1 @c -*- coding: utf-8; mode: texinfo; -*-
3 @chapter Programming work
6 * Overview of LilyPond architecture::
7 * LilyPond programming languages::
8 * Programming without compiling::
11 * Warnings Errors Progress and Debug Output::
12 * Debugging LilyPond::
13 * Tracing object relationships::
14 * Adding or modifying features::
18 * Understanding pure properties::
20 * Scheme->C interface::
21 * LilyPond miscellany::
24 @node Overview of LilyPond architecture
25 @section Overview of LilyPond architecture
27 LilyPond processes the input file into graphical and musical output in a
28 number of stages. This process, along with the types of routines that
29 accomplish the various stages of the process, is described in this section. A
30 more complete description of the LilyPond architecture and internal program
31 execution is found in Erik Sandberg's
32 @uref{http://lilypond.org/website/pdf/thesis-erik-sandberg.pdf, master's
35 The first stage of LilyPond processing is @emph{parsing}. In the parsing
36 process, music expressions in LilyPond input format are converted to music
37 expressions in Scheme format. In Scheme format, a music expression is a list
38 in tree form, with nodes that indicate the relationships between various music
39 events. The LilyPond parser is written in Bison.
41 The second stage of LilyPond processing is @emph{iterating}. Iterating
42 assigns each music event to a context, which is the environment in which the
43 music will be finally engraved. The context is responsible for all further
44 processing of the music. It is during the iteration stage that contexts are
45 created as necessary to ensure that every note has a Voice type context (e.g.
46 Voice, TabVoice, DrumVoice, CueVoice, MensuralVoice, VaticanaVoice,
47 GregorianTranscriptionVoice), that the Voice type contexts exist in
48 appropriate Staff type contexts, and that parallel Staff type contexts exist
49 in StaffGroup type contexts. In addition, during the iteration stage each
50 music event is assigned a moment, or a time in the music when the event
53 Each type of music event has an associated iterator. Iterators are defined in
54 @file{*-iterator.cc}. During iteration, an
55 event's iterator is called to deliver that music event to the appropriate
58 The final stage of LilyPond processing is @emph{translation}. During
59 translation, music events are prepared for graphical or midi output. The
60 translation step is accomplished by the polymorphic base class Translator
61 through its two derived classes: Engraver (for graphical output) and
62 Performer (for midi output).
64 Translators are defined in C++ files named @file{*-engraver.cc}
65 and @file{*-performer.cc}.
66 Much of the work of translating is handled by Scheme functions,
67 which is one of the keys to LilyPond's exceptional flexibility.
69 @sourceimage{architecture-diagram,,,png}
72 @node LilyPond programming languages
73 @section LilyPond programming languages
75 Programming in LilyPond is done in a variety of programming languages. Each
76 language is used for a specific purpose or purposes. This section describes
77 the languages used and provides links to reference manuals and tutorials for
78 the relevant language.
82 The core functionality of LilyPond is implemented in C++.
84 C++ is so ubiquitous that it is difficult to identify either a reference
85 manual or a tutorial. Programmers unfamiliar with C++ will need to spend some
86 time to learn the language before attempting to modify the C++ code.
88 The C++ code calls Scheme/GUILE through the GUILE interface, which is
90 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html, GUILE
95 The LilyPond lexer is implemented in Flex, an implementation of the Unix lex
96 lexical analyser generator. Resources for Flex can be found
97 @uref{http://flex.sourceforge.net/, here}.
101 The LilyPond parser is implemented in Bison, a GNU parser generator. The
102 Bison homepage is found at @uref{http://www.gnu.org/software/bison/,
103 gnu.org}. The manual (which includes both a reference and tutorial) is
104 @uref{http://www.gnu.org/software/bison/manual/index.html, available} in a
109 GNU Make is used to control the compiling process and to build the
110 documentation and the website. GNU Make documentation is available at
111 @uref{http://www.gnu.org/software/make/manual/, the GNU website}.
113 @subsection GUILE or Scheme
115 GUILE is the dialect of Scheme that is used as LilyPond's extension language.
116 Many extensions to LilyPond are written entirely in GUILE. The
117 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html,
118 GUILE Reference Manual} is available online.
120 @uref{http://mitpress.mit.edu/sicp/full-text/book/book.html, Structure and
121 Interpretation of Computer Programs}, a popular textbook used to teach
122 programming in Scheme is available in its entirety online.
124 An introduction to Guile/Scheme as used in LilyPond can be found in the
125 @rextend{Scheme tutorial}.
129 MetaFont is used to create the music fonts used by LilyPond. A MetaFont
130 tutorial is available at @uref{http://metafont.tutorial.free.fr/, the
131 METAFONT tutorial page}.
133 @subsection PostScript
135 PostScript is used to generate graphical output. A brief PostScript tutorial
136 is @uref{http://local.wasp.uwa.edu.au/~pbourke/dataformats/postscript/,
137 available online}. The
138 @uref{http://www.adobe.com/products/postscript/pdfs/PLRM.pdf, PostScript Language
139 Reference} is available online in PDF format.
143 Python is used for XML2ly and is used for building the documentation and the
146 Python documentation is available at @uref{http://www.python.org/doc/,
149 @node Programming without compiling
150 @section Programming without compiling
152 Much of the development work in LilyPond takes place by changing @file{*.ly} or
153 @file{*.scm} files. These changes can be made without compiling LilyPond. Such
154 changes are described in this section.
157 @subsection Modifying distribution files
159 Much of LilyPond is written in Scheme or LilyPond input files. These
160 files are interpreted when the program is run, rather than being compiled
161 when the program is built, and are present in all LilyPond distributions.
162 You will find @file{.ly} files in the @file{ly/} directory and the Scheme files in the
163 @file{scm/} directory. Both Scheme files and @file{.ly} files can be modified and
164 saved with any text editor. It's probably wise to make a backup copy of
165 your files before you modify them, although you can reinstall if the
166 files become corrupted.
168 Once you've modified the files, you can test the changes just by running
169 LilyPond on some input file. It's a good idea to create a file that
170 demonstrates the feature you're trying to add. This file will eventually
171 become a regression test and will be part of the LilyPond distribution.
173 @subsection Desired file formatting
175 Files that are part of the LilyPond distribution have Unix-style line
176 endings (LF), rather than DOS (CR+LF) or MacOS 9 and earlier (CR). Make
177 sure you use the necessary tools to ensure that Unix-style line endings are
178 preserved in the patches you create.
180 Tab characters should not be included in files for distribution. All
181 indentation should be done with spaces. Most editors have settings to
182 allow the setting of tab stops and ensuring that no tab characters are
183 included in the file.
185 Scheme files and LilyPond files should be written according to standard
186 style guidelines. Scheme file guidelines can be found at
187 @uref{http://community.schemewiki.org/?scheme-style}. Following these
188 guidelines will make your code easier to read. Both you and others that
189 work on your code will be glad you followed these guidelines.
191 For LilyPond files, you should follow the guidelines for LilyPond snippets
192 in the documentation. You can find these guidelines at
193 @ref{Texinfo introduction and usage policy}.
195 @node Finding functions
196 @section Finding functions
198 When making changes or fixing bugs in LilyPond, one of the initial
199 challenges is finding out where in the code tree the functions to
200 be modified live. With nearly 3000 files in the source tree,
201 trial-and-error searching is generally ineffective. This section
202 describes a process for finding interesting code.
204 @subsection Using the ROADMAP
206 The file ROADMAP is located in the main directory of the lilypond source.
207 ROADMAP lists all of the directories in the LilyPond source tree, along
208 with a brief description of the kind of files found in each directory.
209 This can be a very helpful tool for deciding which directories to search
210 when looking for a function.
213 @subsection Using grep to search
215 Having identified a likely subdirectory to search, the grep utility can
216 be used to search for a function name. The format of the grep command is
219 grep -i functionName subdirectory/*
222 This command will search all the contents of the directory subdirectory/
223 and display every line in any of the files that contains
224 functionName. The @option{-i} option makes @command{grep} ignore
225 case -- this can be very useful if you are not yet familiar with
226 our capitalization conventions.
228 The most likely directories to grep for function names are @file{scm/} for
229 scheme files, ly/ for lilypond input (@file{*.ly}) files, and @file{lily/} for C++
233 @subsection Using git grep to search
235 If you have used git to obtain the source, you have access to a
236 powerful tool to search for functions. The command:
239 git grep functionName
242 will search through all of the files that are present in the git
243 repository looking for functionName. It also presents the results
244 of the search using @code{less}, so the results are displayed one page
247 @subsection Searching on the git repository at Savannah
249 You can also use the equivalent of git grep on the Savannah server.
254 Go to http://git.sv.gnu.org/gitweb/?p=lilypond.git
257 In the pulldown box that says commit, select grep.
260 Type functionName in the search box, and hit enter/return
264 This will initiate a search of the remote git repository.
270 This section describes style guidelines for LilyPond
277 * Naming conventions::
286 @subsection Languages
288 C++ and Python are preferred. Python code should use PEP 8.
292 @subsection Filenames
294 Definitions of classes that are only accessed via pointers (*) or
295 references (&) shall not be included as include files.
301 ".cc" Implementation files
302 ".icc" Inline definition files
303 ".tcc" non inline Template defs
307 (setq auto-mode-alist
308 (append '(("\\.make$" . makefile-mode)
309 ("\\.cc$" . c++-mode)
310 ("\\.icc$" . c++-mode)
311 ("\\.tcc$" . c++-mode)
312 ("\\.hh$" . c++-mode)
313 ("\\.pod$" . text-mode)
318 The class Class_name is coded in @q{class-name.*}
322 @subsection Indentation
324 Standard GNU coding style is used.
326 @subsubheading Indenting files with @code{fixcc.py} (recommended)
328 LilyPond provides a python script that will adjust the indentation
329 and spacing on a @code{.cc} or @code{.hh} file to very near the
333 scripts/auxiliar/fixcc.py FILENAME
336 This can be run on all files at once, but this is not recommended
337 for normal contributors or developers.
340 scripts/auxiliar/fixcc.py \
341 $(find flower lily -name '*cc' -o -name '*hh' | grep -v /out)
345 @subsubheading Indenting with emacs
347 The following hooks will produce indentation which is similar to
348 our official indentation as produced with @code{fixcc.py}.
351 (add-hook 'c++-mode-hook
354 (setq indent-tabs-mode nil))
357 If you like using font-lock, you can also add this to your
361 (setq font-lock-maximum-decoration t)
362 (setq c++-font-lock-keywords-3
364 c++-font-lock-keywords-3
365 '(("\\b\\(a-zA-Z_?+_\\)\\b" 1 font-lock-variable-name-face) ("\\b\\(A-Z?+a-z_?+\\)\\b" 1 font-lock-type-face))
370 @subheading Indenting with vim
372 Although emacs indentation is the GNU standard, acceptable
373 indentation can usually be accomplished with vim. Some hints for
385 filetype plugin indent on
387 set ignorecase smartcase
390 set statusline=%F%m%r%h%w\ %@{&ff@}\ %Y\ [ASCII=\%03.3b]\ [HEX=\%02.2B]\ %04l,%04v\ %p%%\ [LEN=%L]
393 " Remove trailing whitespace on write
394 autocmd BufWritePre * :%s/\s\+$//e
397 With this @file{.vimrc}, files can be reindented automatically by
398 highlighting the lines to be indented in visual mode (use V to
399 enter visual mode) and pressing @code{=}.
401 A @file{scheme.vim} file will help improve the indentation. This
402 one was suggested by Patrick McCarty. It should be saved in
403 @file{~/.vim/after/syntax/scheme.vim}.
406 " Additional Guile-specific 'forms'
407 syn keyword schemeSyntax define-public define*-public
408 syn keyword schemeSyntax define* lambda* let-keywords*
409 syn keyword schemeSyntax defmacro defmacro* define-macro
410 syn keyword schemeSyntax defmacro-public defmacro*-public
411 syn keyword schemeSyntax use-modules define-module
412 syn keyword schemeSyntax define-method define-class
414 " Additional LilyPond-specific 'forms'
415 syn keyword schemeSyntax define-markup-command define-markup-list-command
416 syn keyword schemeSyntax define-safe-public define-music-function
417 syn keyword schemeSyntax def-grace-function
419 " All of the above should influence indenting too
420 set lw+=define-public,define*-public
421 set lw+=define*,lambda*,let-keywords*
422 set lw+=defmacro,defmacro*,define-macro
423 set lw+=defmacro-public,defmacro*-public
424 set lw+=use-modules,define-module
425 set lw+=define-method,define-class
426 set lw+=define-markup-command,define-markup-list-command
427 set lw+=define-safe-public,define-music-function
428 set lw+=def-grace-function
430 " These forms should not influence indenting
434 " Try to highlight all ly: procedures
435 syn match schemeFunc "ly:[^) ]\+"
439 @node Naming conventions
440 @subsection Naming Conventions
442 Naming conventions have been established for LilyPond
445 @subheading Classes and Types
447 Classes begin with an uppercase letter, and words
448 in class names are separated with @code{_}:
456 Member variable names end with an underscore:
464 Macro names should be written in uppercase completely,
465 with words separated by @code{_}:
471 @subheading Variables
473 Variable names should be complete words, rather than abbreviations.
474 For example, it is preferred to use @code{thickness} rather than
475 @code{th} or @code{t}.
477 Multi-word variable names in C++ should have the words separated
478 by the underscore character (@q{_}):
481 cxx_multiword_variable
484 Multi-word variable names in Scheme should have the words separated
488 scheme-multiword-variable
492 @subsection Broken code
494 Do not write broken code. This includes hardwired dependencies,
495 hardwired constants, slow algorithms and obvious limitations. If
496 you can not avoid it, mark the place clearly, and add a comment
497 explaining shortcomings of the code.
499 Ideally, the comment marking the shortcoming would include
500 TODO, so that it is marked for future fixing.
502 We reject broken-in-advance on principle.
506 @subsection Code comments
508 Comments may not be needed if descriptive variable names are used
509 in the code and the logic is straightforward. However, if the
510 logic is difficult to follow, and particularly if non-obvious
511 code has been included to resolve a bug, a comment describing
512 the logic and/or the need for the non-obvious code should be included.
514 There are instances where the current code could be commented better.
515 If significant time is required to understand the code as part of
516 preparing a patch, it would be wise to add comments reflecting your
517 understanding to make future work easier.
520 @node Handling errors
521 @subsection Handling errors
523 As a general rule, you should always try to continue computations,
524 even if there is some kind of error. When the program stops, it
525 is often very hard for a user to pinpoint what part of the input
526 causes an error. Finding the culprit is much easier if there is
527 some viewable output.
529 So functions and methods do not return errorcodes, they never
530 crash, but report a programming_error and try to carry on.
532 Error and warning messages need to be localized.
536 @subsection Localization
538 This document provides some guidelines to help programmers write
540 messages. To help translations, user messages must follow
541 uniform conventions. Follow these rules when coding for LilyPond.
542 Hopefully, this can be replaced by general GNU guidelines in the
543 future. Even better would be to have an English (en_BR, en_AM)
544 guide helping programmers writing consistent messages for all GNU
547 Non-preferred messages are marked with `+'. By convention,
548 ungrammatical examples are marked with `*'. However, such ungrammatical
549 examples may still be preferred.
554 Every message to the user should be localized (and thus be marked
555 for localization). This includes warning and error messages.
558 Do not localize/gettextify:
562 `programming_error ()'s
565 `programming_warning ()'s
571 output strings (PostScript, TeX, etc.)
576 Messages to be localized must be encapsulated in `_ (STRING)' or
577 `_f (FORMAT, ...)'. E.g.:
580 warning (_ ("need music in a score"));
581 error (_f ("cannot open file: `%s'", file_name));
584 In some rare cases you may need to call `gettext ()' by hand. This
585 happens when you pre-define (a list of) string constants for later
586 use. In that case, you'll probably also need to mark these string
587 constants for translation, using `_i (STRING)'. The `_i' macro is
588 a no-op, it only serves as a marker for `xgettext'.
591 char const* messages[] = @{
592 _i ("enable debugging output"),
593 _i ("ignore lilypond version"),
600 puts (gettext (messages i));
604 See also @file{flower/getopt-long.cc} and @file{lily/main.cc}.
607 Do not use leading or trailing whitespace in messages. If you need
608 whitespace to be printed, prepend or append it to the translated
612 message ("Calculating line breaks..." + " ");
616 Error or warning messages displayed with a file name and line
617 number never start with a capital, eg,
620 foo.ly: 12: not a duration: 3
623 Messages containing a final verb, or a gerund (`-ing'-form) always
624 start with a capital. Other (simpler) messages start with a
630 Not declaring: `foo'.
634 Avoid abbreviations or short forms, use `cannot' and `do not'
635 rather than `can't' or `don't'
636 To avoid having a number of different messages for the same
637 situation, well will use quoting like this `"message: `%s'"' for all
638 strings. Numbers are not quoted:
641 _f ("cannot open file: `%s'", name_str)
642 _f ("cannot find character number: %d", i)
646 Think about translation issues. In a lot of cases, it is better to
647 translate a whole message. English grammar must not be imposed on the
648 translator. So, instead of
651 stem at + moment.str () + does not fit in beam
657 _f ("stem at %s does not fit in beam", moment.str ())
661 Split up multi-sentence messages, whenever possible. Instead of
664 warning (_f ("out of tune! Can't find: `%s'", "Key_engraver"));
665 warning (_f ("cannot find font `%s', loading default", font_name));
671 warning (_ ("out of tune:"));
672 warning (_f ("cannot find: `%s', "Key_engraver"));
673 warning (_f ("cannot find font: `%s', font_name));
674 warning (_f ("Loading default font"));
678 If you must have multiple-sentence messages, use full punctuation.
679 Use two spaces after end of sentence punctuation. No punctuation
680 (esp. period) is used at the end of simple messages.
683 _f ("Non-matching braces in text `%s', adding braces", text)
684 _ ("Debug output disabled. Compiled with NPRINT.")
685 _f ("Huh? Not a Request: `%s'. Ignoring.", request)
689 Do not modularize too much; words frequently cannot be translated
690 without context. It is probably safe to treat most occurrences of
691 words like stem, beam, crescendo as separately translatable words.
694 When translating, it is preferable to put interesting information
695 at the end of the message, rather than embedded in the middle.
696 This especially applies to frequently used messages, even if this
697 would mean sacrificing a bit of eloquence. This holds for original
698 messages too, of course.
701 en: cannot open: `foo.ly'
702 + nl: kan `foo.ly' niet openen (1)
703 kan niet openen: `foo.ly'* (2)
704 niet te openen: `foo.ly'* (3)
708 The first nl message, although grammatically and stylistically
709 correct, is not friendly for parsing by humans (even if they speak
710 dutch). I guess we would prefer something like (2) or (3).
713 Do not run make po/po-update with GNU gettext < 0.10.35
718 @node Warnings Errors Progress and Debug Output
719 @section Warnings, Errors, Progress and Debug Output
721 @unnumberedsubsec Available log levels
723 LilyPond has several loglevels, which specify how verbose the output on
724 the console should be:
726 @item NONE: No output at all, even on failure
727 @item ERROR: Only error messages
728 @item WARN: Only error messages and warnings
729 @item BASIC_PROGRESS: Warnings, errors and basic progress (success, etc.)
730 @item PROGRESS: Warnings, errors and full progress messages
731 @item INFO: Warnings, errors, progress and more detailed information (default)
732 @item DEBUG: All messages, including full debug messages (very verbose!)
735 The loglevel can either be set with the environment variable
736 @code{LILYPOND_LOGLEVEL} or on the command line with the @option{--loglevel=...}
739 @unnumberedsubsec Functions for debug and log output
741 LilyPond has two different types of error and log functions:
745 If a warning or error is caused by an identified position in the input file,
746 e.g. by a grob or by a music expression, the functions of the @code{Input}
747 class provide logging functionality that prints the position of the message
748 in addition to the message.
751 If a message can not be associated with a particular position in an input file,
752 e.g. the output file cannot be written, then the functions in the
753 @code{flower/include/warn.hh} file will provide logging functionality that
754 only prints out the message, but no location.
758 There are also Scheme functions to access all of these logging functions from
759 scheme. In addition, the Grob class contains some convenience wrappers for
760 even easier access to these functions.
762 The message and debug functions in @code{warn.hh} also have an optional
763 argument @code{newline}, which specifies whether the message should always
764 start on a new line or continue a previous message.
765 By default, @code{progress_indication} does NOT start on a new line, but rather
766 continue the previous output. They also do not have a particular input
767 position associated, so there are no progress functions in the Input class.
768 All other functions by default start their output on a new line.
770 The error functions come in three different flavors: fatal error messages,
771 programming error messages and normal error messages. Errors written
772 by the @code{error ()} function will cause LilyPond to exit immediately,
773 errors by @code{Input::error ()} will continue the compilation, but
774 return a non-zero return value of the lilypond call (i.e. indicate an
775 unsuccessful program execution). All other errors will be printed on the
776 console, but not exit LilyPond or indicate an unsuccessful return code.
777 Their only differences to a warnings are the displayed text and that
778 they will be shown with loglevel @code{ERROR}.
780 If the Scheme option @code{warning-as-error} is set, any warning will be
781 treated as if @code{Input::error} was called.
784 @unnumberedsubsec All logging functions at a glance
786 @multitable @columnfractions 0.16 0.42 0.42
788 @tab C++, no location
789 @tab C++ from input location
792 @tab @code{error ()}, @code{programming_error (msg)}, @code{non_fatal_error (msg)}
793 @tab @code{Input::error (msg)}, @code{Input::programming_error (msg)}
796 @tab @code{warning (msg)}
797 @tab @code{Input::warning (msg)}
800 @tab @code{basic_progress (msg)}
804 @tab @code{progress_indication (msg)}
808 @tab @code{message (msg)}
809 @tab @code{Input::message (msg)}
812 @tab @code{debug_output (msg)}
813 @tab @code{Input::debug_output (msg)}
819 @tab Scheme, music expression
822 @tab @code{Grob::programming_error (msg)}
826 @tab @code{Grob::warning (msg)}
827 @tab @code{(ly:music-warning music msg)}
839 @tab @code{(ly:music-message music msg)}
848 @tab Scheme, no location
849 @tab Scheme, input location
853 @tab @code{(ly:error msg args)}, @code{(ly:programming-error msg args)}
856 @tab @code{(ly:warning msg args)}
857 @tab @code{(ly:input-warning input msg args)}
860 @tab @code{(ly:basic-progress msg args)}
864 @tab @code{(ly:progress msg args)}
868 @tab @code{(ly:message msg args)}
869 @tab @code{(ly:input-message input msg args)}
872 @tab @code{(ly:debug msg args)}
880 @node Debugging LilyPond
881 @section Debugging LilyPond
883 The most commonly used tool for debugging LilyPond is the GNU
884 debugger gdb. The gdb tool is used for investigating and debugging
885 core Lilypond code written in C++. Another tool is available for
886 debugging Scheme code using the Guile debugger. This section
887 describes how to use both gdb and the Guile Debugger.
890 * Debugging overview::
891 * Debugging C++ code::
892 * Debugging Scheme code::
895 @node Debugging overview
896 @subsection Debugging overview
898 Using a debugger simplifies troubleshooting in at least two ways.
900 First, breakpoints can be set to pause execution at any desired point.
901 Then, when execution has paused, debugger commands can be issued to
902 explore the values of various variables or to execute functions.
904 Second, the debugger can display a stack trace, which shows the
905 sequence in which functions have been called and the arguments
906 passed to the called functions.
908 @node Debugging C++ code
909 @subsection Debugging C++ code
911 The GNU debugger, gdb, is the principal tool for debugging C++ code.
913 @subheading Compiling LilyPond for use with gdb
915 In order to use gdb with LilyPond, it is necessary to compile
916 LilyPond with debugging information. This is the current default
917 mode of compilation. Often debugging becomes more complicated
918 when the compiler has optimised variables and function calls away.
919 In that case it may be helpful to run the following command in the
920 main LilyPond source directory:
923 ./configure --disable-optimising
927 This will create a version of LilyPond with minimal optimization
928 which will allow the debugger to access all variables and step
929 through the source code in-order. It may not accurately reproduce
930 bugs encountered with the optimized version, however.
932 You should not do @var{make install} if you want to use a debugger
933 with LilyPond. The @var{make install} command will strip debugging
934 information from the LilyPond binary.
936 @subheading Typical gdb usage
938 Once you have compiled the Lilypond image with the necessary
939 debugging information it will have been written to a location in a
940 subfolder of your current working directory:
946 This is important as you will need to let gdb know where to find the
947 image containing the symbol tables. You can invoke gdb from the
948 command line using the following:
954 This loads the LilyPond symbol tables into gdb. Then, to run
955 LilyPond on @file{test.ly} under the debugger, enter the following:
964 As an alternative to running gdb at the command line you may try
965 a graphical interface to gdb such as ddd:
971 You can also use sets of standard gdb commands stored in a .gdbinit
972 file (see next section).
974 @subheading Typical .gdbinit files
976 The behavior of gdb can be readily customized through the use of a
977 @var{.gdbinit} file. A @var{.gdbinit} file is a file named
978 @var{.gdbinit} (notice the @qq{.} at the beginning of the file name)
979 that is placed in a user's home directory.
981 The @var{.gdbinit} file below is from Han-Wen. It sets breakpoints
982 for all errors and defines functions for displaying scheme objects
983 (ps), grobs (pgrob), and parsed music expressions (pmusic).
986 file $LILYPOND_GIT/build/out/bin/lilypond
988 b Grob::programming_error
991 print ly_display_scm($arg0)
994 print ly_display_scm($arg0->self_scm_)
995 print ly_display_scm($arg0->mutable_property_alist_)
996 print ly_display_scm($arg0->immutable_property_alist_)
997 print ly_display_scm($arg0->object_alist_)
1000 print ly_display_scm($arg0->self_scm_)
1001 print ly_display_scm($arg0->mutable_property_alist_)
1002 print ly_display_scm($arg0->immutable_property_alist_)
1006 @node Debugging Scheme code
1007 @subsection Debugging Scheme code
1009 Scheme code can be developed using the Guile command line
1010 interpreter @code{top-repl}. You can either investigate
1011 interactively using just Guile or you can use the debugging
1012 tools available within Guile.
1014 @subheading Using Guile interactively with LilyPond
1016 In order to experiment with Scheme programming in the LilyPond
1017 environment, it is necessary to have a Guile interpreter that
1018 has all the LilyPond modules loaded. This requires the following
1021 First, define a Scheme symbol for the active module in the @file{.ly} file:
1024 #(module-define! (resolve-module '(guile-user))
1025 'lilypond-module (current-module))
1028 Now place a Scheme function in the @file{.ly} file that gives an
1029 interactive Guile prompt:
1035 When the @file{.ly} file is compiled, this causes the compilation to be
1036 interrupted and an interactive guile prompt to appear. Once the
1037 guile prompt appears, the LilyPond active module must be set as the
1038 current guile module:
1041 guile> (set-current-module lilypond-module)
1044 You can demonstrate these commands are operating properly by typing the name
1045 of a LilyPond public scheme function to check it has been defined:
1048 guile> fret-diagram-verbose-markup
1049 #<procedure fret-diagram-verbose-markup (layout props marking-list)>
1052 If the LilyPond module has not been correctly loaded, an error
1053 message will be generated:
1056 guile> fret-diagram-verbose-markup
1057 ERROR: Unbound variable: fret-diagram-verbose-markup
1058 ABORT: (unbound-variable)
1061 Once the module is properly loaded, any valid LilyPond Scheme
1062 expression can be entered at the interactive prompt.
1064 After the investigation is complete, the interactive guile
1065 interpreter can be exited:
1071 The compilation of the @file{.ly} file will then continue.
1073 @subheading Using the Guile debugger
1075 To set breakpoints and/or enable tracing in Scheme functions, put
1078 \include "guile-debugger.ly"
1081 in your input file after any scheme procedures you have defined in
1082 that file. This will invoke the Guile command-line after having set
1083 up the environment for the debug command-line. When your input file
1084 is processed, a guile prompt will be displayed. You may now enter
1085 commands to set up breakpoints and enable tracing by the Guile debugger.
1087 @subheading Using breakpoints
1089 At the guile prompt, you can set breakpoints with
1090 the @code{set-break!} procedure:
1093 guile> (set-break! my-scheme-procedure)
1096 Once you have set the desired breakpoints, you exit the guile repl frame
1103 Then, when one of the scheme routines for which you have set
1104 breakpoints is entered, guile will interrupt execution in a debug
1105 frame. At this point you will have access to Guile debugging
1106 commands. For a listing of these commands, type:
1112 Alternatively you may code the breakpoints in your Lilypond source
1113 file using a command such as:
1116 #(set-break! my-scheme-procedure)
1119 immediately after the @code{\include} statement. In this case the
1120 breakpoint will be set straight after you enter the @code{(quit)}
1121 command at the guile prompt.
1123 Embedding breakpoint commands like this is particularly useful if
1124 you want to look at how the Scheme procedures in the @file{.scm}
1125 files supplied with LilyPond work. To do this, edit the file in
1126 the relevant directory to add this line near the top:
1129 (use-modules (scm guile-debugger))
1132 Now you can set a breakpoint after the procedure you are interested
1133 in has been declared. For example, if you are working on routines
1134 called by @var{print-book-with} in @file{lily-library.scm}:
1137 (define (print-book-with book process-procedure)
1138 (let* ((paper (ly:parser-lookup '$defaultpaper))
1139 (layout (ly:parser-lookup '$defaultlayout))
1140 (outfile-name (get-outfile-name book)))
1141 (process-procedure book paper layout outfile-name)))
1143 (define-public (print-book-with-defaults book)
1144 (print-book-with book ly:book-process))
1146 (define-public (print-book-with-defaults-as-systems book)
1147 (print-book-with book ly:book-process-to-systems))
1150 At this point in the code you could add this to set a breakpoint at
1154 (set-break! print-book-with)
1157 @subheading Tracing procedure calls and evaluator steps
1159 Two forms of trace are available:
1162 (set-trace-call! my-scheme-procedure)
1168 (set-trace-subtree! my-scheme-procedure)
1171 @code{set-trace-call!} causes Scheme to log a line to the standard
1172 output to show when the procedure is called and when it exits.
1174 @code{set-trace-subtree!} traces every step the Scheme evaluator
1175 performs in evaluating the procedure.
1177 @node Tracing object relationships
1178 @section Tracing object relationships
1180 Understanding the LilyPond source often boils down to figuring out what
1181 is happening to the Grobs. Where (and why) are they being created,
1182 modified and destroyed? Tracing Lily through a debugger in order to
1183 identify these relationships can be time-consuming and tedious.
1185 In order to simplify this process, a facility has been added to
1186 display the grobs that are created and the properties that are set
1187 and modified. Although it can be complex to get set up, once set up
1188 it easily provides detailed information about the life of grobs
1189 in the form of a network graph.
1191 Each of the steps necessary to use the graphviz utility
1196 @item Installing graphviz
1198 In order to create the graph of the object relationships, it is
1199 first necessary to install Graphviz. Graphviz is available for a
1200 number of different platforms:
1203 @uref{http://www.graphviz.org/Download..php}
1206 @item Modifying config.make
1208 In order for the Graphviz tool to work, config.make must be modified.
1209 It is probably a good idea to first save a copy of config.make under
1212 In order to have the required functionality available, LilyPond
1213 needs to be compiled with the option @option{-DDEBUG}. You can
1214 achieve this by configuring with
1217 ./configure --enable-checking
1220 @item Rebuilding LilyPond
1222 The executable code of LilyPond must be rebuilt from scratch:
1225 make -C lily clean && make -C lily
1228 @item Create a graphviz-compatible @file{.ly} file
1230 In order to use the graphviz utility, the @file{.ly} file must include
1231 @file{ly/graphviz-init.ly}, and should then specify the
1232 grobs and symbols that should be tracked. An example of this
1233 is found in @file{input/regression/graphviz.ly}.
1235 @item Run lilypond with output sent to a log file
1237 The Graphviz data is sent to stderr by lilypond, so it is
1238 necessary to redirect stderr to a logfile:
1241 lilypond graphviz.ly 2> graphviz.log
1244 @item Edit the logfile
1246 The logfile has standard lilypond output, as well as the Graphviz
1247 output data. Delete everything from the beginning of the file
1248 up to but not including the first occurrence of @code{digraph}.
1250 Also, delete the final lilypond message about success from the end
1253 @item Process the logfile with @code{dot}
1255 The directed graph is created from the log file with the program
1259 dot -Tpdf graphviz.log > graphviz.pdf
1264 The pdf file can then be viewed with any pdf viewer.
1266 When compiled with @option{-DDEBUG}, lilypond may run slower
1267 than normal. The original configuration can be restored by rerunning
1268 @code{./configure} with @option{--disable-checking}. Then
1269 rebuild lilypond with
1272 make -C lily clean && make -C lily
1276 @node Adding or modifying features
1277 @section Adding or modifying features
1279 When a new feature is to be added to LilyPond, it is necessary to
1280 ensure that the feature is properly integrated to maintain
1281 its long-term support. This section describes the steps necessary
1282 for feature addition and modification.
1287 * Write regression tests::
1288 * Write convert-ly rule::
1289 * Automatically update documentation::
1290 * Manually update documentation::
1291 * Edit changes.tely::
1292 * Verify successful build::
1293 * Verify regression tests::
1294 * Post patch for comments::
1296 * Closing the issues::
1299 @node Write the code
1300 @subsection Write the code
1302 You should probably create a new git branch for writing the code, as that
1303 will separate it from the master branch and allow you to continue
1304 to work on small projects related to master.
1306 Please be sure to follow the rules for programming style discussed
1307 earlier in this chapter.
1310 @node Write regression tests
1311 @subsection Write regression tests
1313 In order to demonstrate that the code works properly, you will
1314 need to write one or more regression tests. These tests are
1315 typically @file{.ly} files that are found in @file{input/regression}.
1317 Regression tests should be as brief as possible to demonstrate the
1318 functionality of the code.
1320 Regression tests should generally cover one issue per test. Several
1321 short, single-issue regression tests are preferred to a single, long,
1322 multiple-issue regression test.
1324 If the change in the output is small or easy to overlook, use bigger
1325 staff size -- 40 or more (up to 100 in extreme cases). Size 30 means
1326 "pay extra attention to details in general".
1328 Use existing regression tests as templates to demonstrate the type of
1329 header information that should be included in a regression test.
1332 @node Write convert-ly rule
1333 @subsection Write convert-ly rule
1335 If the modification changes the input syntax, a convert-ly rule
1336 should be written to automatically update input files from older
1339 convert-ly rules are found in python/convertrules.py
1341 If possible, the convert-ly rule should allow automatic updating
1342 of the file. In some cases, this will not be possible, so the
1343 rule will simply point out to the user that the feature needs
1346 @subsubheading Updating version numbers
1348 If a development release occurs between you writing your patch and
1349 having it approved+pushed, you will need to update the version
1350 numbers in your tree. This can be done with:
1353 scripts/auxiliar/update-patch-version old.version.number new.version.number
1356 It will change all files in git, so use with caution and examine
1360 @node Automatically update documentation
1361 @subsection Automatically update documentation
1363 @command{convert-ly} should be used to update the documentation,
1364 the snippets, and the regression tests. This not only makes the
1365 necessary syntax changes, it also tests the @command{convert-ly}
1368 The automatic updating is performed by moving to the top-level
1369 source directory, then running:
1372 scripts/auxiliar/update-with-convert-ly.sh
1375 If you did an out-of-tree build, pass in the relative path:
1378 LILYPOND_BUILD_DIR=../build-lilypond/ scripts/auxiliar/update-with-convert-ly.sh
1382 @node Manually update documentation
1383 @subsection Manually update documentation
1385 Where the convert-ly rule is not able to automatically update the inline
1386 lilypond code in the documentation (i.e. if a NOT_SMART rule is used), the
1387 documentation must be manually updated. The inline snippets that require
1388 changing must be changed in the English version of the docs and all
1389 translated versions. If the inline code is not changed in the
1390 translated documentation, the old snippets will show up in the
1391 English version of the documentation.
1393 Where the convert-ly rule is not able to automatically update snippets
1394 in Documentation/snippets/, those snippets must be manually updated.
1395 Those snippets should be copied to Documentation/snippets/new. The
1396 comments at the top of the snippet describing its automatic generation
1397 should be removed. All translated texidoc strings should be removed.
1398 The comment @qq{% begin verbatim} should be removed. The syntax of
1399 the snippet should then be manually edited.
1401 Where snippets in Documentation/snippets are made obsolete, the snippet
1402 should be copied to Documentation/snippets/new. The comments and
1403 texidoc strings should be removed as described above. Then the body
1404 of the snippet should be changed to:
1408 This snippet is deprecated as of version X.Y.Z and
1409 will be removed from the documentation.
1414 where X.Y.Z is the version number for which the convert-ly rule was
1417 Update the snippet files by running:
1420 scripts/auxiliar/makelsr.py
1423 Where the convert-ly rule is not able to automatically update regression
1424 tests, the regression tests in input/regression should be manually
1427 Although it is not required, it is helpful if the developer
1428 can write relevant material for inclusion in the Notation
1429 Reference. If the developer does not feel qualified to write
1430 the documentation, a documentation editor will be able to
1431 write it from the regression tests. In this case the developer
1432 should raise a new issue with the Type=Documentation tag containing
1433 a reference to the original issue number and/or the committish of
1434 the pushed patch so that the need for new documention is not
1437 Any text that is added to or removed from the documentation should
1438 be changed only in the English version.
1441 @node Edit changes.tely
1442 @subsection Edit changes.tely
1444 An entry should be added to Documentation/changes.tely to describe
1445 the feature changes to be implemented. This is especially important
1446 for changes that change input file syntax.
1448 Hints for changes.tely entries are given at the top of the file.
1450 New entries in changes.tely go at the top of the file.
1452 The changes.tely entry should be written to show how the new change
1453 improves LilyPond, if possible.
1456 @node Verify successful build
1457 @subsection Verify successful build
1459 When the changes have been made, successful completion must be
1467 When these commands complete without error, the patch is
1468 considered to function successfully.
1470 Developers on Windows who are unable to build LilyPond should
1471 get help from a GNU/Linux or OSX developer to do the make tests.
1474 @node Verify regression tests
1475 @subsection Verify regression tests
1477 In order to avoid breaking LilyPond, it is important to verify that
1478 the regression tests succeed, and that no unwanted changes are
1479 introduced into the output. This process is described in
1480 @ref{Regtest comparison}.
1482 @subheading Typical developer's edit/compile/test cycle
1490 make [-j@var{X} CPU_COUNT=@var{X}] test-baseline
1491 make [-j@var{X} CPU_COUNT=@var{X}] check
1495 Edit/compile/test cycle:
1498 @emph{## edit source files, then...}
1500 make clean @emph{## only if needed (see below)}
1501 make [-j@var{X}] @emph{## only if needed (see below)}
1502 make [-j@var{X} CPU_COUNT=@var{X}] test-redo @emph{## redo files differing from baseline}
1503 make [-j@var{X} CPU_COUNT=@var{X}] check
1514 If you modify any source files that have to be compiled (such as
1515 @file{.cc} or @file{.hh} files in @file{flower/} or @file{lily/}),
1516 then you must run @command{make} before @command{make test-redo},
1517 so @command{make} can compile the modified files and relink all
1518 the object files. If you only modify files which are interpreted,
1519 like those in the @file{scm/} and @file{ly/} directories, then
1520 @command{make} is not needed before @command{make test-redo}.
1522 Also, if you modify any font definitions in the @file{mf/}
1523 directory then you must run @command{make clean} and
1524 @command{make} before running @command{make test-redo}. This will
1525 recompile everything, whether modified or not, and takes a lot
1528 Running @command{make@tie{}check} will leave an HTML page
1529 @file{out/test-results/index.html}. This page shows all the
1530 important differences that your change introduced, whether in the
1531 layout, MIDI, performance or error reporting.
1533 You only need to use @command{make test-clean} to start from
1534 scratch, prior to running @command{make@tie{}test-baseline}. To
1535 check new modifications, all that is needed is to repeat
1536 @command{make@tie{}test-redo} and @command{make@tie{}test-check}
1537 (not forgetting @command{make} if needed).
1542 @node Post patch for comments
1543 @subsection Post patch for comments
1545 See @ref{Uploading a patch for review}.
1549 @subsection Push patch
1551 Once all the comments have been addressed, the patch can be pushed.
1553 If the author has push privileges, the author will push the patch.
1554 Otherwise, a developer with push privileges will push the patch.
1557 @node Closing the issues
1558 @subsection Closing the issues
1560 Once the patch has been pushed, all the relevant issues should be
1563 On Rietveld, the author should log in and close the issue either by
1564 using the @q{Edit Issue} link, or by clicking the circled x icon
1565 to the left of the issue name.
1567 If the changes were in response to a feature request on the Google
1568 issue tracker for LilyPond, the author should change the status to
1569 Fixed and a tag @q{fixed_x_y_z} should be added, where the patch was
1570 fixed in version x.y.z. If
1571 the author does not have privileges to change the status, an email
1572 should be sent to bug-lilypond requesting the BugMeister to change
1576 @node Iterator tutorial
1577 @section Iterator tutorial
1579 TODO -- this is a placeholder for a tutorial on iterators
1581 Iterators are routines written in C++ that process music expressions
1582 and sent the music events to the appropriate engravers and/or
1585 See a short example discussing iterators and their duties in
1586 @ref{Articulations on EventChord}.
1589 @node Engraver tutorial
1590 @section Engraver tutorial
1592 Engravers are C++ classes that catch music events and
1593 create the appropriate grobs for display on the page. Though the
1594 majority of engravers are responsible for the creation of a single grob,
1595 in some cases (e.g. @code{New_fingering_engraver}), several different grobs
1598 Engravers listen for events and acknowledge grobs. Events are passed to
1599 the engraver in time-step order during the iteration phase. Grobs are
1600 made available to the engraver when they are created by other engravers
1601 during the iteration phase.
1605 * Useful methods for information processing::
1606 * Translation process::
1607 * Preventing garbage collection for SCM member variables::
1608 * Listening to music events::
1609 * Acknowledging grobs::
1610 * Engraver declaration/documentation::
1613 @node Useful methods for information processing
1614 @subsection Useful methods for information processing
1616 An engraver inherits the following public methods from the Translator
1617 base class, which can be used to process listened events and acknowledged
1621 @item @code{virtual void initialize ()}
1622 @item @code{void start_translation_timestep ()}
1623 @item @code{void process_music ()}
1624 @item @code{void process_acknowledged ()}
1625 @item @code{void stop_translation_timestep ()}
1626 @item @code{virtual void finalize ()}
1629 These methods are listed in order of translation time, with
1630 @code{initialize ()} and @code{finalize ()} bookending the whole
1631 process. @code{initialize ()} can be used for one-time initialization
1632 of context properties before translation starts, whereas
1633 @code{finalize ()} is often used to tie up loose ends at the end of
1634 translation: for example, an unterminated spanner might be completed
1635 automatically or reported with a warning message.
1638 @node Translation process
1639 @subsection Translation process
1641 At each timestep in the music, translation proceeds by calling the
1642 following methods in turn:
1644 @code{start_translation_timestep ()} is called before any user
1645 information enters the translators, i.e., no property operations
1646 (\set, \override, etc.) or events have been processed yet.
1648 @code{process_music ()} and @code{process_acknowledged ()} are called
1649 after all events in the current time step have been heard, or all
1650 grobs in the current time step have been acknowledged. The latter
1651 tends to be used exclusively with engravers which only acknowledge
1652 grobs, whereas the former is the default method for main processing
1655 @code{stop_translation_timestep ()} is called after all user
1656 information has been processed prior to beginning the translation for
1660 @node Preventing garbage collection for SCM member variables
1661 @subsection Preventing garbage collection for SCM member variables
1663 In certain cases, an engraver might need to ensure private Scheme
1664 variables (with type SCM) do not get swept away by Guile's garbage
1665 collector: for example, a cache of the previous key signature which
1666 must persist between timesteps. The method
1667 @code{virtual derived_mark () const} can be used in such cases:
1670 Engraver_name::derived_mark ()
1672 scm_gc_mark (private_scm_member_)
1677 @node Listening to music events
1678 @subsection Listening to music events
1680 External interfaces to the engraver are implemented by protected
1681 macros including one or more of the following:
1684 @item @code{DECLARE_TRANSLATOR_LISTENER (event_name)}
1685 @item @code{IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)}
1689 where @var{event_name} is the type of event required to provide the
1690 input the engraver needs and @var{Engraver_name} is the name of the
1693 Following declaration of a listener, the method is implemented as follows:
1696 IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)
1698 Engraver_name::listen_event_name (Stream event *event)
1700 ...body of listener method...
1705 @node Acknowledging grobs
1706 @subsection Acknowledging grobs
1708 Some engravers also need information from grobs as they are created
1709 and as they terminate. The mechanism and methods to obtain this
1710 information are set up by the macros:
1713 @item @code{DECLARE_ACKNOWLEDGER (grob_interface)}
1714 @item @code{DECLARE_END_ACKNOWLEDGER (grob_interface)}
1717 where @var{grob_interface} is an interface supported by the
1718 grob(s) which should be acknowledged. For example, the following
1719 code would declare acknowledgers for a @code{NoteHead} grob (via the
1720 @code{note-head-interface}) and any grobs which support the
1721 @code{side-position-interface}:
1724 @code{DECLARE_ACKNOWLEDGER (note_head)}
1725 @code{DECLARE_ACKNOWLEDGER (side_position)}
1728 The @code{DECLARE_END_ACKNOWLEDGER ()} macro sets up a spanner-specific
1729 acknowledger which will be called whenever a spanner ends.
1731 Following declaration of an acknowledger, the method is coded as follows:
1735 Engraver_name::acknowledge_interface_name (Grob_info info)
1737 ...body of acknowledger method...
1741 Acknowledge functions are called in the order engravers are
1742 @code{\consist}-ed (the only exception is if you set
1743 @code{must-be-last} to @code{#t}).
1745 If useful things are to be done to the acknowledged grobs, this
1746 should be deferred until all the acknowledging has finished, i.e.,
1747 store the acknowledged grobs and process the information in a
1748 @code{process-acknowledged ()} or @code{stop-translation-timestep ()}
1752 @node Engraver declaration/documentation
1753 @subsection Engraver declaration/documentation
1755 An engraver must have a public macro
1758 @item @code{TRANSLATOR_DECLARATIONS (Engraver_name)}
1762 where @code{Engraver_name} is the name of the engraver. This
1763 defines the common variables and methods used by every engraver.
1765 At the end of the engraver file, one or both of the following
1766 macros are generally called to document the engraver in the
1767 Internals Reference:
1770 @item @code{ADD_ACKNOWLEDGER (Engraver_name, grob_interface)}
1771 @item @code{ADD_TRANSLATOR (Engraver_name, Engraver_doc,
1772 Engraver_creates, Engraver_reads, Engraver_writes)}
1776 where @code{Engraver_name} is the name of the engraver, @code{grob_interface}
1777 is the name of the interface that will be acknowledged,
1778 @code{Engraver_doc} is a docstring for the engraver,
1779 @code{Engraver_creates} is the set of grobs created by the engraver,
1780 @code{Engraver_reads} is the set of properties read by the engraver,
1781 and @code{Engraver_writes} is the set of properties written by
1784 The @code{ADD_ACKNOWLEDGER} and @code{ADD_TRANSLATOR} macros use a
1785 non-standard indentation system. Each interface, grob, read property,
1786 and write property is on its own line, and the closing parenthesis
1787 and semicolon for the macro all occupy a separate line beneath the final
1788 interface or write property. See existing engraver files for more
1792 @node Callback tutorial
1793 @section Callback tutorial
1795 TODO -- This is a placeholder for a tutorial on callback functions.
1798 @node Understanding pure properties
1799 @section Understanding pure properties
1802 * Purity in LilyPond::
1803 * Writing a pure function::
1804 * How purity is defined and stored::
1805 * Where purity is used::
1810 Pure properties are some of the most difficult properties to understand
1811 in LilyPond but, once understood, it is much easier to work with
1812 horizontal spacing. This document provides an overview of what it means
1813 for something to be @q{pure} in LilyPond, what this purity guarantees,
1814 and where pure properties are stored and used. It finishes by
1815 discussing a few case studies for the pure programmer to save you some
1816 time and to prevent you some major headaches.
1819 @node Purity in LilyPond
1820 @subsection Purity in LilyPond
1821 Pure properties in LilyPond that do not have any @q{side effects}.
1822 That is, looking up a pure property should never result in calls to the
1823 following functions:
1825 @item @code{set_property}
1826 @item @code{set_object}
1827 @item @code{suicide}
1829 This means that, if the property is calculated via a callback, this callback
1830 must not only avoid the functions above but make sure that any functions
1831 it calls also avoid the functions above. Also, to date in LilyPond, a pure
1832 function will always return the same value before line breaking (or, more
1833 precisely, before any version of @code{break_into_pieces} is called). This
1834 convention makes it possible to cache pure functions and be more flexible
1835 about the order in which functions are called. For example; Stem #'length has
1836 a pure property that will @emph{never} trigger one of the functions listed
1837 above and will @emph{always} return the same value before line breaking,
1838 independent of where it is called. Sometimes, this will be the actual length
1839 of the Stem. But sometimes it will not. For example; stem that links up
1840 with a beam will need its end set to the Y position of the beam at the stem's
1841 X position. However, the beam's Y positions can only be known after the score
1842 is broken up in to several systems (a beam that has a shallow slope on a
1843 compressed line of music, for example, may have a steeper one on an
1844 uncompressed line). Thus, we only call the impure version of the properties
1845 once we are @emph{absolutely certain} that all of the parameters needed to
1846 calculate their final value have been calculated. The pure version provides a
1847 useful estimate of what this Stem length (or any property) will be, and
1848 the art of creating good pure properties is trying to get the estimation
1849 as close to the actual value as possible.
1851 Of course, like Gregory Peck and Tintin, some Grobs will have properties
1852 that will always be pure. For example, the height of a note-head in
1853 not-crazy music will never depend on line breaking or other parameters
1854 decided late in the typesetting process. Inversely, in rare cases,
1855 certain properties are difficult to estimate with pure values. For
1856 example, the height of a Hairpin at a certain cross-section of its
1857 horizontal span is difficult to know without knowing the horizontal
1858 distance that the hairpin spans, and LilyPond provides an
1859 over-estimation by reporting the pure height as the entire height of the
1862 Purity, like for those living in a convent, is more like a contract than
1863 an @emph{a priori}. If you write a pure-function, you are promising
1864 the user (and the developer who may have to clean up after you) that
1865 your function will not be dependent on factors that change at different
1866 stages of the compilation process (compilation of a score, not of
1869 One last oddity is that purity, in LilyPond, is currently limited
1870 exclusively to things that have to do with Y-extent and positioning.
1871 There is no concept of @q{pure X} as, by design, X is always the
1872 independent variable (i.e. from column X1 to column X2, what will be the
1873 Y height of a given grob). Furthermore, there is no purity for
1874 properties like color, text, and other things for which a meaningful notion
1875 of estimation is either not necessary or has not yet been found. For example,
1876 even if a color were susceptible to change at different points of the
1877 compilation process, it is not clear what a pure estimate of this color
1878 would be or how this pure color could be used. Thus, in this document and
1879 in the source, you will see purity discussed almost interchangeably with
1880 Y-axis positioning issues.
1883 @node Writing a pure function
1884 @subsection Writing a pure function
1885 Pure functions take, at a minimum, three arguments: the @var{grob}, the
1886 starting column at which the function is being evaluated (hereafter
1887 referred to as @var{start}), and the end column at which the grob is
1888 being evaluated (hereafter referred to as @var{end}). For items,
1889 @var{start} and @var{end} must be provided (meaning they are not optional)
1890 but will not have a meaningful impact on the result, as items only occupy
1891 one column and will thus yield a value or not (if they are not in the range
1892 from @var{start} to @var{end}). For spanners however, @var{start} and
1893 @var{end} are important, as we may can get a better pure estimation of a
1894 slice of the spanner than considering it on the whole. This is useful
1895 during line breaking, for example, when we want to estimate the Y-extent
1896 of a spanner broken at given starting and ending columns.
1899 @node How purity is defined and stored
1900 @subsection How purity is defined and stored
1901 Purity is defined in LilyPond with the creation of an unpure-pure container
1902 (unpure is not a word, but hey, neither was Lilypond until the 90s). For example:
1908 #(define (bar grob start end)
1911 \override Stem #'length = #(ly:make-unpure-pure-container foo bar)
1914 Note that items can only ever have two pure heights: their actual pure height
1915 if they are between @q{start} and @q{end}, or an empty interval if they are
1916 not. Thus, their pure property is cached to speed LilyPond up. Pure
1917 heights for spanners are generally not cached as they change depending
1918 on the start and end values. They are only cached in certain particular
1919 cases. Before writing a lot of caching code, make sure that it is a
1920 value that will be reused a lot.
1923 @node Where purity is used
1924 @subsection Where purity is used
1925 Pure Y values must be used in any functions that are called before
1926 line breaking. Examples of this can be seen in
1927 @code{Separation_items::boxes} to construct horizontal skylines and in
1928 @code{Note_spacing::stem_dir_correction} to correct for optical
1929 illusions in spacing. Pure properties are also used in the calculation
1930 of other pure properties. For example, the @code{Axis_group_interface}
1931 has pure functions that look up other pure functions.
1933 Purity is also implicitly used in any functions that should only ever
1934 return pure values. For example, extra-spacing-height is only ever used
1935 before line-breaking and thus should never use values that would only be
1936 available after line breaking. In this case, there is no need to create
1937 callbacks with pure equivalents because these functions, by design, need
1940 To know if a property will be called before and/or after line-breaking
1941 is sometimes tricky and can, like all things in coding, be found by
1942 using a debugger and/or adding @var{printf} statements to see where they
1943 are called in various circumstances.
1947 @subsection Case studies
1948 In each of these case studies, we expose a problem in pure properties, a
1949 solution, and the pros and cons of this solution.
1951 @subheading Time signatures
1952 A time signature needs to prevent accidentals from passing over or under
1953 it, but its extent does not necessarily extend to the Y-position of
1954 accidentals. LilyPond's horizontal spacing sometimes makes a line of
1955 music compact and, when doing so, allows certain columns to pass over
1956 each other if they will not collide. This type of passing over is not
1957 desirable with time signatures in traditional engraving. But how do we
1958 know if this passing over will happen before line breaking, as we are
1959 not sure what the X positions will be? We need a pure estimation of how
1960 much extra spacing height the time signatures would need to prevent this
1961 form of passing over without making this height so large as to
1962 overly-distort the Y-extent of an system, which could result in a very
1963 @q{loose} looking score with lots of horizontal space between columns.
1964 So, to approximate this extra spacing height, we use the Y-extent of a
1965 time signature's next-door-neighbor grobs via the pure-from-neighbor
1969 @item pros: By extending the extra spacing height of a time signature to
1970 that of its next-door-neighbors, we make sure that grobs to the right of
1971 it that could pass above or below it do not.
1973 @item cons: This over-estimation of the vertical height could prevent
1974 snug vertical spacing of systems, as the system will be registered as
1975 being taller at the point of the time signature than it actually is.
1976 This approach can be used for clefs and bar lines as well.
1980 As described above, Stems need pure height approximations when they are
1981 beamed, as we do not know the beam positions before line breaking. To
1982 estimate this pure height, we take all the stems in a beam and find
1983 their pure heights as if they were not beamed. Then, we find the union
1984 of all these pure heights and take the intersection between this
1985 interval (which is large) and an interval going from the note-head of a
1986 stem to infinity in the direction of the stem so that the interval stops
1990 @item pros: This is guaranteed to be at least as long as the beamed
1991 stem, as a beamed stem will never go over the ideal length of the
1992 extremal beam of a stem.
1994 @item cons: Certain stems will be estimated as being too long, which
1995 leads to the same problem of too-much-vertical-height as described
2001 @node Debugging tips
2002 @subsection Debugging tips
2003 A few questions to ask yourself when working with pure properties:
2006 @item Is the property really pure? Are you sure that its value could
2007 not be changed later in the compiling process due to other changes?
2009 @item Can the property be made to correspond even more exactly with the
2010 eventual impure property?
2012 @item For a spanner, is the pure property changing correctly depending
2013 on the starting and ending points of the spanner?
2015 @item For an Item, will the item's pure height need to act in horizontal
2016 spacing but not in vertical spacing? If so, use extra-spacing-height
2017 instead of pure height.
2022 @node LilyPond scoping
2023 @section LilyPond scoping
2025 The Lilypond language has a concept of scoping, i.e. you can do:
2031 (display (+ foo 2)))
2034 @noindent with @code{\paper}, @code{\midi} and @code{\header} being
2035 nested scope inside the @file{.ly} file-level scope. @w{@code{foo = 1}}
2036 is translated in to a scheme variable definition.
2038 This implemented using modules, with each scope being an anonymous
2039 module that imports its enclosing scope's module.
2041 Lilypond's core, loaded from @file{.scm} files, is usually placed in the
2042 @code{lily} module, outside the @file{.ly} level. In the case of
2049 we want to reuse the built-in definitions, without changes effected in
2050 user-level @file{a.ly} leaking into the processing of @file{b.ly}.
2052 The user-accessible definition commands have to take care to avoid
2053 memory leaks that could occur when running multiple files. All
2054 information belonging to user-defined commands and markups is stored in
2055 a manner that allows it to be garbage-collected when the module is
2056 dispersed, either by being stored module-locally, or in weak hash
2060 @node Scheme->C interface
2061 @section Scheme->C interface
2063 Most of the C functions interfacing with Guile/Scheme used in LilyPond
2064 are described in the API Reference of the
2065 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html,
2066 GUILE Reference Manual}.
2068 The remaining functions are defined in @file{lily/lily-guile.cc},
2069 @file{lily/include/lily-guile.hh} and
2070 @file{lily/include/lily-guile-macros.hh}.
2071 Although their names are meaningful there's a few things you should know
2080 @subsection Comparison
2082 This is the trickiest part of the interface.
2084 Mixing Scheme values with C comparison operators won't produce any crash
2085 or warning when compiling but must be avoided:
2088 scm_string_p (scm_value) == SCM_BOOL_T
2091 As we can read in the reference, @code{scm_string_p} returns a Scheme
2092 value: either @code{#t} or @code{#f} which are written @code{SCM_BOOL_T}
2093 and @code{SCM_BOOL_F} in C. This will work, but it is not following
2094 to the API guidelines. For further information, read this discussion:
2097 @uref{http://lists.gnu.org/archive/html/lilypond-devel/2011-08/msg00646.html}
2100 There are functions in the Guile reference that returns C values
2101 instead of Scheme values. In our example, a function called
2102 @code{scm_is_string} (described after @code{string?} and @code{scm_string_p})
2103 returns the C value 0 or 1.
2105 So the best solution was simply:
2108 scm_is_string (scm_value)
2111 There a simple solution for almost every common comparison. Another example:
2112 we want to know if a Scheme value is a non-empty list. Instead of:
2115 (scm_is_true (scm_list_p (scm_value)) && scm_value != SCM_EOL)
2118 one can usually use:
2121 scm_is_pair (scm_value)
2124 since a list of at least one member is a pair. This test is
2125 cheap; @code{scm_list_p} is actually quite more complex since it makes
2126 sure that its argument is neither a `dotted list' where the last pair
2127 has a non-null @code{cdr}, nor a circular list. There are few
2128 situations where the complexity of those tests make sense.
2130 Unfortunately, there is not a @code{scm_is_[something]} function for
2131 everything. That's one of the reasons why LilyPond has its own Scheme
2132 interface. As a rule of thumb, tests that are cheap enough to be
2133 worth inlining tend to have such a C interface. So there is
2134 @code{scm_is_pair} but not @code{scm_is_list}, and @code{scm_is_eq}
2135 but not @code{scm_is_equal}.
2137 @subheading General definitions
2139 @subsubheading bool to_boolean (SCM b)
2141 Return @code{true} if @var{b} is @code{SCM_BOOL_T}, else return @code{false}.
2143 This should be used instead of @code{scm_is_true} and
2144 @code{scm_is_false} for properties since in Lilypond, unset properties
2145 are read as an empty list, and by convention unset Boolean properties
2146 default to false. Since both @code{scm_is_true} and
2147 @code{scm_is_false} only compare with @code{##f} in line with what
2148 Scheme's conditionals do, they are not really useful for checking the
2149 state of a Boolean property.
2151 @subsubheading bool ly_is_[something] (args)
2153 Behave the same as scm_is_[something] would do if it existed.
2155 @subsubheading bool is_[type] (SCM s)
2157 Test whether the type of @var{s} is [type].
2158 [type] is a LilyPond-only set of values (direction, axis...). More
2159 often than not, the code checks Lilypond specific C++-implemented
2162 @subsubheading [Type *] unsmob<Type> (SCM s)
2164 This tries converting a Scheme object to a pointer of the desired
2165 kind. If the Scheme object is of the wrong type, a pointer value
2166 of@w{ }@code{0} is returned, making this suitable for a Boolean test.
2169 @subsection Conversion
2171 @subheading General definitions
2173 @subsubheading bool to_boolean (SCM b)
2175 Return @code{true} if @var{b} is @code{SCM_BOOL_T}, else return @code{false}.
2177 This should be used instead of @code{scm_is_true} and @code{scm_is_false}
2178 for properties since empty lists are sometimes used to unset them.
2180 @subsubheading [C type] ly_scm2[C type] (SCM s)
2182 Behave the same as scm_to_[C type] would do if it existed.
2184 @subsubheading [C type] robust_scm2[C type] (SCM s, [C type] d)
2186 Behave the same as scm_to_[C type] would do if it existed.
2187 Return @var{d} if type verification fails.
2190 @node LilyPond miscellany
2191 @section LilyPond miscellany
2193 This is a place to dump information that may be of use to developers
2194 but doesn't yet have a proper home. Ideally, the length of this section
2195 would become zero as items are moved to other homes.
2199 * Spacing algorithms::
2200 * Info from Han-Wen email::
2201 * Music functions and GUILE debugging::
2202 * Articulations on EventChord::
2205 @node Spacing algorithms
2206 @subsection Spacing algorithms
2208 Here is information from an email exchange about spacing algorithms.
2210 On Thu, 2010-02-04 at 15:33 -0500, Boris Shingarov wrote:
2211 I am experimenting with some modifications to the line breaking code,
2212 and I am stuck trying to understand how some of it works. So far my
2213 understanding is that Simple_spacer operates on a vector of Grobs, and
2214 it is a well-known Constrained-QP problem (rods = constraints, springs
2215 = quadratic function to minimize). What I don't understand is, if the
2216 spacer operates at the level of Grobs, which are built at an earlier
2217 stage in the pipeline, how are the changes necessitated by differences
2218 in line breaking, taken into account? in other words, if I take the
2219 last measure of a line and place it on the next line, it is not just a
2220 matter of literally moving that graphic to where the start of the next
2221 line is, but I also need to draw a clef, key signature, and possibly
2222 other fundamental things -- but at that stage in the rendering
2223 pipeline, is it not too late??
2225 Joe Neeman answered:
2227 We create lots of extra grobs (eg. a BarNumber at every bar line) but
2228 most of them are not drawn. See the break-visibility property in
2231 Here is another e-mail exchange. Janek Warchoł asked for a starting point
2232 to fixing 1301 (change clef colliding with notes). Neil Puttock replied:
2234 The clef is on a loose column (it floats before the head), so the
2235 first place I'd look would be lily/spacing-loose-columns.cc (and
2236 possibly lily/spacing-determine-loose-columns.cc).
2237 I'd guess the problem is the way loose columns are spaced between
2238 other columns: in this snippet, the columns for the quaver and tuplet
2239 minim are so close together that the clef's column gets dumped on top
2240 of the quaver (since it's loose, it doesn't influence the spacing).
2242 @node Info from Han-Wen email
2243 @subsection Info from Han-Wen email
2245 In 2004, Douglas Linhardt decided to try starting a document that would
2246 explain LilyPond architecture and design principles. The material below
2247 is extracted from that email, which can be found at
2248 @uref{http://thread.gmane.org/gmane.comp.gnu.lilypond.devel/2992}.
2249 The headings reflect questions from Doug or comments from Han-Wen;
2250 the body text are Han-Wen's answers.
2252 @subheading Figuring out how things work.
2254 I must admit that when I want to know how a program works, I use grep
2255 and emacs and dive into the source code. The comments and the code
2256 itself are usually more revealing than technical documents.
2258 @subheading What's a grob, and how is one used?
2260 Graphical object - they are created from within engravers, either as
2261 Spanners (derived class) -slurs, beams- or Items (also a derived
2262 class) -notes, clefs, etc.
2264 There are two other derived classes System (derived from Spanner,
2265 containing a "line of music") and Paper_column (derived from Item, it
2266 contains all items that happen at the same moment). They are separate
2267 classes because they play a special role in the linebreaking process.
2269 @subheading What's a smob, and how is one used?
2271 A C(++) object that is encapsulated so it can be used as a Scheme
2272 object. See GUILE info, "19.3 Defining New Types (Smobs)"
2274 @subheading When is each C++ class constructed and used?
2281 In the parser.yy see the macro calls MAKE_MUSIC_BY_NAME().
2286 Constructed during "interpreting" phase.
2291 Executive branch of Contexts, plugins that create grobs, usually one
2292 engraver per grob type. Created together with context.
2302 These are not C++ classes per se. The idea of a Grob interface hasn't
2303 crystallized well. ATM, an interface is a symbol, with a bunch of grob
2304 properties. They are not objects that are created or destroyed.
2309 Objects that walk through different music classes, and deliver events
2310 in a synchronized way, so that notes that play together are processed
2311 at the same moment and (as a result) end up on the same horizontal position.
2313 Created during interpreting phase.
2315 BTW, the entry point for interpreting is ly:run-translator
2316 (ly_run_translator on the C++ side)
2320 @subheading Can you get to Context properties from a Music object?
2322 You can create music object with a Scheme function that reads context
2323 properties (the \applycontext syntax). However, that function is
2324 executed during Interpreting, so you can not really get Context
2325 properties from Music objects, since music objects are not directly
2326 connected to Contexts. That connection is made by the Music_iterators
2328 @subheading Can you get to Music properties from a Context object?
2330 Yes, if you are given the music object within a Context
2331 object. Normally, the music objects enter Contexts in synchronized
2332 fashion, and the synchronization is done by Music_iterators.
2334 @subheading What is the relationship between C++ classes and Scheme objects?
2336 Smobs are C++ objects in Scheme. Scheme objects (lists, functions) are
2337 manipulated from C++ as well using the GUILE C function interface
2340 @subheading How do Scheme procedures get called from C++ functions?
2342 scm_call_*, where * is an integer from 0 to 4.
2343 Also scm_c_eval_string (), scm_eval ()
2345 @subheading How do C++ functions get called from Scheme procedures?
2347 Export a C++ function to Scheme with LY_DEFINE.
2349 @subheading What is the flow of control in the program?
2351 Good question. Things used to be clear-cut, but we have Scheme
2352 and SMOBs now, which means that interactions do not follow a very
2353 rigid format anymore. See below for an overview, though.
2355 @subheading Does the parser make Scheme procedure calls or C++ function calls?
2357 Both. And the Scheme calls can call C++ and vice versa. It's nested,
2358 with the SCM datatype as lubrication between the interactions
2360 (I think the word "lubrication" describes the process better than the
2361 traditional word "glue")
2363 @subheading How do the front-end and back-end get started?
2365 Front-end: a file is parsed, the rest follows from that. Specifically,
2367 Parsing leads to a Music + Music_output_def object (see parser.yy,
2368 definition of toplevel_expression )
2370 A Music + Music_output_def object leads to a Global_context object (see
2371 ly_run_translator ())
2373 During interpreting, Global_context + Music leads to a bunch of
2374 Contexts (see Global_translator::run_iterator_on_me ()).
2376 After interpreting, Global_context contains a Score_context (which
2377 contains staves, lyrics etc.) as a child. Score_context::get_output ()
2378 spews a Music_output object (either a Paper_score object for notation
2379 or Performance object for MIDI).
2381 The Music_output object is the entry point for the backend (see
2382 ly_render_output ()).
2384 The main steps of the backend itself are in
2389 @file{paper-score.cc} , Paper_score::process_
2392 @file{system.cc} , System::get_lines()
2395 The step, where things go from grobs to output, is in
2396 System::get_line(): each grob delivers a Stencil (a Device
2397 independent output description), which is interpreted by our
2398 outputting backends (@file{scm/output-tex.scm} and
2399 @file{scm/output-ps.scm}) to produce TeX and PS.
2403 Interactions between grobs and putting things into .tex and .ps files
2404 have gotten a little more complex lately. Jan has implemented
2405 page-breaking, so now the backend also involves Paper_book,
2406 Paper_lines and other things. This area is still heavily in flux, and
2407 perhaps not something you should want to look at.
2409 @subheading How do the front-end and back-end communicate?
2411 There is no communication from backend to front-end. From front-end to
2412 backend is simply the program flow: music + definitions gives
2413 contexts, contexts yield output, after processing, output is written
2416 @subheading Where is the functionality associated with KEYWORDs?
2418 See @file{my-lily-lexer.cc} (keywords, there aren't that many)
2419 and @file{ly/*.ly} (most of the other backslashed @code{/\words} are identifiers)
2421 @subheading What Contexts/Properties/Music/etc. are available when they are processed?
2423 What do you mean exactly with this question?
2425 See @file{ly/engraver-init.ly} for contexts,
2426 see @file{scm/define-*.scm} for other objects.
2428 @subheading How do you decide if something is a Music, Context, or Grob property?
2429 Why is part-combine-status a Music property when it seems (IMO)
2430 to be related to the Staff context?
2432 The Music_iterators and Context communicate through two channels
2434 Music_iterators can set and read context properties, idem for
2435 Engravers and Contexts
2437 Music_iterators can send "synthetic" music events (which aren't in
2438 the input) to a context. These are caught by Engravers. This is
2439 mostly a one way communication channel.
2441 part-combine-status is part of such a synthetic event, used by
2442 Part_combine_iterator to communicate with Part_combine_engraver.
2445 @subheading Deciding between context and music properties
2447 I'm adding a property to affect how \autochange works. It seems to
2448 me that it should be a context property, but the Scheme autochange
2449 procedure has a Music argument. Does this mean I should use
2452 \autochange is one of these extra strange beasts: it requires
2453 look-ahead to decide when to change staves. This is achieved by
2454 running the interpreting step twice (see
2455 @file{scm/part-combiner.scm} , at the bottom), and
2456 storing the result of the first step (where to switch
2457 staves) in a Music property. Since you want to influence that
2458 where-to-switch list, your must affect the code in
2459 make-autochange-music (@file{scm/part-combiner.scm}).
2460 That code is called directly from the parser and there are no
2461 official "parsing properties" yet, so there is no generic way
2462 to tune \autochange. We would have to invent something new
2463 for this, or add a separate argument,
2466 \autochange #around-central-C ..music..
2470 where around-central-C is some function that is called from
2471 make-autochange-music.
2473 @subheading More on context and music properties
2475 From Neil Puttock, in response to a question about transposition:
2477 Context properties (using \set & \unset) are tied to engravers: they
2478 provide information relevant to the generation of graphical objects.
2480 Since transposition occurs at the music interpretation stage, it has
2481 no direct connection with engravers: the pitch of a note is fixed
2482 before a notehead is created. Consider the following minimal snippet:
2488 This generates (simplified) a NoteEvent, with its pitch and duration
2489 as event properties,
2495 (ly:make-duration 2 0 1 1)
2497 (ly:make-pitch 0 0 0)
2500 which the Note_heads_engraver hears. It passes this information on to
2501 the NoteHead grob it creates from the event, so the head's correct
2502 position and duration-log can be determined once it's ready for
2505 If we transpose the snippet,
2508 \transpose c d @{ c' @}
2511 the pitch is changed before it reaches the engraver (in fact, it
2512 happens just after the parsing stage with the creation of a
2513 TransposedMusic music object):
2519 (ly:make-duration 2 0 1 1)
2521 (ly:make-pitch 0 1 0)
2524 You can see an example of a music property relevant to transposition:
2528 \transpose c d @{ c'2 \withMusicProperty #'untransposable ##t c' @}
2531 -> the second c' remains untransposed.
2533 Take a look at @file{lily/music.cc} to see where the transposition takes place.
2536 @subheading How do I tell about the execution environment?
2538 I get lost figuring out what environment the code I'm looking at is in when it
2539 executes. I found both the C++ and Scheme autochange code. Then I was trying
2540 to figure out where the code got called from. I finally figured out that the
2541 Scheme procedure was called before the C++ iterator code, but it took me a
2542 while to figure that out, and I still didn't know who did the calling in the
2543 first place. I only know a little bit about Flex and Bison, so reading those
2544 files helped only a little bit.
2546 @emph{Han-Wen:} GDB can be of help here. Set a breakpoint in C++, and run. When you
2547 hit the breakpoint, do a backtrace. You can inspect Scheme objects
2548 along the way by doing
2551 p ly_display_scm(obj)
2554 this will display OBJ through GUILE.
2556 @node Music functions and GUILE debugging
2557 @subsection Music functions and GUILE debugging
2559 Ian Hulin was trying to do some debugging in music functions, and
2560 came up with the following question (edited and adapted to current
2564 I'm working on the Guile Debugger Stuff, and would like to try
2565 debugging a music function definition such as:
2569 #(define-music-function () ()
2570 #@{ \tag instrumental-part @{\mark \default@} #@} )
2573 It appears @code{conditionalMark} does not get set up as an
2574 equivalent of a Scheme
2577 (define conditionalMark = define-music-function () () ...
2581 although something gets defined because Scheme apparently recognizes
2584 #(set-break! conditionalMark)
2588 later on in the file without signalling any Guile errors.
2590 However the breakpoint trap is never encountered as
2591 @code{define-music-function} passed things on to
2592 @code{ly:make-music-function}, which is really C++ code
2593 @code{ly_make_music_function}, so Guile never finds out about the
2597 The answer in the mailing list archive at that time was less than
2598 helpful. The question already misidentifies the purpose of
2599 @code{ly:make-music-function} which is only called once at the
2600 time of @emph{defining} @code{conditionalMark} but is not involved
2601 in its later @emph{execution}.
2603 Here is the real deal:
2605 A music function is not the same as a GUILE function. It boxes
2606 both a proper Scheme function (with argument list and body from
2607 the @code{define-music-function} definition) along with a call
2608 signature representing the @emph{types} of both function and
2611 Those components can be reextracted using
2612 @code{ly:music-function-extract} and
2613 @code{ly:music-function-signature}, respectively.
2615 When LilyPond's parser encounters a music function call in its
2616 input, it reads, interprets, and verifies the arguments
2617 individually according to the call signature and @emph{then} calls
2618 the proper Scheme function.
2620 While it is actually possible these days to call a music function
2621 @emph{as if} it were a Scheme function itself, this pseudo-call
2622 uses its own wrapping code matching the argument list @emph{as a
2623 whole} to the call signature, substituting omitted optional
2624 arguments with defaults and verifying the result type.
2626 So putting a breakpoint on the music function itself will still
2627 not help with debugging uses of the function using LilyPond
2630 However, either calling mechanism ultimately calls the proper
2631 Scheme function stored as part of the music function, and that is
2632 where the breakpoint belongs:
2635 #(set-break! (ly:music-function-extract conditionalMark))
2638 will work for either calling mechanism.
2640 @node Articulations on EventChord
2641 @subsection Articulations on EventChord
2643 From David Kastrup's email
2644 @uref{http://lists.gnu.org/archive/html/lilypond-devel/2012-02/msg00189.html}:
2646 LilyPond's typesetting does not act on music expressions and music
2647 events. It acts exclusively on stream events. It is the act of
2648 iterators to convert a music expression into a sequence of stream events
2649 played in time order.
2651 The EventChord iterator is pretty simple: it just takes its "elements"
2652 field when its time comes up, turns every member into a StreamEvent and
2653 plays that through the typesetting process. The parser currently
2654 appends all postevents belonging to a chord at the end of "elements",
2655 and thus they get played at the same point of time as the elements of
2656 the chord. Due to this design, you can add per-chord articulations or
2657 postevents or even assemble chords with a common stem by using parallel
2658 music providing additional notes/events: the typesetter does not see a
2659 chord structure or postevents belonging to a chord, it just sees a
2660 number of events occuring at the same point of time in a Voice context.
2662 So all one needs to do is let the EventChord iterator play articulations
2663 after elements, and then adding to articulations in EventChord is
2664 equivalent to adding them to elements (except in cases where the order