scheme +in a music file, so we add a @code{#} at the beginning of each Scheme expression. All of the top-level Scheme expressions in a LilyPond input file can -be combined into a single Scheme expression by the use of the +be combined into a single Scheme expression by use of the @code{begin} statement: @example @@ -679,8 +777,28 @@ twentyFour = #(* 2 twelve) @end example @noindent -which would result in the number 24 being stored in the -LilyPond (and Scheme) variable @code{twentyFour}. +which would result in the number @emph{24} being stored in the LilyPond +(and Scheme) variable @code{twentyFour}. + +Scheme allows modifying complex expressions in-place and LilyPond makes +use of this @q{in-place modification} when using music functions. But +when music expressions are stored in variables rather than entered +directly the usual expectation, when passing them to music functions, +would be that the original value is unmodified. So when referencing a +music variable with leading backslash (such as @code{\twentyFour}), +LilyPond creates a copy of that variable's music value for use in the +surrounding music expression rather than using the variable's value +directly. + +Therefore, Scheme music expressions written with the @code{#} syntax +should be used for material that is created @q{from scratch} (or that is +explicitly copied) rather than being used, instead, to directly +reference material. + +@seealso +Extending: +@ref{LilyPond Scheme syntax}. + @node Input variables and Scheme @subsection Input variables and Scheme @@ -721,7 +839,7 @@ traLaLa = @{ c'4 d'4 @} is internally converted to a Scheme definition: @example -(define traLaLa @var{Scheme value of `@code{... }'}) +(define traLaLa @var{Scheme value of `@code{@dots{}}'}) @end example This means that LilyPond variables and Scheme variables may be freely @@ -733,46 +851,80 @@ imported in a @code{\score} block by means of a second variable @lilypond[verbatim] traLaLa = { c'4 d'4 } -%% dummy action to deal with parser lookahead -#(display "this needs to be here, sorry!") - #(define newLa (map ly:music-deep-copy (list traLaLa traLaLa))) #(define twice (make-sequential-music newLa)) -{ \twice } +\twice @end lilypond @c Due to parser lookahead -In this example, the assignment happens after the parser has -verified that nothing interesting happens after -@code{traLaLa = @{ ... @}}. Without the dummy statement in the -above example, the @code{newLa} definition is executed before -@code{traLaLa} is defined, leading to a syntax error. +This is actually a rather interesting example. The assignment will only +take place after the parser has ascertained that nothing akin to +@code{\addlyrics} follows, so it needs to check what comes next. It +reads @code{#} and the following Scheme expression @emph{without} +evaluating it, so it can go ahead with the assignment, and +@emph{afterwards} execute the Scheme code without problem. + +@node Importing Scheme in LilyPond +@subsection Importing Scheme in LilyPond +@funindex $ +@funindex # The above example shows how to @q{export} music expressions from the input to the Scheme interpreter. The opposite is also possible. By -wrapping a Scheme value in the function @code{ly:export}, a Scheme +placing it after @code{$}, a Scheme value is interpreted as if it were entered in LilyPond syntax. Instead of defining @code{\twice}, the example above could also have been written as @example -... -@{ #(ly:export (make-sequential-music (list newLa))) @} +@dots{} +$(make-sequential-music newLa) @end example -Scheme code is evaluated as soon as the parser encounters it. To -define some Scheme code in a macro (to be called later), use -@ref{Void functions}, or +You can use @code{$} with a Scheme expression anywhere you could use +@code{\@var{name}} after having assigned the Scheme expression to a +variable @var{name}. This replacement happens in the @q{lexer}, so +LilyPond is not even aware of the difference. + +One drawback, however, is that of timing. If we had been using @code{$} +instead of @code{#} for defining @code{newLa} in the above example, the +following Scheme definition would have failed because @code{traLaLa} +would not yet have been defined. For an explanation of this timing +problem, @ref{LilyPond Scheme syntax}. + +@funindex $@@ +@funindex #@@ +A further convenience can be the @q{list splicing} operators @code{$@@} +and @code{#@@} for inserting the elements of a list in the surrounding +context. Using those, the last part of the example could have been +written as + +@example +@dots{} +@{ #@@newLa @} +@end example + +Here, every element of the list stored in @code{newLa} is taken in +sequence and inserted into the list, as if we had written + +@example +@{ #(first newLa) #(second newLa) @} +@end example + +Now in all of these forms, the Scheme code is evaluated while the +input is still being consumed, either in the lexer or in the parser. +If you need it to be executed at a later point of time, check out +@ref{Void scheme functions}, or store it in a procedure: @example #(define (nopc) (ly:set-option 'point-and-click #f)) -... +@dots{} #(nopc) @{ c'4 @} @end example @@ -780,7 +932,7 @@ define some Scheme code in a macro (to be called later), use @knownissues Mixing Scheme and LilyPond variables is not possible with the -@code{--safe} option. +@option{--safe} option. @node Object properties @@ -796,7 +948,7 @@ the alist with both a key and a value. The LilyPond syntax for doing this is: @example -\override Stem #'thickness = #2.6 +\override Stem.thickness = #2.6 @end example This instruction adjusts the appearance of stems. An alist entry @@ -821,14 +973,23 @@ while @code{twentyFour} is a variable. @node LilyPond compound variables @subsection LilyPond compound variables -@subheading Offsets +@menu +* Offsets:: +* Fractions:: +* Extents:: +* Property alists:: +* Alist chains:: +@end menu + +@node Offsets +@unnumberedsubsubsec Offsets -Two-dimensional offsets (X and Y coordinates) are stored as @code{pairs}. +Two-dimensional offsets (X and Y coordinates) are stored as @emph{pairs}. The @code{car} of the offset is the X coordinate, and the @code{cdr} is the Y coordinate. @example -\override TextScript #'extra-offset = #'(1 . 2) +\override TextScript.extra-offset = #'(1 . 2) @end example This assigns the pair @code{(1 . 2)} to the @code{extra-offset} @@ -838,7 +999,19 @@ this command moves the object 1 staff space to the right, and 2 spaces up. Procedures for working with offsets are found in @file{scm/lily-library.scm}. -@subheading Extents +@node Fractions +@unnumberedsubsubsec Fractions + +Fractions as used by LilyPond are again stored as @emph{pairs}, this +time of unsigned integers. While Scheme can represent rational numbers +as a native type, musically @samp{2/4} and @samp{1/2} are not the same, +and we need to be able to distinguish between them. Similarly there are +no negative @q{fractions} in LilyPond's mind. So @code{2/4} in LilyPond +means @code{(2 . 4)} in Scheme, and @code{#2/4} in LilyPond means +@code{1/2} in Scheme. + +@node Extents +@unnumberedsubsubsec Extents Pairs are also used to store intervals, which represent a range of numbers from the minimum (the @code{car}) to the maximum (the @code{cdr}). @@ -851,7 +1024,8 @@ Procedures for working with intervals are found in @file{scm/lily-library.scm}. These procedures should be used when possible to ensure consistency of code. -@subheading Property alists +@node Property alists +@unnumberedsubsubsec Property alists A property alist is a LilyPond data structure that is an alist whose keys are properties and whose values are Scheme expressions that give @@ -859,7 +1033,8 @@ the desired value for the property. LilyPond properties are Scheme symbols, such as @code{'thickness}. -@subheading Alist chains +@node Alist chains +@unnumberedsubsubsec Alist chains An alist chain is a list containing property alists. @@ -948,7 +1123,7 @@ to create complicated music functions. When writing a music function it is often instructive to inspect how a music expression is stored internally. This can be done with the -music function @code{\displayMusic} +music function @code{\displayMusic}. @example @{ @@ -964,74 +1139,115 @@ will display 'SequentialMusic 'elements (list (make-music - 'EventChord - 'elements + 'NoteEvent + 'articulations (list (make-music - 'NoteEvent - 'duration - (ly:make-duration 2 0 1 1) - 'pitch - (ly:make-pitch 0 0 0)) - (make-music 'AbsoluteDynamicEvent 'text - "f"))))) + "f")) + 'duration + (ly:make-duration 2 0 1/1) + 'pitch + (ly:make-pitch 0 0 0)))) @end example By default, LilyPond will print these messages to the console along with all the other messages. To split up these messages and save -the results of @code{\display@{STUFF@}}, redirect the output to -a file. +the results of @code{\display@{STUFF@}}, you can specify an optional +output port to use: @example -lilypond file.ly >display.txt +@{ + \displayMusic #(open-output-file "display.txt") @{ c'4\f @} +@} @end example -With a bit of reformatting, the above information is easier to read, +This will overwrite a previous output file whenever it is called; if you +need to write more than one expression, you would use a variable for +your port and reuse it: +@example +@{ + port = #(open-output-file "display.txt") + \displayMusic \port @{ c'4\f @} + \displayMusic \port @{ d'4 @} + #(close-output-port port) +@} +@end example + +Guile's manual describes ports in detail. Closing the port is actually +only necessary if you need to read the file before LilyPond finishes; in +the first example, we did not bother to do so. + +A bit of reformatting makes the above information easier to read: @example (make-music 'SequentialMusic - 'elements (list (make-music 'EventChord - 'elements (list (make-music 'NoteEvent - 'duration (ly:make-duration 2 0 1 1) - 'pitch (ly:make-pitch 0 0 0)) - (make-music 'AbsoluteDynamicEvent - 'text "f"))))) + 'elements (list + (make-music 'NoteEvent + 'articulations (list + (make-music 'AbsoluteDynamicEvent + 'text + "f")) + 'duration (ly:make-duration 2 0 1/1) + 'pitch (ly:make-pitch 0 0 0)))) @end example -A @code{@{ ... @}} music sequence has the name @code{SequentialMusic}, -and its inner expressions are stored as a list in its @code{'elements} -property. A note is represented as an @code{EventChord} expression, -containing a @code{NoteEvent} object (storing the duration and -pitch properties) and any extra information (in this case, an -@code{AbsoluteDynamicEvent} with a @code{"f"} text property. +A @code{@{ @dots{} @}} music sequence has the name +@code{SequentialMusic}, and its inner expressions are stored as a list +in its @code{'elements} property. A note is represented as a +@code{NoteEvent} object (storing the duration and pitch properties) with +attached information (in this case, an @code{AbsoluteDynamicEvent} with +a @code{"f"} text property) stored in its @code{articulations} property. +@funindex{\void} +@code{\displayMusic} returns the music it displays, so it will get +interpreted as well as displayed. To avoid interpretation, write +@code{\void} before @code{\displayMusic}. @node Music properties @subsection Music properties +@ignore TODO -- make sure we delineate between @emph{music} properties, @emph{context} properties, and @emph{layout} properties. These are potentially confusing. +@end ignore -The @code{NoteEvent} object is the first object of the -@code{'elements} property of @code{someNote}. +Let's look at an example: @example someNote = c' \displayMusic \someNote ===> +(make-music + 'NoteEvent + 'duration + (ly:make-duration 2 0 1/1) + 'pitch + (ly:make-pitch 0 0 0)) +@end example + +The @code{NoteEvent} object is the representation of @code{someNote}. +Straightforward. How about putting c' in a chord? + +@example +someNote = +\displayMusic \someNote +===> (make-music 'EventChord 'elements (list (make-music 'NoteEvent 'duration - (ly:make-duration 2 0 1 1) + (ly:make-duration 2 0 1/1) 'pitch (ly:make-pitch 0 0 0)))) @end example +Now the @code{NoteEvent} object is the first object of the +@code{'elements} property of @code{someNote}. + The @code{display-scheme-music} function is the function used by @code{\displayMusic} to display the Scheme representation of a music expression. @@ -1042,13 +1258,13 @@ expression. (make-music 'NoteEvent 'duration - (ly:make-duration 2 0 1 1) + (ly:make-duration 2 0 1/1) 'pitch (ly:make-pitch 0 0 0)) @end example Then the note pitch is accessed through the @code{'pitch} property -of the @code{NoteEvent} object, +of the @code{NoteEvent} object. @example #(display-scheme-music @@ -1058,7 +1274,7 @@ of the @code{NoteEvent} object, (ly:make-pitch 0 0 0) @end example -The note pitch can be changed by setting this @code{'pitch} property, +The note pitch can be changed by setting this @code{'pitch} property. @funindex \displayLilyMusic @@ -1068,7 +1284,7 @@ The note pitch can be changed by setting this @code{'pitch} property, (ly:make-pitch 0 1 0)) ;; set the pitch to d'. \displayLilyMusic \someNote ===> -d' +d'4 @end example @@ -1076,7 +1292,7 @@ d' @subsection Doubling a note with slurs (example) Suppose we want to create a function that translates input like -@code{a} into @code{a( a)}. We begin by examining the internal +@code{a} into @code{@{ a( a) @}}. We begin by examining the internal representation of the desired result. @example @@ -1086,70 +1302,65 @@ representation of the desired result. 'SequentialMusic 'elements (list (make-music - 'EventChord - 'elements + 'NoteEvent + 'articulations (list (make-music - 'NoteEvent - 'duration - (ly:make-duration 2 0 1 1) - 'pitch - (ly:make-pitch 0 5 0)) - (make-music 'SlurEvent 'span-direction - -1))) + -1)) + 'duration + (ly:make-duration 2 0 1/1) + 'pitch + (ly:make-pitch 0 5 0)) (make-music - 'EventChord - 'elements + 'NoteEvent + 'articulations (list (make-music - 'NoteEvent - 'duration - (ly:make-duration 2 0 1 1) - 'pitch - (ly:make-pitch 0 5 0)) - (make-music 'SlurEvent 'span-direction - 1))))) + 1)) + 'duration + (ly:make-duration 2 0 1/1) + 'pitch + (ly:make-pitch 0 5 0)))) @end example The bad news is that the @code{SlurEvent} expressions -must be added @q{inside} the note (or more precisely, -inside the @code{EventChord} expression). +must be added @q{inside} the note (in its @code{articulations} +property). -Now we examine the input, +Now we examine the input. @example +\displayMusic a' +===> (make-music - 'SequentialMusic - 'elements - (list (make-music - 'EventChord - 'elements - (list (make-music - 'NoteEvent - 'duration - (ly:make-duration 2 0 1 1) - 'pitch - (ly:make-pitch 0 5 0)))))) + 'NoteEvent + 'duration + (ly:make-duration 2 0 1/1) + 'pitch + (ly:make-pitch 0 5 0)))) @end example -So in our function, we need to clone this expression (so that we -have two notes to build the sequence), add a @code{SlurEvent} to the -@code{'elements} property of each one, and finally make a -@code{SequentialMusic} with the two @code{EventChords}. +So in our function, we need to clone this expression (so that we have +two notes to build the sequence), add a @code{SlurEvent} to the +@code{'articulations} property of each one, and finally make a +@code{SequentialMusic} with the two @code{NoteEvent} elements. For adding to a +property, it is useful to know that an unset property is read out as +@code{'()}, the empty list, so no special checks are required before we +put another element at the front of the @code{articulations} property. @example -doubleSlur = #(define-music-function (parser location note) (ly:music?) +doubleSlur = #(define-music-function (note) (ly:music?) "Return: @{ note ( note ) @}. - `note' is supposed to be an EventChord." + `note' is supposed to be a single note." (let ((note2 (ly:music-deep-copy note))) - (set! (ly:music-property note 'elements) + (set! (ly:music-property note 'articulations) (cons (make-music 'SlurEvent 'span-direction -1) - (ly:music-property note 'elements))) - (set! (ly:music-property note2 'elements) + (ly:music-property note 'articulations))) + (set! (ly:music-property note2 'articulations) (cons (make-music 'SlurEvent 'span-direction 1) - (ly:music-property note2 'elements))) + (ly:music-property note2 'articulations))) (make-music 'SequentialMusic 'elements (list note note2)))) @end example @@ -1158,11 +1369,14 @@ doubleSlur = #(define-music-function (parser location note) (ly:music?) @subsection Adding articulation to notes (example) The easy way to add articulation to notes is to merge two music -expressions into one context, as explained in -@ruser{Creating contexts}. However, suppose that we want to write -a music function that does this. - -A @code{$variable} inside the @code{#@{...#@}} notation is like +expressions into one context. +However, suppose that we want to write a music function that does this. +This will have the additional advantage that we can use that music +function to add an articulation (like a fingering instruction) to a +single note inside of a chord which is not possible if we just merge +independent music. + +A @code{$variable} inside the @code{#@{@dots{}#@}} notation is like a regular @code{\variable} in classical LilyPond notation. We know that @@ -1172,155 +1386,162 @@ know that @noindent will not work in LilyPond. We could avoid this problem by attaching -the articulation to a fake note, +the articulation to an empty chord, @example -@{ << \music s1*0-.-> @} +@{ << \music <> -. -> >> @} @end example @noindent but for the sake of this example, we will learn how to do this in -Scheme. We begin by examining our input and desired output, +Scheme. We begin by examining our input and desired output. @example % input \displayMusic c4 ===> (make-music - 'EventChord - 'elements - (list (make-music - 'NoteEvent - 'duration - (ly:make-duration 2 0 1 1) - 'pitch - (ly:make-pitch -1 0 0)))) + 'NoteEvent + 'duration + (ly:make-duration 2 0 1/1) + 'pitch + (ly:make-pitch -1 0 0)))) ===== % desired output \displayMusic c4-> ===> (make-music - 'EventChord - 'elements + 'NoteEvent + 'articulations (list (make-music - 'NoteEvent - 'duration - (ly:make-duration 2 0 1 1) - 'pitch - (ly:make-pitch -1 0 0)) - (make-music 'ArticulationEvent 'articulation-type - "marcato"))) + "accent")) + 'duration + (ly:make-duration 2 0 1/1) + 'pitch + (ly:make-pitch -1 0 0)) @end example -We see that a note (@code{c4}) is represented as an @code{EventChord} -expression, with a @code{NoteEvent} expression in its elements list. To -add a marcato articulation, an @code{ArticulationEvent} expression must -be added to the elements property of the @code{EventChord} -expression. +We see that a note (@code{c4}) is represented as an @code{NoteEvent} +expression. To add an accent articulation, an @code{ArticulationEvent} +expression must be added to the @code{articulations} property of the +@code{NoteEvent} expression. To build this function, we begin with @example -(define (add-marcato event-chord) - "Add a marcato ArticulationEvent to the elements of `event-chord', - which is supposed to be an EventChord expression." - (let ((result-event-chord (ly:music-deep-copy event-chord))) - (set! (ly:music-property result-event-chord 'elements) - (cons (make-music 'ArticulationEvent - 'articulation-type "marcato") - (ly:music-property result-event-chord 'elements))) - result-event-chord)) +(define (add-accent note-event) + "Add an accent ArticulationEvent to the articulations of `note-event', + which is supposed to be a NoteEvent expression." + (set! (ly:music-property note-event 'articulations) + (cons (make-music 'ArticulationEvent + 'articulation-type "accent") + (ly:music-property note-event 'articulations))) + note-event) @end example The first line is the way to define a function in Scheme: the function -name is @code{add-marcato}, and has one variable called -@code{event-chord}. In Scheme, the type of variable is often clear +name is @code{add-accent}, and has one variable called +@code{note-event}. In Scheme, the type of variable is often clear from its name. (this is good practice in other programming languages, too!) @example -"Add a marcato..." +"Add an accent@dots{}" @end example @noindent is a description of what the function does. This is not strictly necessary, but just like clear variable names, it is good practice. -@example -(let ((result-event-chord (ly:music-deep-copy event-chord))) -@end example - -@code{let} is used to declare local variables. Here we use one local -variable, named @code{result-event-chord}, to which we give the value -@code{(ly:music-deep-copy event-chord)}. @code{ly:music-deep-copy} is -a function specific to LilyPond, like all functions prefixed by -@code{ly:}. It is use to make a copy of a music -expression. Here we copy @code{event-chord} (the parameter of the -function). Recall that our purpose is to add a marcato to an -@code{EventChord} expression. It is better to not modify the -@code{EventChord} which was given as an argument, because it may be -used elsewhere. - -Now we have a @code{result-event-chord}, which is a -@code{NoteEventChord} expression and is a copy of -@code{event-chord}. We add the marcato to its @code{'elements} -list property. +You may wonder why we modify the note event directly instead of working +on a copy (@code{ly:music-deep-copy} can be used for that). The reason +is a silent contract: music functions are allowed to modify their +arguments: they are either generated from scratch (like user input) or +are already copied (referencing a music variable with @samp{\name} or +music from immediate Scheme expressions @samp{$(@dots{})} provides a +copy). Since it would be inefficient to create unnecessary copies, the +return value from a music function is @emph{not} copied. So to heed +that contract, you must not use any arguments more than once, and +returning it counts as one use. + +In an earlier example, we constructed music by repeating a given music +argument. In that case, at least one repetition had to be a copy of its +own. If it weren't, strange things may happen. For example, if you use +@code{\relative} or @code{\transpose} on the resulting music containing +the same elements multiple times, those will be subjected to +relativation or transposition multiple times. If you assign them to a +music variable, the curse is broken since referencing @samp{\name} will +again create a copy which does not retain the identity of the repeated +elements. + +Now while the above function is not a music function, it will normally +be used within music functions. So it makes sense to heed the same +contract we use for music functions: the input may be modified for +producing the output, and the caller is responsible for creating copies +if it still needs the unchanged argument itself. If you take a look at +LilyPond's own functions like @code{music-map}, you'll find that they +stick with the same principles. + +Where were we? Now we have a @code{note-event} we may modify, not +because of using @code{ly:music-deep-copy} but because of a long-winded +explanation. We add the accent to its @code{'articulations} list +property. @example (set! place new-value) @end example -Here, what we want to set (the @q{place}) is the @code{'elements} -property of @code{result-event-chord} expression. +Here, what we want to set (the @q{place}) is the @code{'articulations} +property of @code{note-event} expression. @example -(ly:music-property result-event-chord 'elements) +(ly:music-property note-event 'articulations) @end example @code{ly:music-property} is the function used to access music properties -(the @code{'elements}, @code{'duration}, @code{'pitch}, etc, that we +(the @code{'articulations}, @code{'duration}, @code{'pitch}, etc, that we see in the @code{\displayMusic} output above). The new value is the -former @code{'elements} property, with an extra item: the +former @code{'articulations} property, with an extra item: the @code{ArticulationEvent} expression, which we copy from the @code{\displayMusic} output, @example (cons (make-music 'ArticulationEvent - 'articulation-type "marcato") - (ly:music-property result-event-chord 'elements)) + 'articulation-type "accent") + (ly:music-property result-event-chord 'articulations)) @end example -@code{cons} is used to add an element to a list without modifying -the original list. This is what we want: the same list as before, -plus the new @code{ArticulationEvent} expression. The order -inside the @code{'elements} property is not important here. +@code{cons} is used to add an element to the front of a list without +modifying the original list. This is what we want: the same list as +before, plus the new @code{ArticulationEvent} expression. The order +inside the @code{'articulations} property is not important here. -Finally, once we have added the marcato articulation to its @code{elements} -property, we can return @code{result-event-chord}, hence the last line of -the function. +Finally, once we have added the accent articulation to its +@code{articulations} property, we can return @code{note-event}, hence +the last line of the function. -Now we transform the @code{add-marcato} function into a music -function, +Now we transform the @code{add-accent} function into a music function (a +matter of some syntactic sugar and a declaration of the type of its +argument). @example -addMarcato = #(define-music-function (parser location event-chord) +addAccent = #(define-music-function (note-event) (ly:music?) - "Add a marcato ArticulationEvent to the elements of `event-chord', - which is supposed to be an EventChord expression." - (let ((result-event-chord (ly:music-deep-copy event-chord))) - (set! (ly:music-property result-event-chord 'elements) - (cons (make-music 'ArticulationEvent - 'articulation-type "marcato") - (ly:music-property result-event-chord 'elements))) - result-event-chord)) + "Add an accent ArticulationEvent to the articulations of `note-event', + which is supposed to be a NoteEvent expression." + (set! (ly:music-property note-event 'articulations) + (cons (make-music 'ArticulationEvent + 'articulation-type "accent") + (ly:music-property note-event 'articulations))) + note-event) @end example -We may verify that this music function works correctly, +We then verify that this music function works correctly: @example -\displayMusic \addMarcato c4 +\displayMusic \addAccent c4 @end example @@ -1337,7 +1558,7 @@ We may verify that this music function works correctly, We have seen how LilyPond output can be heavily modified using commands like -@code{\override TextScript #'extra-offset = ( 1 . -1)}. But +@code{\override TextScript.extra-offset = ( 1 . -1)}. But we have even more power if we use Scheme. For a full explanation of this, see the @ref{Scheme tutorial}, and @ref{Interfaces for programmers}. @@ -1351,13 +1572,13 @@ TODO Find a simple example @ignore @lilypond[quote,verbatim,ragged-right] -padText = #(define-music-function (parser location padding) (number?) +padText = #(define-music-function (padding) (number?) #{ - \once \override TextScript #'padding = #$padding + \once \override TextScript.padding = #padding #}) -\relative c''' { - c4^"piu mosso" b a b +\relative { + c'''4^"piu mosso" b a b \padText #1.8 c4^"piu mosso" d e f \padText #2.6 @@ -1374,17 +1595,17 @@ We can use it to create new commands: @lilypond[quote,verbatim,ragged-right] -tempoPadded = #(define-music-function (parser location padding tempotext) - (number? string?) +tempoPadded = #(define-music-function (padding tempotext) + (number? markup?) #{ - \once \override Score.MetronomeMark #'padding = $padding - \tempo \markup { \bold $tempotext } + \once \override Score.MetronomeMark.padding = #padding + \tempo \markup { \bold #tempotext } #}) -\relative c'' { +\relative { \tempo \markup { "Low tempo" } - c4 d e f g1 - \tempoPadded #4.0 #"High tempo" + c''4 d e f g1 + \tempoPadded #4.0 "High tempo" g4 f e d c1 } @end lilypond @@ -1393,9 +1614,9 @@ tempoPadded = #(define-music-function (parser location padding tempotext) Even music expressions can be passed in: @lilypond[quote,verbatim,ragged-right] -pattern = #(define-music-function (parser location x y) (ly:music? ly:music?) +pattern = #(define-music-function (x y) (ly:music? ly:music?) #{ - $x e8 a b $y b a e + #x e8 a b #y b a e #}) \relative c''{