# LilyPond — Extending

 This file explains how to extend the functionality of LilyPond version 2.18.2.
 For more information about how this manual fits with the other documentation, or to read this manual in other formats, see Manuals. If you are missing any manuals, the complete documentation can be found at http://www.lilypond.org/.

# 1. Scheme tutorial

LilyPond uses the Scheme programming language, both as part of the input syntax, and as internal mechanism to glue modules of the program together. This section is a very brief overview of entering data in Scheme. If you want to know more about Scheme, see http://www.schemers.org.

LilyPond uses the GNU Guile implementation of Scheme, which is based on the Scheme “R5RS” standard. If you are learning Scheme to use with LilyPond, working with a different implementation (or referring to a different standard) is not recommended. Information on guile can be found at http://www.gnu.org/software/guile/. The “R5RS” Scheme standard is located at http://www.schemers.org/Documents/Standards/R5RS/.

## 1.1 Introduction to Scheme

We begin with an introduction to Scheme. For this brief introduction, we will use the GUILE interpreter to explore how the language works. Once we are familiar with Scheme, we will show how the language can be integrated in LilyPond files.

### 1.1.1 Scheme sandbox

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 ‘guile’. On some systems, notably Windows, you may need to set the environment variable GUILE_LOAD_PATH to the directory ../usr/share/guile/1.8 in the LilyPond installation. For the full path to this directory see Other sources of information. Alternatively, Windows users may simply choose ‘Run’ from the Start menu and enter ‘guile’.

However, a hands-on Scheme sandbox with all of Lilypond loaded is available with this command line:

lilypond scheme-sandbox


Once the sandbox is running, you will receive a guile prompt:

 guile> 

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 ‘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.

### 1.1.2 Scheme variables

Scheme variables can have any valid scheme value, including a Scheme procedure.

Scheme variables are created with define:

 guile> (define a 2) guile> 

Scheme variables can be evaluated at the guile prompt simply by typing the variable name:

 guile> a 2 guile> 

Scheme variables can be printed on the display by using the display function:

 guile> (display a) 2guile> 

Note that both the value 2 and the guile prompt guile showed up on the same line. This can be avoided by calling the newline procedure or displaying a newline character.

 guile> (display a)(newline) 2 guile> (display a)(display "\n") 2 guile> 

Once a variable has been created, its value can be changed with set!:

 guile> (set! a 12345) guile> a 12345 guile> 

### 1.1.3 Scheme simple data types

The most basic concept in a language is data typing: numbers, character strings, lists, etc. Here is a list of simple Scheme data types that are often used with LilyPond.

Booleans

Boolean values are True or False. The Scheme for True is #t and False is #f.

Numbers

Numbers are entered in the standard fashion, 1 is the (integer) number one, while -1.5 is a floating point number (a non-integer number).

Strings

Strings are enclosed in double quotes:

"this is a string"


Strings may span several lines:

"this
is
a string"


and the newline characters at the end of each line will be included in the string.

Newline characters can also be added by including \n in the string.

"this\nis a\nmultiline string"


Quotation marks and backslashes are added to strings by preceding them with a backslash. The string \a said "b" is entered as

"\\a said \"b\""


There are additional Scheme data types that are not discussed here. For a complete listing see the Guile reference guide, http://www.gnu.org/software/guile/manual/html_node/Simple-Data-Types.html.

### 1.1.4 Scheme compound data types

There are also compound data types in Scheme. The types commonly used in LilyPond programming include pairs, lists, alists, and hash tables.

#### Pairs

The foundational compound data type of Scheme is the pair. As might be expected from its name, a pair is two values glued together. The operator used to form a pair is called cons.

 guile> (cons 4 5) (4 . 5) guile> 

Note that the pair is displayed as two items surrounded by parentheses and separated by whitespace, a period (.), and more whitespace. The period is not a decimal point, but rather an indicator of the pair.

Pairs can also be entered as literal values by preceding them with a single quote character.

 guile> '(4 . 5) (4 . 5) guile> 

The two elements of a pair may be any valid Scheme value:

 guile> (cons #t #f) (#t . #f) guile> '("blah-blah" . 3.1415926535) ("blah-blah" . 3.1415926535) guile> 

The first and second elements of the pair can be accessed by the Scheme procedures car and cdr, respectively.

 guile> (define mypair (cons 123 "hello there") … ) guile> (car mypair) 123 guile> (cdr mypair) "hello there" guile> 

Note: cdr is pronounced "could-er", according Sussman and Abelson, see http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-14.html#footnote_Temp_133

#### Lists

A very common Scheme data structure is the list. Formally, a list is defined as either the empty list (represented as '(), or a pair whose cdr is a list.

There are many ways of creating lists. Perhaps the most common is with the list procedure:

 guile> (list 1 2 3 "abc" 17.5) (1 2 3 "abc" 17.5) 

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.

A list can also be entered as a literal list by enclosing its elements in parentheses, and adding a quote:

 guile> '(17 23 "foo" "bar" "bazzle") (17 23 "foo" "bar" "bazzle") 

Lists are a central part of Scheme. In, fact, Scheme is considered a dialect of lisp, where ‘lisp’ is an abbreviation for ‘List Processing’. Scheme expressions are all lists.

#### Association lists (alists)

A special type of list is an association list or alist. An alist is used to store data for easy retrieval.

Alists are lists whose elements are pairs. The car of each element is called the key, and the cdr of each element is called the value. The Scheme procedure assoc is used to retrieve an entry from the alist, and cdr is used to retrieve the value:

 guile> (define my-alist '((1 . "A") (2 . "B") (3 . "C"))) guile> my-alist ((1 . "A") (2 . "B") (3 . "C")) guile> (assoc 2 my-alist) (2 . "B") guile> (cdr (assoc 2 my-alist)) "B" guile> 

Alists are widely used in LilyPond to store properties and other data.

#### 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 of Scheme value, not just integers.

Hash tables are more efficient than alists if there is a lot of data to store and the data changes very infrequently.

The syntax to create hash tables is a bit complex, but you can see examples of it in the LilyPond source.

 guile> (define h (make-hash-table 10)) guile> h # guile> (hashq-set! h 'key1 "val1") "val1" guile> (hashq-set! h 'key2 "val2") "val2" guile> (hashq-set! h 3 "val3") "val3" 

Values are retrieved from hash tables with hashq-ref.

 guile> (hashq-ref h 3) "val3" guile> (hashq-ref h 'key2) "val2" guile> 

Keys and values are retrieved as a pair with hashq-get-handle. This is a preferred way, because it will return #f if a key is not found.

 guile> (hashq-get-handle h 'key1) (key1 . "val1") guile> (hashq-get-handle h 'frob) #f guile> 

### 1.1.5 Calculations in Scheme

Scheme can be used to do calculations. It uses prefix syntax. Adding 1 and 2 is written as (+ 1 2) rather than the traditional 1+2.

 guile> (+ 1 2) 3 

Calculations may be nested; the result of a function may be used for another calculation.

 guile> (+ 1 (* 3 4)) 13 

These calculations are examples of evaluations; an expression like (* 3 4) is replaced by its value 12.

Scheme calculations are sensitive to the differences between integers and non-integers. Integer calculations are exact, while non-integers are calculated to the appropriate limits of precision:

 guile> (/ 7 3) 7/3 guile> (/ 7.0 3.0) 2.33333333333333 

When the scheme interpreter encounters an expression that is a list, the first element of the list is treated as a procedure to be evaluated with the arguments of the remainder of the list. Therefore, all operators in Scheme are prefix operators.

If the first element of a Scheme expression that is a list passed to the interpreter is not an operator or procedure, an error will occur:

 guile> (1 2 3) Backtrace: In current input: 52: 0* [1 2 3] :52:1: In expression (1 2 3): :52:1: Wrong type to apply: 1 ABORT: (misc-error) guile> 

Here you can see that the interpreter was trying to treat 1 as an operator or procedure, and it couldn’t. Hence the error is "Wrong type to apply: 1".

Therefore, to create a list we need to use the list operator, or to quote the list so that the interpreter will not try to evaluate it.

 guile> (list 1 2 3) (1 2 3) guile> '(1 2 3) (1 2 3) guile> 

This is an error that can appear as you are working with Scheme in LilyPond.

### 1.1.6 Scheme procedures

Scheme procedures are executable scheme expressions that return a value resulting from their execution. They can also manipulate variables defined outside of the procedure.

#### Defining procedures

Procedures are defined in Scheme with define

(define (function-name arg1 arg2 ... argn)
scheme-expression-that-gives-a-return-value)


For example, we could define a procedure to calculate the average:

 guile> (define (average x y) (/ (+ x y) 2)) guile> average # 

Once a procedure is defined, it is called by putting the procedure name and the arguments in a list. For example, we can calculate the average of 3 and 12:

 guile> (average 3 12) 15/2 

#### Predicates

Scheme procedures that return boolean values are often called predicates. By convention (but not necessity), predicate names typically end in a question mark:

 guile> (define (less-than-ten? x) (< x 10)) guile> (less-than-ten? 9) #t guile> (less-than-ten? 15) #f 

#### Return values

Scheme procedures always return a return value, which is the value of the last expression executed in the procedure. The return value can be any valid Scheme value, including a complex data structure or a procedure.

Sometimes the user would like to have multiple Scheme expressions in a procedure. There are two ways that multiple expressions can be combined. The first is the begin procedure, which allows multiple expressions to be evaluated, and returns the value of the last expression.

 guile> (begin (+ 1 2) (- 5 8) (* 2 2)) 4 

The second way to combine multiple expressions is in a let block. In a let block, a series of bindings are created, and then a sequence of expressions that can include those bindings is evaluated. The return value of the let block is the return value of the last statement in the let block:

 guile> (let ((x 2) (y 3) (z 4)) (display (+ x y)) (display (- z 4)) … (+ (* x y) (/ z x))) 508 

### 1.1.7 Scheme conditionals

#### if

Scheme has an if procedure:

(if test-expression true-expression false-expression)


test-expression is an expression that returns a boolean value. If test-expression returns #t, the if procedure returns the value of true-expression, otherwise it returns the value of false-expression.

 guile> (define a 3) guile> (define b 5) guile> (if (> a b) "a is greater than b" "a is not greater than b") "a is not greater than b" 

#### cond

Another conditional procedure in scheme is cond:

(cond (test-expression-1 result-expression-sequence-1)
(test-expression-2 result-expression-sequence-2)
…
(test-expression-n result-expression-sequence-n))


For example:

 guile> (define a 6) guile> (define b 8) guile> (cond ((< a b) "a is less than b") ... ((= a b) "a equals b") ... ((> a b) "a is greater than b")) "a is less than b" 

## 1.2 Scheme in LilyPond

### 1.2.1 LilyPond Scheme syntax

The Guile interpreter is part of LilyPond, which means that Scheme can be included in LilyPond input files. There are several methods for including Scheme in LilyPond.

The simplest way is to use a hash mark # before a Scheme expression.

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, LilyPond grammar. Once it knows that a particular syntax rule applies, it executes actions associated with it.

The hash mark # 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 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 # for calling Scheme whenever this is feasible.

Another way to call the Scheme interpreter from LilyPond is the use of dollar $ 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 xxx_IDENTIFIER in the syntax) for it. It creates a 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 *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 \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 $ can lead to surprises, Input variables and Scheme. Using # where the parser supports it is usually preferable. Inside of music expressions, expressions created using # are interpreted as music. However, they are not copied before use. If they are part of some structure that might still get used, you may need to use ly:music-deep-copy explicitly.

There are also ‘list splicing’ operators $@ and #@ 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: #(define (average a b c) (/ (+ a b c) 3))  Note that LilyPond comments (% and %{ %}) cannot be used within Scheme code, even in a LilyPond input file, because the Guile interpreter, not the LilyPond lexer, is reading the Scheme expression. Comments in Guile Scheme are entered as follows: ; this is a single-line comment #! This a (non-nestable) Guile-style block comment But these are rarely used by Schemers and never in LilyPond source code !#  For the rest of this section, we will assume that the data is entered in a music file, so we add #s 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 begin statement: #(begin (define foo 0) (define bar 1))  ### 1.2.2 LilyPond variables LilyPond variables are stored internally in the form of Scheme variables. Thus, twelve = 12  is equivalent to #(define twelve 12)  This means that LilyPond variables are available for use in Scheme expressions. For example, we could use twentyFour = #(* 2 twelve)  which would result in the number 24 being stored in the LilyPond (and Scheme) variable twentyFour. The usual way to refer to Lilypond variables, LilyPond Scheme syntax, is to call them using a backslash, i.e., \twentyFour. Since this creates a copy of the value for most of LilyPond’s internal types, in particular music expressions, music functions don’t usually create copies of material they change. For this reason, music expressions given with # should usually not contain material that is not either created from scratch or explicitly copied rather than directly referenced. ### 1.2.3 Input variables and Scheme The input format supports the notion of variables: in the following example, a music expression is assigned to a variable with the name traLaLa. traLaLa = { c'4 d'4 }  There is also a form of scoping: in the following example, the \layout block also contains a traLaLa variable, which is independent of the outer \traLaLa. traLaLa = { c'4 d'4 } \layout { traLaLa = 1.0 }  In effect, each input file is a scope, and all \header, \midi, and \layout blocks are scopes nested inside that toplevel scope. Both variables and scoping are implemented in the GUILE module system. An anonymous Scheme module is attached to each scope. An assignment of the form: traLaLa = { c'4 d'4 }  is internally converted to a Scheme definition: (define traLaLa Scheme value of …')  This means that LilyPond variables and Scheme variables may be freely mixed. In the following example, a music fragment is stored in the variable traLaLa, and duplicated using Scheme. The result is imported in a \score block by means of a second variable twice: traLaLa = { c'4 d'4 } #(define newLa (map ly:music-deep-copy (list traLaLa traLaLa))) #(define twice (make-sequential-music newLa)) \twice  This is actually a rather interesting example. The assignment will only take place after the parser has ascertained that nothing akin to \addlyrics follows, so it needs to check what comes next. It reads # and the following Scheme expression without evaluating it, so it can go ahead with the assignment, and afterwards execute the Scheme code without problem. ### 1.2.4 Importing Scheme in LilyPond The above example shows how to ‘export’ music expressions from the input to the Scheme interpreter. The opposite is also possible. By placing it after $, a Scheme value is interpreted as if it were entered in LilyPond syntax. Instead of defining \twice, the example above could also have been written as

…
$(make-sequential-music newLa)  You can use $ with a Scheme expression anywhere you could use \name after having assigned the Scheme expression to a variable name. This replacement happens in the ‘lexer’, so Lilypond is not even aware of the difference.

One drawback, however, is that of timing. If we had been using $ instead of # for defining newLa in the above example, the following Scheme definition would have failed because traLaLa would not yet have been defined. For an explanation of this timing problem, LilyPond Scheme syntax. A further convenience can be the ‘list splicing’ operators $@ and #@ for inserting the elements of a list in the surrounding context. Using those, the last part of the example could have been written as

…
{ #@newLa }


Here, every element of the list stored in newLa is taken in sequence and inserted into the list, as if we had written

{ #(first newLa) #(second newLa) }


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 Void scheme functions, or store it in a procedure:

#(define (nopc)
(ly:set-option 'point-and-click #f))

…
#(nopc)
{ c'4 }


#### Known issues and warnings

Mixing Scheme and LilyPond variables is not possible with the ‘--safe’ option.

### 1.2.5 Object properties

Object properties are stored in LilyPond in the form of alist-chains, which are lists of alists. Properties are set by adding values at the beginning of the property list. Properties are read by retrieving values from the alists.

Setting a new value for a property requires assigning a value to the alist with both a key and a value. The LilyPond syntax for doing this is:

\override Stem.thickness = #2.6


This instruction adjusts the appearance of stems. An alist entry '(thickness . 2.6) is added to the property list of the Stem object. thickness is measured relative to the thickness of staff lines, so these stem lines will be 2.6 times the width of staff lines. This makes stems almost twice as thick as their normal size. To distinguish between variables defined in input files (like twentyFour in the example above) and variables of internal objects, we will call the latter ‘properties’ and the former ‘variables.’ So, the stem object has a thickness property, while twentyFour is a variable.

### 1.2.6 LilyPond compound variables

#### Offsets

Two-dimensional offsets (X and Y coordinates) are stored as pairs. The car of the offset is the X coordinate, and the cdr is the Y coordinate.

\override TextScript.extra-offset = #'(1 . 2)


This assigns the pair (1 . 2) to the extra-offset property of the TextScript object. These numbers are measured in staff-spaces, so this command moves the object 1 staff space to the right, and 2 spaces up.

Procedures for working with offsets are found in ‘scm/lily-library.scm’.

#### Fractions

Fractions as used by LilyPond are again stored as pairs, this time of unsigned integers. While Scheme can represent rational numbers as a native type, musically ‘2/4’ and ‘1/2’ are not the same, and we need to be able to distinguish between them. Similarly there are no negative ‘fractions’ in LilyPond’s mind. So 2/4 in LilyPond means (2 . 4) in Scheme, and #2/4 in LilyPond means 1/2 in Scheme.

#### Extents

Pairs are also used to store intervals, which represent a range of numbers from the minimum (the car) to the maximum (the cdr). Intervals are used to store the X- and Y- extents of printable objects. For X extents, the car is the left hand X coordinate, and the cdr is the right hand X coordinate. For Y extents, the car is the bottom coordinate, and the cdr is the top coordinate.

Procedures for working with intervals are found in ‘scm/lily-library.scm’. These procedures should be used when possible to ensure consistency of code.

#### 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 the desired value for the property.

LilyPond properties are Scheme symbols, such as 'thickness.

#### Alist chains

An alist chain is a list containing property alists.

The set of all properties that will apply to a grob is typically stored as an alist chain. In order to find the value for a particular property that a grob should have, each alist in the chain is searched in order, looking for an entry containing the property key. The first alist entry found is returned, and the value is the property value.

The Scheme procedure chain-assoc-get is normally used to get grob property values.

### 1.2.7 Internal music representation

Internally, music is represented as a Scheme list. The list contains various elements that affect the printed output. Parsing is the process of converting music from the LilyPond input representation to the internal Scheme representation.

When a music expression is parsed, it is converted into a set of Scheme music objects. The defining property of a music object is that it takes up time. The time it takes up is called its duration. Durations are expressed as a rational number that measures the length of the music object in whole notes.

A music object has three kinds of types:

• music name: Each music expression has a name. For example, a note leads to a NoteEvent, and \simultaneous leads to a SimultaneousMusic. A list of all expressions available is in the Internals Reference manual, under Music expressions.
• ‘type’ or interface: Each music name has several ‘types’ or interfaces, for example, a note is an event, but it is also a note-event, a rhythmic-event, and a melodic-event. All classes of music are listed in the Internals Reference, under Music classes.
• C++ object: Each music object is represented by an object of the C++ class Music.

The actual information of a music expression is stored in properties. For example, a NoteEvent has pitch and duration properties that store the pitch and duration of that note. A list of all properties available can be found in the Internals Reference, under Music properties.

A compound music expression is a music object that contains other music objects in its properties. A list of objects can be stored in the elements property of a music object, or a single ‘child’ music object in the element property. For example, SequentialMusic has its children in elements, and GraceMusic has its single argument in element. The body of a repeat is stored in the element property of RepeatedMusic, and the alternatives in elements.

## 1.3 Building complicated functions

This section explains how to gather the information necessary to create complicated music functions.

### 1.3.1 Displaying music expressions

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 \displayMusic

{
\displayMusic { c'4\f }
}


will display

(make-music
'SequentialMusic
'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))))


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 \display{STUFF}, redirect the output to a file.

lilypond file.ly >display.txt


With a combined bit of Lilypond and Scheme magic, you can actually let Lilypond direct just this output to a file of its own:

{
#(with-output-to-file "display.txt"
(lambda () #{ \displayMusic { c'4\f } #}))
}


A bit of reformatting makes the above information easier to read:

(make-music 'SequentialMusic
'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))))


A { … } music sequence has the name SequentialMusic, and its inner expressions are stored as a list in its 'elements property. A note is represented as a NoteEvent object (storing the duration and pitch properties) with attached information (in this case, an AbsoluteDynamicEvent with a "f" text property) stored in its articulations property.

\displayMusic returns the music it displays, so it will get interpreted as well as displayed. To avoid interpretation, write \void before \displayMusic.

### 1.3.2 Music properties

TODO – make sure we delineate between music properties, context properties, and layout properties. These are potentially confusing.

Let’s look at an example:

someNote = c'
\displayMusic \someNote
===>
(make-music
'NoteEvent
'duration
(ly:make-duration 2 0 1/1)
'pitch
(ly:make-pitch 0 0 0))


The NoteEvent object is the representation of someNote. Straightforward. How about putting c’ in a chord?

someNote = <c'>
\displayMusic \someNote
===>
(make-music
'EventChord
'elements
(list (make-music
'NoteEvent
'duration
(ly:make-duration 2 0 1/1)
'pitch
(ly:make-pitch 0 0 0))))


Now the NoteEvent object is the first object of the 'elements property of someNote.

The display-scheme-music function is the function used by \displayMusic to display the Scheme representation of a music expression.

#(display-scheme-music (first (ly:music-property someNote 'elements)))
===>
(make-music
'NoteEvent
'duration
(ly:make-duration 2 0 1/1)
'pitch
(ly:make-pitch 0 0 0))


Then the note pitch is accessed through the 'pitch property of the NoteEvent object,

#(display-scheme-music
(ly:music-property (first (ly:music-property someNote 'elements))
'pitch))
===>
(ly:make-pitch 0 0 0)


The note pitch can be changed by setting this 'pitch property,

#(set! (ly:music-property (first (ly:music-property someNote 'elements))
'pitch)
(ly:make-pitch 0 1 0)) ;; set the pitch to d'.
\displayLilyMusic \someNote
===>
d'


### 1.3.3 Doubling a note with slurs (example)

Suppose we want to create a function that translates input like a into { a( a) }. We begin by examining the internal representation of the desired result.

\displayMusic{ a'( a') }
===>
(make-music
'SequentialMusic
'elements
(list (make-music
'NoteEvent
'articulations
(list (make-music
'SlurEvent
'span-direction
-1))
'duration
(ly:make-duration 2 0 1/1)
'pitch
(ly:make-pitch 0 5 0))
(make-music
'NoteEvent
'articulations
(list (make-music
'SlurEvent
'span-direction
1))
'duration
(ly:make-duration 2 0 1/1)
'pitch
(ly:make-pitch 0 5 0))))


The bad news is that the SlurEvent expressions must be added ‘inside’ the note (in its articulations property).

Now we examine the input,

\displayMusic a'
===>
(make-music
'NoteEvent
'duration
(ly:make-duration 2 0 1/1)
'pitch
(ly:make-pitch 0 5 0))))


So in our function, we need to clone this expression (so that we have two notes to build the sequence), add a SlurEvent to the 'articulations property of each one, and finally make a SequentialMusic with the two NoteEvent elements. For adding to a property, it is useful to know that an unset property is read out as '(), the empty list, so no special checks are required before we put another element at the front of the articulations property.

doubleSlur = #(define-music-function (parser location note) (ly:music?)
"Return: { note ( note ) }.
note' is supposed to be a single note."
(let ((note2 (ly:music-deep-copy note)))
(set! (ly:music-property note 'articulations)
(cons (make-music 'SlurEvent 'span-direction -1)
(ly:music-property note 'articulations)))
(set! (ly:music-property note2 'articulations)
(cons (make-music 'SlurEvent 'span-direction 1)
(ly:music-property note2 'articulations)))
(make-music 'SequentialMusic 'elements (list note note2))))


### 1.3.4 Adding articulation to notes (example)

The easy way to add articulation to notes is to merge two music 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 $variable inside the #{…#} notation is like a regular \variable in classical LilyPond notation. We know that { \music -. -> }  will not work in LilyPond. We could avoid this problem by attaching the articulation to an empty chord, { << \music <> -. -> >> }  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, % input \displayMusic c4 ===> (make-music 'NoteEvent 'duration (ly:make-duration 2 0 1/1) 'pitch (ly:make-pitch -1 0 0)))) ===== % desired output \displayMusic c4-> ===> (make-music 'NoteEvent 'articulations (list (make-music 'ArticulationEvent 'articulation-type "accent")) 'duration (ly:make-duration 2 0 1/1) 'pitch (ly:make-pitch -1 0 0))  We see that a note (c4) is represented as an NoteEvent expression. To add an accent articulation, an ArticulationEvent expression must be added to the articulations property of the NoteEvent expression. To build this function, we begin with (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)  The first line is the way to define a function in Scheme: the function name is add-accent, and has one variable called note-event. In Scheme, the type of variable is often clear from its name. (this is good practice in other programming languages, too!) "Add an accent…"  is a description of what the function does. This is not strictly necessary, but just like clear variable names, it is good practice. You may wonder why we modify the note event directly instead of working on a copy (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 ‘\name’ or music from immediate Scheme expressions ‘$(…)’ provides a copy). Since it would be inefficient to create unnecessary copies, the return value from a music function is 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 \relative or \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 ‘\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 music-map, you’ll find that they stick with the same principles.

Where were we? Now we have a note-event we may modify, not because of using ly:music-deep-copy but because of a long-winded explanation. We add the accent to its 'articulations list property.

(set! place new-value)


Here, what we want to set (the ‘place’) is the 'articulations property of note-event expression.

(ly:music-property note-event 'articulations)


ly:music-property is the function used to access music properties (the 'articulations, 'duration, 'pitch, etc, that we see in the \displayMusic output above). The new value is the former 'articulations property, with an extra item: the ArticulationEvent expression, which we copy from the \displayMusic output,

(cons (make-music 'ArticulationEvent
'articulation-type "accent")
(ly:music-property result-event-chord 'articulations))


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 ArticulationEvent expression. The order inside the 'articulations property is not important here.

Finally, once we have added the accent articulation to its articulations property, we can return note-event, hence the last line of the function.

Now we transform the add-accent function into a music function (a matter of some syntactic sugar and a declaration of the type of its sole ‘real’ argument).

addAccent = #(define-music-function (parser location note-event)
(ly:music?)
"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)


We may verify that this music function works correctly,

\displayMusic \addAccent c4


# 2. Interfaces for programmers

Advanced tweaks may be performed by using Scheme. If you are not familiar with Scheme, you may wish to read our Scheme tutorial.

## 2.1 LilyPond code blocks

Creating music expressions in Scheme can be tedious, as they are heavily nested and the resulting Scheme code is large. For some simple tasks this can be avoided by using LilyPond code blocks, which enable common LilyPond syntax to be used within Scheme.

LilyPond code blocks look like

  #{ LilyPond code #}


Here is a trivial example:

ritpp = #(define-event-function (parser location) ()
#{ ^"rit." \pp #}
)

{ c'4 e'4\ritpp g'2 }


LilyPond code blocks can be used anywhere where you can write Scheme code. The Scheme reader actually is changed for accommodating LilyPond code blocks and can deal with embedded Scheme expressions starting with $ and #. The reader extracts the LilyPond code block and generates a runtime call to the LilyPond parser to interpret the LilyPond code. Scheme expressions embedded in the LilyPond code are evaluated in the lexical environment of the LilyPond code block, so all local variables and function parameters available at the point the LilyPond code block is written may be accessed. Variables defined in other Scheme modules, like the modules containing \header and \layout blocks, are not accessible as Scheme variables, i.e. prefixed with #, but they are accessible as LilyPond variables, i.e. prefixed with \. If location (see Scheme functions) refers to a valid input location (which it usually does inside of music/Scheme functions), all music generated inside the code block has its ‘origin’ set to location. A LilyPond code block may contain anything that you can use on the right side of an assignment. In addition, an empty LilyPond block corresponds to a void music expression, and a LilyPond block containing multiple music events gets turned into a sequential music expression. ## 2.2 Scheme functions Scheme functions are Scheme procedures that can create Scheme expressions from input written in LilyPond syntax. They can be called in pretty much all places where using # for specifying a value in Scheme syntax is allowed. While Scheme has functions of its own, this chapter is concerned with syntactic functions, functions that receive arguments specified in LilyPond syntax. ### 2.2.1 Scheme function definitions The general form for defining scheme functions is: function = #(define-scheme-function (parser location arg1 arg2 …) (type1? type2? …) body)  where  parser needs to be literally parser in order to give LilyPond code blocks (#{…#}) access to the parser. location needs to be literally location in order to provide access to the input location object, which is used to provide error messages with file names and line numbers. argN nth argument typeN? a Scheme type predicate for which argN must return #t. There is also a special form (predicate? default) for specifying optional arguments. If the actual argument is missing when the function is being called, the default value is substituted instead. Default values are evaluated at definition time (including LilyPond code blocks!), so if you need a default calculated at runtime, instead write a special value you can easily recognize. If you write the predicate in parentheses but don’t follow it with a default value, #f is used as the default. Default values are not verified with predicate? at either definition or run time: it is your responsibility to deal with the values you specify. Default values that happen to be music expressions are copied while setting origin to the location parameter. body A sequence of Scheme forms evaluated in order, the last one being used as the return value of the scheme function. It may contain LilyPond code blocks enclosed in hashed braces ( #{…#} ), like described in LilyPond code blocks. Within LilyPond code blocks, use # to reference function arguments (eg., ‘#arg1’) or to start an inline Scheme expression containing function arguments (eg., ‘#(cons arg1 arg2)’). Where normal Scheme expressions using # don’t do the trick, you might need to revert to immediate Scheme expressions using $, for example as ‘music’. If your function returns a music expression, it is given a useful value of origin. Suitability of arguments for the predicates is determined by actually calling the predicate after LilyPond has already converted them into a Scheme expression. As a consequence, the argument can be specified in Scheme syntax if desired (introduced with # or as the result of calling a scheme function), but LilyPond will also convert a number of LilyPond constructs into Scheme before actually checking the predicate on them. Currently, those include music, postevents, simple strings (with or without quotes), numbers, full markups and markup lists, score, book, bookpart, context definition and output definition blocks. For some kinds of expression (like most music not enclosed in braces) LilyPond needs to look further than the expression itself in order to determine its end. If such an expression were considered for an optional argument by evaluating its predicate, LilyPond would not be able to ‘backup’ when it decides the expression does not fit the parameter. So some forms of music might need to be enclosed in braces to make them acceptable in some circumstances. Some other ambiguities LilyPond sorts out by checking with predicate functions: is ‘-3’ a fingering postevent or a negative number? Is "a" 4 in lyric mode a string followed by a number, or a lyric event of duration 4? LilyPond tries the argument predicate on successive interpretations until success, with an order designed to minimize inconsistent interpretations and lookahead. For example, a predicate accepting both music expressions and pitches will consider c'' to be a pitch rather than a music expression. Immediately following durations or postevents might not work with that interpretation. So it’s best to avoid overly permissive predicates like scheme? when the application rather calls for more specific argument types. For a list of available predefined type predicates, see Predefined type predicates. #### See also Notation Reference: Predefined type predicates. Installed Files: ‘lily/music-scheme.cc’, ‘scm/c++.scm’, ‘scm/lily.scm’. ### 2.2.2 Scheme function usage Scheme functions can be called pretty much anywhere where a Scheme expression starting with # can be written. You call a scheme function by writing its name preceded by \, followed by its arguments. Once an optional argument predicate does not match an argument, LilyPond skips this and all following optional arguments, replacing them with their specified default, and ‘backs up’ the argument that did not match to the place of the next mandatory argument. Since the backed up argument needs to go somewhere, optional arguments are not actually considered optional unless followed by a mandatory argument. There is one exception: if you write \default in the place of an optional argument, this and all following optional arguments are skipped and replaced by their default. This works even when no mandatory argument follows since \default does not need to get backed up. The mark and key commands make use of that trick to provide their default behavior when just followed by \default. Apart from places where a Scheme value is required, there are a few places where # expressions are currently accepted and evaluated for their side effects but otherwise ignored. Mostly those are the places where an assignment would be acceptable as well. Since it is a bad idea to return values that can be misinterpreted in some context, you should use normal scheme functions only for those cases where you always return a useful value, and use void scheme functions (see section Void scheme functions) otherwise. ### 2.2.3 Void scheme functions Sometimes a procedure is executed in order to perform an action rather than return a value. Some programming languages (like C and Scheme) use functions for either concept and just discard the returned value (usually by allowing any expression to act as statement, ignoring the result). This is clever but error-prone: most C compilers nowadays offer warnings for various non-“void” expressions being discarded. For many functions executing an action, the Scheme standards declare the return value to be unspecified. LilyPond’s Scheme interpreter Guile has a unique value *unspecified* that it usually (such when using set! directly on a variable) but unfortunately not consistently returns in such cases. Defining a LilyPond function with define-void-function makes sure that this special value (the only value satisfying the predicate void?) will be returned. noPointAndClick = #(define-void-function (parser location) () (ly:set-option 'point-and-click #f)) … \noPointAndClick % disable point and click  If you want to evaluate an expression only for its side-effect and don’t want any value it may return interpreted, you can do so by prefixing it with \void: \void #(hashq-set! some-table some-key some-value)  That way, you can be sure that LilyPond will not assign meaning to the returned value regardless of where it encounters it. This will also work for music functions such as \displayMusic. ## 2.3 Music functions Music functions are Scheme procedures that can create music expressions automatically, and can be used to greatly simplify the input file. ### 2.3.1 Music function definitions The general form for defining music functions is: function = #(define-music-function (parser location arg1 arg2 …) (type1? type2? …) body)  quite in analogy to Scheme function definitions. More often than not, body will be a LilyPond code block. For a list of available type predicates, see Predefined type predicates. #### See also Notation Reference: Predefined type predicates. Installed Files: ‘lily/music-scheme.cc’, ‘scm/c++.scm’, ‘scm/lily.scm’. ### 2.3.2 Music function usage Music functions may currently be used in several places. Depending on where they are used, restrictions apply in order to be able to parse them unambiguously. The result a music function returns must be compatible with the context in which it is called. • At top level in a music expression. No restriction apply here. • As a post-event, explicitly started with a direction indicator (one of -, ^, and _). In this case, you can’t use an open music expression as the last argument, one that would end with a music expression able to accept additional postevents. • As a chord constituent. The returned expression must be of rhythmic-event type, most likely a NoteEvent. The special rules for trailing arguments make it possible to write polymorphic functions like \tweak that can be applied to different constructs. ### 2.3.3 Simple substitution functions Simple substitution functions are music functions whose output music expression is written in LilyPond format and contains function arguments in the output expression. They are described in Substitution function examples. ### 2.3.4 Intermediate substitution functions Intermediate substitution functions involve a mix of Scheme code and LilyPond code in the music expression to be returned. Some \override commands require an argument consisting of a pair of numbers (called a cons cell in Scheme). The pair can be directly passed into the music function, using a pair? variable: manualBeam = #(define-music-function (parser location beg-end) (pair?) #{ \once \override Beam.positions = #beg-end #}) \relative c' { \manualBeam #'(3 . 6) c8 d e f }  Alternatively, the numbers making up the pair can be passed as separate arguments, and the Scheme code used to create the pair can be included in the music expression: manualBeam = #(define-music-function (parser location beg end) (number? number?) #{ \once \override Beam.positions = #(cons beg end) #}) \relative c' { \manualBeam #3 #6 c8 d e f }  Properties are maintained conceptually using one stack per property per grob per context. Music functions may need to override one or several properties for the duration of the function, restoring them to their previous value before exiting. However, normal overrides pop and discard the top of the current property stack before pushing to it, so the previous value of the property is lost when it is overridden. When the previous value must be preserved, prefix the \override command with \temporary, like this: \temporary \override …  The use of \temporary causes the (usually set) pop-first property in the override to be cleared, so the previous value is not popped off the property stack before pushing the new value onto it. When a subsequent \revert pops off the temporarily overriden value, the previous value will re-emerge. In other words, calling \temporary \override and \revert in succession on the same property will have a net effect of zero. Similarly, pairing \temporary and \undo on the same music containing overrides will have a net effect of zero. Here is an example of a music function which makes use of this. The use of \temporary ensures the values of the cross-staff and style properties are restored on exit to whatever values they had when the crossStaff function was called. Without \temporary the default values would have been set on exit. crossStaff = #(define-music-function (parser location notes) (ly:music?) (_i "Create cross-staff stems") #{ \temporary \override Stem.cross-staff = #cross-staff-connect \temporary \override Flag.style = #'no-flag #notes \revert Stem.cross-staff \revert Flag.style #})  ### 2.3.5 Mathematics in functions Music functions can involve Scheme programming in addition to simple substitution, AltOn = #(define-music-function (parser location mag) (number?) #{ \override Stem.length = #(* 7.0 mag) \override NoteHead.font-size = #(inexact->exact (* (/ 6.0 (log 2.0)) (log mag))) #}) AltOff = { \revert Stem.length \revert NoteHead.font-size } \relative c' { c2 \AltOn #0.5 c4 c \AltOn #1.5 c c \AltOff c2 }  This example may be rewritten to pass in music expressions, withAlt = #(define-music-function (parser location mag music) (number? ly:music?) #{ \override Stem.length = #(* 7.0 mag) \override NoteHead.font-size = #(inexact->exact (* (/ 6.0 (log 2.0)) (log mag))) #music \revert Stem.length \revert NoteHead.font-size #}) \relative c' { c2 \withAlt #0.5 { c4 c } \withAlt #1.5 { c c } c2 }  ### 2.3.6 Functions without arguments In most cases a function without arguments should be written with a variable, dolce = \markup{ \italic \bold dolce }  However, in rare cases it may be useful to create a music function without arguments, displayBarNum = #(define-music-function (parser location) () (if (eq? #t (ly:get-option 'display-bar-numbers)) #{ \once \override Score.BarNumber.break-visibility = ##f #} #{#}))  To actually display bar numbers where this function is called, invoke lilypond with lilypond -d display-bar-numbers FILENAME.ly  ### 2.3.7 Void music functions A music function must return a music expression. If you want to execute a function only for its side effect, you should use define-void-function. But there may be cases where you sometimes want to produce a music expression, and sometimes not (like in the previous example). Returning a void music expression via #{ #} will achieve that. ## 2.4 Event functions To use a music function in the place of an event, you need to write a direction indicator before it. But sometimes, this does not quite match the syntax of constructs you want to replace. For example, if you want to write dynamics commands, those are usually attached without direction indicator, like c'\pp. Here is a way to write arbitrary dynamics: dyn=#(define-event-function (parser location arg) (markup?) (make-dynamic-script arg)) \relative c' { c\dyn pfsss }  You could do the same using a music function, but then you always would have to write a direction indicator before calling it, like c-\dyn pfsss. ## 2.5 Markup functions Markups are implemented as special Scheme functions which produce a Stencil object given a number of arguments. ### 2.5.1 Markup construction in Scheme Markup expressions are internally represented in Scheme using the markup macro: (markup expr)  To see a markup expression in its Scheme form, use the \displayScheme command: \displayScheme \markup { \column { \line { \bold \italic "hello" \raise #0.4 "world" } \larger \line { foo bar baz } } }  Compiling the code above will send the following to the display console: (markup #:line (#:column (#:line (#:bold (#:italic "hello") #:raise 0.4 "world") #:larger (#:line (#:simple "foo" #:simple "bar" #:simple "baz")))))  To prevent the markup from printing on the page, use ‘\void \displayScheme markup’. Also, as with the \displayMusic command, the output of \displayScheme can be saved to an external file. See Displaying music expressions. This example demonstrates the main translation rules between regular LilyPond markup syntax and Scheme markup syntax. Using #{ … #} for entering in LilyPond syntax will often be most convenient, but we explain how to use the markup macro to get a Scheme-only solution.  LilyPond Scheme \markup markup1 (markup markup1) \markup { markup1 markup2 … } (markup markup1 markup2 … ) \markup-command #:markup-command \variable variable \center-column { … } #:center-column ( … ) string "string" #scheme-arg scheme-arg The whole Scheme language is accessible inside the markup macro. For example, You may use function calls inside markup in order to manipulate character strings. This is useful when defining new markup commands (see New markup command definition). #### Known issues and warnings The markup-list argument of commands such as #:line, #:center, and #:column cannot be a variable or the result of a function call.  (markup #:line (function-that-returns-markups))  is invalid. One should use the make-line-markup, make-center-markup, or make-column-markup functions instead,  (markup (make-line-markup (function-that-returns-markups)))  ### 2.5.2 How markups work internally In a markup like \raise #0.5 "text example"  \raise is actually represented by the raise-markup function. The markup expression is stored as (list raise-markup 0.5 (list simple-markup "text example"))  When the markup is converted to printable objects (Stencils), the raise-markup function is called as (apply raise-markup \layout object list of property alists 0.5 the "text example" markup)  The raise-markup function first creates the stencil for the text example string, and then it raises that Stencil by 0.5 staff space. This is a rather simple example; more complex examples are in the rest of this section, and in ‘scm/define-markup-commands.scm’. ### 2.5.3 New markup command definition This section discusses the definition of new markup commands. #### Markup command definition syntax New markup commands can be defined using the define-markup-command Scheme macro, at top-level.  (define-markup-command (command-name layout props arg1 arg2 …) (arg1-type? arg2-type? …) [ #:properties ((property1 default-value1) …) ] …command body…)  The arguments are command-name the markup command name layout the ‘layout’ definition. props a list of associative lists, containing all active properties. argi ith command argument argi-type? a type predicate for the ith argument If the command uses properties from the props arguments, the #:properties keyword can be used to specify which properties are used along with their default values. Arguments are distinguished according to their type: • a markup, corresponding to type predicate markup?; • a list of markups, corresponding to type predicate markup-list?; • any other scheme object, corresponding to type predicates such as list?, number?, boolean?, etc. There is no limitation on the order of arguments (after the standard layout and props arguments). However, markup functions taking a markup as their last argument are somewhat special as you can apply them to a markup list, and the result is a markup list where the markup function (with the specified leading arguments) has been applied to every element of the original markup list. Since replicating the leading arguments for applying a markup function to a markup list is cheap mostly for Scheme arguments, you avoid performance pitfalls by just using Scheme arguments for the leading arguments of markup functions that take a markup as their last argument. Markup commands have a rather complex life cycle. The body of a markup command definition is responsible for converting the arguments of the markup command into a stencil expression which is returned. Quite often this is accomplished by calling the interpret-markup function on a markup expression, passing the layout and props arguments on to it. Those arguments are usually only known at a very late stage in typesetting. Markup expressions have their components assembled into markup expressions already when \markup in a LilyPond expression or the markup macro in Scheme is expanded. The evaluation and typechecking of markup command arguments happens at the time \markup/markup are interpreted. But the actual conversion of markup expressions into stencil expressions by executing the markup function bodies only happens when interpret-markup is called on a markup expression. #### On properties The layout and props arguments of markup commands bring a context for the markup interpretation: font size, line width, etc. The layout argument allows access to properties defined in paper blocks, using the ly:output-def-lookup function. For instance, the line width (the same as the one used in scores) is read using: (ly:output-def-lookup layout 'line-width)  The props argument makes some properties accessible to markup commands. For instance, when a book title markup is interpreted, all the variables defined in the \header block are automatically added to props, so that the book title markup can access the book title, composer, etc. It is also a way to configure the behaviour of a markup command: for example, when a command uses font size during processing, the font size is read from props rather than having a font-size argument. The caller of a markup command may change the value of the font size property in order to change the behaviour. Use the #:properties keyword of define-markup-command to specify which properties shall be read from the props arguments. The example in next section illustrates how to access and override properties in a markup command. #### A complete example The following example defines a markup command to draw a double box around a piece of text. Firstly, we need to build an approximative result using markups. Consulting the Text markup commands shows us the \box command is useful: \markup \box \box HELLO  Now, we consider that more padding between the text and the boxes is preferable. According to the \box documentation, this command uses a box-padding property, which defaults to 0.2. The documentation also mentions how to override it: \markup \box \override #'(box-padding . 0.6) \box A  Then, the padding between the two boxes is considered too small, so we override it too: \markup \override #'(box-padding . 0.4) \box \override #'(box-padding . 0.6) \box A  Repeating this lengthy markup would be painful. This is where a markup command is needed. Thus, we write a double-box markup command, taking one argument (the text). This draws the two boxes, with some padding.  #(define-markup-command (double-box layout props text) (markup?) "Draw a double box around text." (interpret-markup layout props #{\markup \override #'(box-padding . 0.4) \box \override #'(box-padding . 0.6) \box { #text }#}))  or, equivalently  #(define-markup-command (double-box layout props text) (markup?) "Draw a double box around text." (interpret-markup layout props (markup #:override '(box-padding . 0.4) #:box #:override '(box-padding . 0.6) #:box text)))  text is the name of the command argument, and markup? its type: it identifies it as a markup. The interpret-markup function is used in most of markup commands: it builds a stencil, using layout, props, and a markup. In the second case, this markup is built using the markup scheme macro, see Markup construction in Scheme. The transformation from \markup expression to scheme markup expression is straight-forward. The new command can be used as follow: \markup \double-box A  It would be nice to make the double-box command customizable: here, the box-padding values are hard coded, and cannot be changed by the user. Also, it would be better to distinguish the padding between the two boxes, from the padding between the inner box and the text. So we will introduce a new property, inter-box-padding, for the padding between the two boxes. The box-padding will be used for the inner padding. The new code is now as follows:  #(define-markup-command (double-box layout props text) (markup?) #:properties ((inter-box-padding 0.4) (box-padding 0.6)) "Draw a double box around text." (interpret-markup layout props #{\markup \override #(box-padding . ,inter-box-padding) \box \override #(box-padding . ,box-padding) \box { #text } #}))  Again, the equivalent version using the markup macro would be:  #(define-markup-command (double-box layout props text) (markup?) #:properties ((inter-box-padding 0.4) (box-padding 0.6)) "Draw a double box around text." (interpret-markup layout props (markup #:override (box-padding . ,inter-box-padding) #:box #:override (box-padding . ,box-padding) #:box text)))  Here, the #:properties keyword is used so that the inter-box-padding and box-padding properties are read from the props argument, and default values are given to them if the properties are not defined. Then, these values are used to override the box-padding properties used by the two \box commands. Note the backquote and the comma in the \override argument: they allow you to introduce a variable value into a literal expression. Now, the command can be used in a markup, and the boxes padding be customized: #(define-markup-command (double-box layout props text) (markup?) #:properties ((inter-box-padding 0.4) (box-padding 0.6)) "Draw a double box around text." (interpret-markup layout props #{\markup \override #(box-padding . ,inter-box-padding) \box \override #(box-padding . ,box-padding) \box { #text } #})) \markup \double-box A \markup \override #'(inter-box-padding . 0.8) \double-box A \markup \override #'(box-padding . 1.0) \double-box A  #### Adapting builtin commands A good way to start writing a new markup command, is to take example on a builtin one. Most of the markup commands provided with LilyPond can be found in file ‘scm/define-markup-commands.scm’. For instance, we would like to adapt the \draw-line command, to draw a double line instead. The \draw-line command is defined as follow (documentation stripped):  (define-markup-command (draw-line layout props dest) (number-pair?) #:category graphic #:properties ((thickness 1)) "…documentation…" (let ((th (* (ly:output-def-lookup layout 'line-thickness) thickness)) (x (car dest)) (y (cdr dest))) (make-line-stencil th 0 0 x y)))  To define a new command based on an existing one, copy the definition, and change the command name. The #:category keyword can be safely removed, as it is only used for generating LilyPond documentation, and is of no use for user-defined markup commands.  (define-markup-command (draw-double-line layout props dest) (number-pair?) #:properties ((thickness 1)) "…documentation…" (let ((th (* (ly:output-def-lookup layout 'line-thickness) thickness)) (x (car dest)) (y (cdr dest))) (make-line-stencil th 0 0 x y)))  Then, a property for setting the gap between two lines is added, called line-gap, defaulting e.g. to 0.6:  (define-markup-command (draw-double-line layout props dest) (number-pair?) #:properties ((thickness 1) (line-gap 0.6)) "…documentation…" …  Finally, the code for drawing two lines is added. Two calls to make-line-stencil are used to draw the lines, and the resulting stencils are combined using ly:stencil-add: #(define-markup-command (my-draw-line layout props dest) (number-pair?) #:properties ((thickness 1) (line-gap 0.6)) "..documentation.." (let* ((th (* (ly:output-def-lookup layout 'line-thickness) thickness)) (dx (car dest)) (dy (cdr dest)) (w (/ line-gap 2.0)) (x (cond ((= dx 0) w) ((= dy 0) 0) (else (/ w (sqrt (+ 1 (* (/ dx dy) (/ dx dy)))))))) (y (* (if (< (* dx dy) 0) 1 -1) (cond ((= dy 0) w) ((= dx 0) 0) (else (/ w (sqrt (+ 1 (* (/ dy dx) (/ dy dx)))))))))) (ly:stencil-add (make-line-stencil th x y (+ dx x) (+ dy y)) (make-line-stencil th (- x) (- y) (- dx x) (- dy y))))) \markup \my-draw-line #'(4 . 3) \markup \override #'(line-gap . 1.2) \my-draw-line #'(4 . 3)  ### 2.5.4 New markup list command definition Markup list commands are defined with the define-markup-list-command Scheme macro, which is similar to the define-markup-command macro described in New markup command definition, except that where the latter returns a single stencil, the former returns a list of stencils. In a similar vein, interpret-markup-list is used instead of interpret-markup for converting a markup list into a list of stencils. In the following example, a \paragraph markup list command is defined, which returns a list of justified lines, the first one being indented. The indent width is taken from the props argument. #(define-markup-list-command (paragraph layout props args) (markup-list?) #:properties ((par-indent 2)) (interpret-markup-list layout props #{\markuplist \justified-lines { \hspace #par-indent #args } #}))  The version using just Scheme is more complex: #(define-markup-list-command (paragraph layout props args) (markup-list?) #:properties ((par-indent 2)) (interpret-markup-list layout props (make-justified-lines-markup-list (cons (make-hspace-markup par-indent) args))))  Besides the usual layout and props arguments, the paragraph markup list command takes a markup list argument, named args. The predicate for markup lists is markup-list?. First, the function gets the indent width, a property here named par-indent, from the property list props. If the property is not found, the default value is 2. Then, a list of justified lines is made using the built-in markup list command \justified-lines, which is related to the make-justified-lines-markup-list function. A horizontal space is added at the beginning using \hspace (or the make-hspace-markup function). Finally, the markup list is interpreted using the interpret-markup-list function. This new markup list command can be used as follows: \markuplist { \paragraph { The art of music typography is called \italic {(plate) engraving.} The term derives from the traditional process of music printing. Just a few decades ago, sheet music was made by cutting and stamping the music into a zinc or pewter plate in mirror image. } \override-lines #'(par-indent . 4) \paragraph { The plate would be inked, the depressions caused by the cutting and stamping would hold ink. An image was formed by pressing paper to the plate. The stamping and cutting was completely done by hand. } }  ## 2.6 Contexts for programmers ### 2.6.1 Context evaluation Contexts can be modified during interpretation with Scheme code. The syntax for this is \applyContext function  function should be a Scheme function that takes a single argument: the context in which the \applyContext command is being called. The following code will print the current bar number on the standard output during the compile: \applyContext #(lambda (x) (format #t "\nWe were called in barnumber ~a.\n" (ly:context-property x 'currentBarNumber)))  ### 2.6.2 Running a function on all layout objects The most versatile way of tuning an object is \applyOutput which works by inserting an event into the specified context ( ApplyOutputEvent). Its syntax is \applyOutput Context proc  where proc is a Scheme function, taking three arguments. When interpreted, the function proc is called for every layout object found in the context Context at the current time step, with the following arguments: • the layout object itself, • the context where the layout object was created, and • the context where \applyOutput is processed. In addition, the cause of the layout object, i.e., the music expression or object that was responsible for creating it, is in the object property cause. For example, for a note head, this is a NoteHead event, and for a stem object, this is a Stem object. Here is a function to use for \applyOutput; it blanks note-heads on the center-line and next to it: #(define (blanker grob grob-origin context) (if (and (memq 'note-head-interface (ly:grob-interfaces grob)) (< (abs (ly:grob-property grob 'staff-position)) 2)) (set! (ly:grob-property grob 'transparent) #t))) \relative c' { a'4 e8 <<\applyOutput #'Voice #blanker a c d>> b2 }  To have function interpreted at the Score or Staff level use these forms \applyOutput #'Score #function \applyOutput #'Staff #function  ## 2.7 Callback functions Properties (like thickness, direction, etc.) can be set at fixed values with \override, e.g. \override Stem.thickness = #2.0  Properties can also be set to a Scheme procedure, \override Stem.thickness = #(lambda (grob) (if (= UP (ly:grob-property grob 'direction)) 2.0 7.0)) c b a g b a g b  In this case, the procedure is executed as soon as the value of the property is requested during the formatting process. Most of the typesetting engine is driven by such callbacks. Properties that typically use callbacks include stencil The printing routine, that constructs a drawing for the symbol X-offset The routine that sets the horizontal position X-extent The routine that computes the width of an object The procedure always takes a single argument, being the grob. That procedure may access the usual value of the property, by first calling the function that is the usual callback for that property, which can by found in the Internals Reference or the file ’define-grobs.scm’: \relative c'' { \override Flag #'X-offset = #(lambda (flag) (let ((default (ly:flag::calc-x-offset flag))) (* default 4.0))) c4. d8 a4. g8 }  If routines with multiple arguments must be called, the current grob can be inserted with a grob closure. Here is a setting from AccidentalSuggestion, (X-offset . ,(ly:make-simple-closure (,+ ,(ly:make-simple-closure (list ly:self-alignment-interface::centered-on-x-parent)) ,(ly:make-simple-closure (list ly:self-alignment-interface::x-aligned-on-self)))))  In this example, both ly:self-alignment-interface::x-aligned-on-self and ly:self-alignment-interface::centered-on-x-parent are called with the grob as argument. The results are added with the + function. To ensure that this addition is properly executed, the whole thing is enclosed in ly:make-simple-closure. In fact, using a single procedure as property value is equivalent to (ly:make-simple-closure (ly:make-simple-closure (list proc)))  The inner ly:make-simple-closure supplies the grob as argument to proc, the outer ensures that result of the function is returned, rather than the simple-closure object. From within a callback, the easiest method for evaluating a markup is to use grob-interpret-markup. For example: my-callback = #(lambda (grob) (grob-interpret-markup grob (markup "foo")))  ## 2.8 Inline Scheme code TODO: after this section had been written, LilyPond has improved to the point that finding a simple example where one would have to revert to Scheme has become rather hard. Until this section gets a rewrite, let’s pretend we don’t know. The main disadvantage of \tweak is its syntactical inflexibility. For example, the following produces a syntax error (or rather, it did so at some point in the past). F = \tweak font-size #-3 -\flageolet \relative c'' { c4^\F c4_\F }  Using Scheme, this problem can be avoided. The route to the result is given in Adding articulation to notes (example), especially how to use \displayMusic as a helping guide. F = #(let ((m (make-music 'ArticulationEvent 'articulation-type "flageolet"))) (set! (ly:music-property m 'tweaks) (acons 'font-size -3 (ly:music-property m 'tweaks))) m) \relative c'' { c4^\F c4_\F }  Here, the tweaks properties of the flageolet object m (created with make-music) are extracted with ly:music-property, a new key-value pair to change the font size is prepended to the property list with the acons Scheme function, and the result is finally written back with set!. The last element of the let block is the return value, m itself. ## 2.9 Difficult tweaks There are a few classes of difficult adjustments. • One type of difficult adjustment involves the appearance of spanner objects, such as slurs and ties. Usually, only one spanner object is created at a time, and it can be adjusted with the normal mechanism. However, occasionally a spanner crosses a line break. When this happens, the object is cloned. A separate object is created for every system in which the spanner appears. The new objects are clones of the original object and inherit all properties, including \overrides. In other words, an \override always affects all pieces of a broken spanner. To change only one part of a spanner at a line break, it is necessary to hook into the formatting process. The after-line-breaking callback contains the Scheme procedure that is called after the line breaks have been determined and layout objects have been split over different systems. In the following example, we define a procedure my-callback. This procedure • determines if the spanner has been split across line breaks • if yes, retrieves all the split objects • checks if this grob is the last of the split objects • if yes, it sets extra-offset. This procedure is installed into Tie, so the last part of the broken tie is repositioned. #(define (my-callback grob) (let* ( ;; have we been split? (orig (ly:grob-original grob)) ;; if yes, get the split pieces (our siblings) (siblings (if (ly:grob? orig) (ly:spanner-broken-into orig) '()))) (if (and (>= (length siblings) 2) (eq? (car (last-pair siblings)) grob)) (ly:grob-set-property! grob 'extra-offset '(-2 . 5))))) \relative c'' { \override Tie.after-line-breaking = #my-callback c1 ~ \break c2 ~ c }  When applying this trick, the new after-line-breaking callback should also call the old one, if such a default exists. For example, if using this with Hairpin, ly:spanner::kill-zero-spanned-time should also be called. • Some objects cannot be changed with \override for technical reasons. Examples of those are NonMusicalPaperColumn and PaperColumn. They can be changed with the \overrideProperty function, which works similar to \once \override, but uses a different syntax. \overrideProperty Score.NonMusicalPaperColumn % Grob name . line-break-system-details % Property name . next-padding % Optional subproperty name #20 % Value  Note, however, that \override, applied to NonMusicalPaperColumn and PaperColumn, still works as expected within \context blocks. # 3. LilyPond Scheme interfaces This chapter covers the various tools provided by LilyPond to help Scheme programmers get information into and out of the music streams. TODO – figure out what goes in here and how to organize it # A. GNU Free Documentation License Version 1.3, 3 November 2008  Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.  1. 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where the Example assumes that the current position is at Subsubsection One-Two-Three of a document of the following structure:

• 1. Section One
• 1.1 Subsection One-One
• ...
• 1.2 Subsection One-Two
• 1.2.1 Subsubsection One-Two-One
• 1.2.2 Subsubsection One-Two-Two
• 1.2.3 Subsubsection One-Two-Three     <== Current Position
• 1.2.4 Subsubsection One-Two-Four
• 1.3 Subsection One-Three
• ...
• 1.4 Subsection One-Four

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