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 * Debugging LilyPond::
12 * Tracing object relationships::
13 * Adding or modifying features::
18 * LilyPond miscellany::
21 @node Overview of LilyPond architecture
22 @section Overview of LilyPond architecture
24 LilyPond processes the input file into graphical and musical output in a
25 number of stages. This process, along with the types of routines that
26 accomplish the various stages of the process, is described in this section. A
27 more complete description of the LilyPond architecture and internal program
28 execution is found in Erik Sandberg's
29 @uref{http://lilypond.org/web/images/thesis-erik-sandberg.pdf, master's
32 The first stage of LilyPond processing is @emph{parsing}. In the parsing
33 process, music expressions in LilyPond input format are converted to music
34 expressions in Scheme format. In Scheme format, a music expression is a list
35 in tree form, with nodes that indicate the relationships between various music
36 events. The LilyPond parser is written in Bison.
38 The second stage of LilyPond processing is @emph{iterating}. Iterating
39 assigns each music event to a context, which is the environment in which the
40 music will be finally engraved. The context is responsible for all further
41 processing of the music. It is during the iteration stage that contexts are
42 created as necessary to ensure that every note has a Voice type context (e.g.
43 Voice, TabVoice, DrumVoice, CueVoice, MensuralVoice, VaticanaVoice,
44 GregorianTranscriptionVoice), that the Voice type contexts exist in
45 appropriate Staff type contexts, and that parallel Staff type contexts exist
46 in StaffGroup type contexts. In addition, during the iteration stage each
47 music event is assigned a moment, or a time in the music when the event
50 Each type of music event has an associated iterator. Iterators are defined in
51 *-iterator.cc. During iteration, an
52 event's iterator is called to deliver that music event to the appropriate
55 The final stage of LilyPond processing is @emph{translation}. During
56 translation, music events are prepared for graphical or midi output. The
57 translation step is accomplished by the polymorphic base class Translator
58 through its two derived classes: Engraver (for graphical output) and
59 Performer (for midi output).
61 Translators are defined in C++ files named *-engraver.cc and *-performer.cc.
62 Much of the work of translating is handled by Scheme functions,
63 which is one of the keys to LilyPond's exceptional flexibility.
65 @sourceimage{architecture-diagram,,,png}
68 @node LilyPond programming languages
69 @section LilyPond programming languages
71 Programming in LilyPond is done in a variety of programming languages. Each
72 language is used for a specific purpose or purposes. This section describes
73 the languages used and provides links to reference manuals and tutorials for
74 the relevant language.
78 The core functionality of LilyPond is implemented in C++.
80 C++ is so ubiquitous that it is difficult to identify either a reference
81 manual or a tutorial. Programmers unfamiliar with C++ will need to spend some
82 time to learn the language before attempting to modify the C++ code.
84 The C++ code calls Scheme/GUILE through the GUILE interface, which is
86 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html, GUILE
91 The LilyPond lexer is implemented in Flex, an implementation of the Unix lex
92 lexical analyser generator. Resources for Flex can be found
93 @uref{http://flex.sourceforge.net/, here}.
97 The LilyPond parser is implemented in Bison, a GNU parser generator. The
98 Bison homepage is found at @uref{http://www.gnu.org/software/bison/,
99 gnu.org}. The manual (which includes both a reference and tutorial) is
100 @uref{http://www.gnu.org/software/bison/manual/index.html, available} in a
105 GNU Make is used to control the compiling process and to build the
106 documentation and the website. GNU Make documentation is available at
107 @uref{http://www.gnu.org/software/make/manual/, the GNU website}.
109 @subsection GUILE or Scheme
111 GUILE is the dialect of Scheme that is used as LilyPond's extension language.
112 Many extensions to LilyPond are written entirely in GUILE. The
113 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html,
114 GUILE Reference Manual} is available online.
116 @uref{http://mitpress.mit.edu/sicp/full-text/book/book.html, Structure and
117 Interpretation of Computer Programs}, a popular textbook used to teach
118 programming in Scheme is available in its entirety online.
120 An introduction to Guile/Scheme as used in LilyPond can be found in the
121 @rextend{Scheme tutorial}.
125 MetaFont is used to create the music fonts used by LilyPond. A MetaFont
126 tutorial is available at @uref{http://metafont.tutorial.free.fr/, the
127 METAFONT tutorial page}.
129 @subsection PostScript
131 PostScript is used to generate graphical output. A brief PostScript tutorial
132 is @uref{http://local.wasp.uwa.edu.au/~pbourke/dataformats/postscript/,
133 available online}. The
134 @uref{http://www.adobe.com/devnet/postscript/pdfs/PLRM.pdf, PostScript Lanugage
135 Reference} is available online in PDF format.
139 Python is used for XML2ly and is used for buillding the documentation and the
142 Python documentation is available at @uref{http://www.python.org/doc/,
145 @node Programming without compiling
146 @section Programming without compiling
148 Much of the development work in LilyPond takes place by changing *.ly or
149 *.scm files. These changes can be made without compiling LilyPond. Such
150 changes are described in this section.
153 @subsection Modifying distribution files
155 Much of LilyPond is written in Scheme or LilyPond input files. These
156 files are interpreted when the program is run, rather than being compiled
157 when the program is built, and are present in all LilyPond distributions.
158 You will find .ly files in the ly/ directory and the Scheme files in the
159 scm/ directory. Both Scheme files and .ly files can be modified and
160 saved with any text editor. It's probably wise to make a backup copy of
161 your files before you modify them, although you can reinstall if the
162 files become corrupted.
164 Once you've modified the files, you can test the changes just by running
165 LilyPond on some input file. It's a good idea to create a file that
166 demonstrates the feature you're trying to add. This file will eventually
167 become a regression test and will be part of the LilyPond distribution.
169 @subsection Desired file formatting
171 Files that are part of the LilyPond distribution have Unix-style line
172 endings (LF), rather than DOS (CR+LF) or MacOS 9 and earlier (CR). Make
173 sure you use the necessary tools to ensure that Unix-style line endings are
174 preserved in the patches you create.
176 Tab characters should not be included in files for distribution. All
177 indentation should be done with spaces. Most editors have settings to
178 allow the setting of tab stops and ensuring that no tab characters are
179 included in the file.
181 Scheme files and LilyPond files should be written according to standard
182 style guidelines. Scheme file guidelines can be found at
183 @uref{http://community.schemewiki.org/?scheme-style}. Following these
184 guidelines will make your code easier to read. Both you and others that
185 work on your code will be glad you followed these guidelines.
187 For LilyPond files, you should follow the guidelines for LilyPond snippets
188 in the documentation. You can find these guidelines at
189 @ref{Texinfo introduction and usage policy}.
191 @node Finding functions
192 @section Finding functions
194 When making changes or fixing bugs in LilyPond, one of the initial
195 challenges is finding out where in the code tree the functions to
196 be modified live. With nearly 3000 files in the source tree,
197 trial-and-error searching is generally ineffective. This section
198 describes a process for finding interesting code.
200 @subsection Using the ROADMAP
202 The file ROADMAP is located in the main directory of the lilypond source.
203 ROADMAP lists all of the directories in the LilPond source tree, along
204 with a brief description of the kind of files found in each directory.
205 This can be a very helpful tool for deciding which directories to search
206 when looking for a function.
209 @subsection Using grep to search
211 Having identified a likely subdirectory to search, the grep utility can
212 be used to search for a function name. The format of the grep command is
215 grep -i functionName subdirectory/*
218 This command will search all the contents of the directory subdirectory/
219 and display every line in any of the files that contains
220 functionName. The @code{-i} option makes @command{grep} ignore
221 case -- this can be very useful if you are not yet familiar with
222 our capitalization conventions.
224 The most likely directories to grep for function names are scm/ for
225 scheme files, ly/ for lilypond input (*.ly) files, and lily/ for C++
229 @subsection Using git grep to search
231 If you have used git to obtain the source, you have access to a
232 powerful tool to search for functions. The command:
235 git grep functionName
238 will search through all of the files that are present in the git
239 repository looking for functionName. It also presents the results
240 of the search using @code{less}, so the results are displayed one page
243 @subsection Searching on the git repository at Savannah
245 You can also use the equivalent of git grep on the Savannah server.
250 Go to http://git.sv.gnu.org/gitweb/?p=lilypond.git
253 In the pulldown box that says commit, select grep.
256 Type functionName in the search box, and hit enter/return
260 This will initiate a search of the remote git repository.
271 * Indenting files with fixcc.py::
272 * Indenting files with emacs in script mode::
273 * Indenting with vim::
274 * Naming conventions::
282 @node Handling errors
283 @subsection Handling errors
285 As a general rule, you should always try to continue computations,
286 even if there is some kind of error. When the program stops, it
287 is often very hard for a user to pinpoint what part of the input
288 causes an error. Finding the culprit is much easier if there is
289 some viewable output.
291 So functions and methods do not return errorcodes, they never
292 crash, but report a programming_error and try to carry on.
296 @subsection Languages
298 C++ and Python are preferred. Python code should use PEP 8.
302 @subsection Filenames
304 Definitions of classes that are only accessed via pointers (*) or
305 references (&) shall not be included as include files.
311 ".cc" Implementation files
312 ".icc" Inline definition files
313 ".tcc" non inline Template defs
317 (setq auto-mode-alist
318 (append '(("\\.make$" . makefile-mode)
319 ("\\.cc$" . c++-mode)
320 ("\\.icc$" . c++-mode)
321 ("\\.tcc$" . c++-mode)
322 ("\\.hh$" . c++-mode)
323 ("\\.pod$" . text-mode)
328 The class Class_name is coded in @q{class-name.*}
332 @subsection Indentation
334 Standard GNU coding style is used. In emacs:
337 (add-hook 'c++-mode-hook
338 '(lambda() (c-set-style "gnu")
342 If you like using font-lock, you can also add this to your
346 (setq font-lock-maximum-decoration t)
347 (setq c++-font-lock-keywords-3
349 c++-font-lock-keywords-3
350 '(("\\b\\(a-zA-Z_?+_\\)\\b" 1 font-lock-variable-name-face) ("\\b\\(A-Z?+a-z_?+\\)\\b" 1 font-lock-type-face))
354 Some source files may not currently have proper indenting. If this
355 is the case, it is desirable to fix the improper indenting when the
356 file is modified, with the hope of continually improving the code.
359 @node Indenting files with fixcc.py
360 @subsection Indenting files with fixcc.py
362 LilyPond provides a python script that will correct the indentation
366 scripts/auxiliar/fixcc.py lily/my-test-file.cc
369 Be sure you replace @code{my-test-file.cc} with the name of the file
372 If you are editing a file that contains an ADD_TRANSLATOR or ADD_INTERFACE
373 macro, the fixcc.py script will move the final parenthesis up one line
374 from where it should be. Please check the end of the file before
375 you run fixcc.py, and then put the final parenthesis and semicolon
376 back on a line by themselves.
379 @node Indenting files with emacs in script mode
380 @subsection Indenting files with emacs in script mode
382 @c email to wl@gnu.org when I get here.
384 @warning{this is pending some confirmation on -devel. July 2009 -gp}
386 Command-line script to format stuff with emacs:
390 emacs $1 -batch --eval '(indent-region (point-min) (point-max) nil)' -f save-buffer
393 (that's all on one line)
395 Save it as a shell script, then run on the file(s) you modified.
398 @node Indenting with vim
399 @subsection Indenting with vim
401 Although emacs indentation is the LilyPond standard, acceptable
402 indentation can usually be accomplished with vim. Some hints for
414 filetype plugin indent on
416 set ignorecase smartcase
419 set statusline=%F%m%r%h%w\ %{&ff}\ %Y\ [ASCII=\%03.3b]\ [HEX=\%02.2B]\ %04l,%04v\ %p%%\ [LEN=%L]
422 " Remove trailing whitespace on write
423 autocmd BufWritePre * :%s/\s\+$//e
426 With this .vimrc, files can be reindented automatically by highlihting
427 the lines to be indented in visual mode (use V to enter visual mode)
430 A scheme.vim file will help improve the indentation. This one
431 was suggested by Patrick McCarty. It should be saved in
432 ~/.vim/after/syntax/scheme.vim.
435 " Additional Guile-specific 'forms'
436 syn keyword schemeSyntax define-public define* define-safe-public
437 syn keyword schemeSyntax use-modules define-module
438 syn keyword schemeSyntax defmacro-public define-macro
439 syn keyword schemeSyntax define-markup-command
440 syn keyword schemeSyntax define-markup-list-command
441 syn keyword schemeSyntax let-keywords* lambda* define*-public
442 syn keyword schemeSyntax defmacro* defmacro*-public
444 " All of the above should influence indenting too
445 set lw+=define-public,define*,define-safe-public,use-modules,define-module
446 set lw+=defmacro-public,define-macro
447 set lw+=define-markup-command,define-markup-list-command
448 set lw+=let-keywords*,lambda*,define*-public,defmacro*,defmacro*-public
450 " These forms should not influence indenting
454 " Try to highlight all ly: procedures
455 syn match schemeFunc "ly:[^) ]\+"
459 @node Naming conventions
460 @subsection Naming Conventions
462 @subheading Classes and Types
470 Member variable names end with an underscore:
478 Macro names should be written in uppercase completely.
480 @subheading Variables
482 Variable names should be complete words, rather than abbreviations.
483 For example, it is preferred to use @code{thickness} rather than
484 @code{th} or @code{t}.
486 Multi-word variable names in C++ should have the words separated
487 by the underscore character (@q{_}).
489 Multi-word variable names in Scheme should have the words separated
494 @subsection Broken code
496 Do not write broken code. This includes hardwired dependencies,
497 hardwired constants, slow algorithms and obvious limitations. If
498 you can not avoid it, mark the place clearly, and add a comment
499 explaining shortcomings of the code.
501 We reject broken-in-advance on principle.
505 @subsection Code comments
507 Comments may not be needed if descriptive variable names are used
508 in the code and the logic is straightforward. However, if the
509 logic is difficult to follow, and particularly if non-obvious
510 code has been included to resolve a bug, a comment describing
511 the logic and/or the need for the non-obvious code should be included.
513 There are instances where the current code could be commented better.
514 If significant time is required to understand the code as part of
515 preparing a patch, it would be wise to add comments reflecting your
516 understanding to make future work easier.
522 Messages need to follow Localization.
526 @subsection Localization
528 This document provides some guidelines to help programmers write
530 messages. To help translations, user messages must follow
531 uniform conventions. Follow these rules when coding for LilyPond.
532 Hopefully, this can be replaced by general GNU guidelines in the
533 future. Even better would be to have an English (en_BR, en_AM)
534 guide helping programmers writing consistent messages for all GNU
537 Non-preferred messages are marked with `+'. By convention,
538 ungrammatical examples are marked with `*'. However, such ungrammatical
539 examples may still be preferred.
544 Every message to the user should be localized (and thus be marked
545 for localization). This includes warning and error messages.
548 Do not localize/gettextify:
552 `programming_error ()'s
555 `programming_warning ()'s
561 output strings (PostScript, TeX, etc.)
566 Messages to be localized must be encapsulated in `_ (STRING)' or
567 `_f (FORMAT, ...)'. E.g.:
570 warning (_ ("need music in a score"));
571 error (_f ("cannot open file: `%s'", file_name));
574 In some rare cases you may need to call `gettext ()' by hand. This
575 happens when you pre-define (a list of) string constants for later
576 use. In that case, you'll probably also need to mark these string
577 constants for translation, using `_i (STRING)'. The `_i' macro is
578 a no-op, it only serves as a marker for `xgettext'.
581 char const* messages[] = @{
582 _i ("enable debugging output"),
583 _i ("ignore lilypond version"),
590 puts (gettext (messages i));
594 See also `flower/getopt-long.cc' and `lily/main.cc'.
597 Do not use leading or trailing whitespace in messages. If you need
598 whitespace to be printed, prepend or append it to the translated
602 message ("Calculating line breaks..." + " ");
606 Error or warning messages displayed with a file name and line
607 number never start with a capital, eg,
610 foo.ly: 12: not a duration: 3
613 Messages containing a final verb, or a gerund (`-ing'-form) always
614 start with a capital. Other (simpler) messages start with a
620 Not declaring: `foo'.
624 Avoid abbreviations or short forms, use `cannot' and `do not'
625 rather than `can't' or `don't'
626 To avoid having a number of different messages for the same
627 situation, well will use quoting like this `"message: `%s'"' for all
628 strings. Numbers are not quoted:
631 _f ("cannot open file: `%s'", name_str)
632 _f ("cannot find character number: %d", i)
636 Think about translation issues. In a lot of cases, it is better to
637 translate a whole message. The english grammar must not be imposed
638 on the translator. So, instead of
641 stem at + moment.str () + does not fit in beam
647 _f ("stem at %s does not fit in beam", moment.str ())
651 Split up multi-sentence messages, whenever possible. Instead of
654 warning (_f ("out of tune! Can't find: `%s'", "Key_engraver"));
655 warning (_f ("cannot find font `%s', loading default", font_name));
661 warning (_ ("out of tune:"));
662 warning (_f ("cannot find: `%s', "Key_engraver"));
663 warning (_f ("cannot find font: `%s', font_name));
664 warning (_f ("Loading default font"));
668 If you must have multiple-sentence messages, use full punctuation.
669 Use two spaces after end of sentence punctuation. No punctuation
670 (esp. period) is used at the end of simple messages.
673 _f ("Non-matching braces in text `%s', adding braces", text)
674 _ ("Debug output disabled. Compiled with NPRINT.")
675 _f ("Huh? Not a Request: `%s'. Ignoring.", request)
679 Do not modularize too much; words frequently cannot be translated
680 without context. It is probably safe to treat most occurences of
681 words like stem, beam, crescendo as separately translatable words.
684 When translating, it is preferable to put interesting information
685 at the end of the message, rather than embedded in the middle.
686 This especially applies to frequently used messages, even if this
687 would mean sacrificing a bit of eloquency. This holds for original
688 messages too, of course.
691 en: cannot open: `foo.ly'
692 + nl: kan `foo.ly' niet openen (1)
693 kan niet openen: `foo.ly'* (2)
694 niet te openen: `foo.ly'* (3)
698 The first nl message, although grammatically and stylistically
699 correct, is not friendly for parsing by humans (even if they speak
700 dutch). I guess we would prefer something like (2) or (3).
703 Do not run make po/po-update with GNU gettext < 0.10.35
709 @node Debugging LilyPond
710 @section Debugging LilyPond
712 The most commonly used tool for debugging LilyPond is the GNU
713 debugger gdb. The gdb tool is used for investigating and debugging
714 core Lilypond code written in C++. Another tool is available for
715 debugging Scheme code using the Guile debugger. This section
716 describes how to use both gdb and the Guile Debugger.
719 * Debugging overview::
720 * Debugging C++ code::
721 * Debugging Scheme code::
724 @node Debugging overview
725 @subsection Debugging overview
727 Using a debugger simplifies troubleshooting in at least two ways.
729 First, breakpoints can be set to pause execution at any desired point.
730 Then, when execution has paused, debugger commands can be issued to
731 explore the values of various variables or to execute functions.
733 Second, the debugger can display a stack trace, which shows the
734 sequence in which functions have been called and the arguments
735 passed to the called functions.
737 @node Debugging C++ code
738 @subsection Debugging C++ code
740 The GNU debugger, gdb, is the principal tool for debugging C++ code.
742 @unnumberedsubsubsec Compiling LilyPond for use with gdb
744 In order to use gdb with LilyPond, it is necessary to compile
745 LilyPond with debugging information. This is accomplished by running
746 the following commands in the main LilyPond source directory.
749 ./configure --disable-optimising
753 This will create a version of LilyPond containing debugging
754 information that will allow the debugger to tie the source code
755 to the compiled code.
757 You should not do @var{make install} if you want to use a debugger
758 with LilyPond. The @var{make install} command will strip debugging
759 information from the LilyPond binary.
761 @unnumberedsubsubsec Typical gdb usage
763 Once you have compiled the Lilypond image with the necessary
764 debugging information it will have been written to a location in a
765 subfolder of your current working directory:
771 This is important as you will need to let gdb know where to find the
772 image containing the symbol tables. You can invoke gdb from the
779 This loads the LilyPond symbol tables into gdb. Then, to run
780 LilyPond on @code{test.ly} under the debugger, enter
788 As an alternative to running gdb at the command line you may try
789 a graphical interface to gdb such as ddd
795 You can also use sets of standard gdb commands stored in a .gdbinit
796 file (see next section).
798 @unnumberedsubsubsec Typical .gdbinit files
800 The behavior of gdb can be readily customized through the use of a
801 @var{.gdbinit} file. A @var{.gdbinit} file is a file named
802 @var{.gdbinit} (notice the @qq{.} at the beginning of the file name)
803 that is placed in a user's home directory.
805 The @var{.gdbinit} file below is from Han-Wen. It sets breakpoints
806 for all errors and defines functions for displaying scheme objects
807 (ps), grobs (pgrob), and parsed music expressions (pmusic).
810 file lily/out/lilypond
812 b Grob::programming_error
815 print ly_display_scm($arg0)
818 print ly_display_scm($arg0->self_scm_)
819 print ly_display_scm($arg0->mutable_property_alist_)
820 print ly_display_scm($arg0->immutable_property_alist_)
821 print ly_display_scm($arg0->object_alist_)
824 print ly_display_scm($arg0->self_scm_)
825 print ly_display_scm($arg0->mutable_property_alist_)
826 print ly_display_scm($arg0->immutable_property_alist_)
830 @node Debugging Scheme code
831 @subsection Debugging Scheme code
833 Scheme code can be developed using the Guile command line
834 interpreter @code{top-repl}. You can either investigate
835 interactively using just Guile or you can use the debugging
836 tools available within Guile.
838 @unnumberedsubsubsec Using Guile interactively with LilyPond
840 In order to experiment with Scheme programming in the LilyPond
841 environment, it is necessary to have a Guile interpreter that
842 has all the LilyPond modules loaded. This requires the following
845 First, define a Scheme symbol for the active module in the .ly file:
848 #(module-define! (resolve-module '(guile-user))
849 'lilypond-module (current-module))
852 Now place a Scheme function in the .ly file that gives an
853 interactive Guile prompt:
859 When the .ly file is compiled, this causes the compilation to be
860 interrupted and an interactive guile prompt to appear. Once the
861 guile prompt appears, the LilyPond active module must be set as the
862 current guile module:
865 guile> (set-current-module lilypond-module)
868 You can demonstrate these commands are operating properly by typing the name
869 of a LilyPond public scheme function to check it has been defined:
872 guile> fret-diagram-verbose-markup
873 #<procedure fret-diagram-verbose-markup (layout props marking-list)>
876 If the LilyPond module has not been correctly loaded, an error
877 message will be generated:
880 guile> fret-diagram-verbose-markup
881 ERROR: Unbound variable: fret-diagram-verbose-markup
882 ABORT: (unbound-variable)
885 Once the module is properly loaded, any valid LilyPond Scheme
886 expression can be entered at the interactive prompt.
888 After the investigation is complete, the interactive guile
889 interpreter can be exited:
895 The compilation of the .ly file will then continue.
897 @unnumberedsubsubsec Using the Guile debugger
899 To set breakpoints and/or enable tracing in Scheme functions, put
902 \include "guile-debugger.ly"
905 in your input file after any scheme procedures you have defined in
906 that file. This will invoke the Guile command-line after having set
907 up the environment for the debug command-line. When your input file
908 is processed, a guile prompt will be displayed. You may now enter
909 commands to set up breakpoints and enable tracing by the Guile debugger.
911 @unnumberedsubsubsec Using breakpoints
913 At the guile prompt, you can set breakpoints with
914 the @code{set-break!} procedure:
917 guile> (set-break! my-scheme-procedure)
920 Once you have set the desired breakpoints, you exit the guile repl frame
927 Then, when one of the scheme routines for which you have set
928 breakpoints is entered, guile will interrupt execution in a debug
929 frame. At this point you will have access to Guile debugging
930 commands. For a listing of these commands, type:
936 Alternatively you may code the breakpoints in your Lilypond source
937 file using a command such as:
940 #(set-break! my-scheme-procedure)
943 immediately after the @code{\include} statement. In this case the
944 breakpoint will be set straight after you enter the @code{(quit)}
945 command at the guile prompt.
947 Embedding breakpoint commands like this is particularly useful if
948 you want to look at how the Scheme procedures in the @var{.scm}
949 files supplied with LilyPond work. To do this, edit the file in
950 the relevant directory to add this line near the top:
953 (use-modules (scm guile-debugger))
956 Now you can set a breakpoint after the procedure you are interested
957 in has been declared. For example, if you are working on routines
958 called by @var{print-book-with} in @var{lily-library.scm}:
961 (define (print-book-with parser book process-procedure)
962 (let* ((paper (ly:parser-lookup parser '$defaultpaper))
963 (layout (ly:parser-lookup parser '$defaultlayout))
964 (outfile-name (get-outfile-name parser)))
965 (process-procedure book paper layout outfile-name)))
967 (define-public (print-book-with-defaults parser book)
968 (print-book-with parser book ly:book-process))
970 (define-public (print-book-with-defaults-as-systems parser book)
971 (print-book-with parser book ly:book-process-to-systems))
975 At this point in the code you could add this to set a breakpoint at
979 (set-break! print-book-with)
982 @unnumberedsubsubsec Tracing procedure calls and evaluator steps
984 Two forms of trace are available:
987 (set-trace-call! my-scheme-procedure)
993 (set-trace-subtree! my-scheme-procedure)
996 @code{set-trace-call!} causes Scheme to log a line to the standard
997 output to show when the procedure is called and when it exits.
999 @code{set-trace-subtree!} traces every step the Scheme evaluator
1000 performs in evaluating the procedure.
1002 @node Tracing object relationships
1003 @section Tracing object relationships
1005 Understanding the LilyPond source often boils down to figuring out what
1006 is happening to the Grobs. Where (and why) are they being created,
1007 modified and destroyed? Tracing Lily through a debugger in order to
1008 identify these relationships can be time-consuming and tedious.
1010 In order to simplify this process, a facility has been added to
1011 display the grobs that are created and the properties that are set
1012 and modified. Although it can be complex to get set up, once set up
1013 it easily provides detailed information about the life of grobs
1014 in the form of a network graph.
1016 Each of the steps necessary to use the graphviz utility
1021 @item Installing graphviz
1023 In order to create the graph of the object relationships, it is
1024 first necessary to install Graphviz. graphviz is available for a
1025 number of different platforms:
1028 @uref{http://www.graphviz.org/Download..php}
1031 @item Modifying config.make
1033 In order for the Graphviz tool to work, config.make must be modified.
1034 It is probably a good idea to first save a copy of config.make under
1035 a different name. Then, edit config.make by removing every occurence
1038 @item Rebuilding LilyPond
1040 The executable code of LilyPond must be rebuilt from scratch:
1043 make -C lily clean && make -C lily
1046 @item Create a graphviz-compatible .ly file
1048 In order to use the graphviz utility, the .ly file must include
1049 @file{ly/graphviz-init.ly}, and should then specify the
1050 grobs and symbols that should be tracked. An example of this
1051 is found in @file{input/regression/graphviz.ly}.
1053 @item Run lilypond with output sent to a log file
1055 The Graphviz data is sent to stderr by lilypond, so it is
1056 necessary to redirect stderr to a logfile:
1059 lilypond graphviz.ly 2> graphviz.log
1062 @item Edit the logfile
1064 The logfile has standard lilypond output, as well as the Graphviz
1065 output data. Delete everything from the beginning of the file
1066 up to but not including the first occurence of @code{digraph}.
1068 @item Process the logfile with @code{dot}
1070 The directed graph is created from the log file with the program
1074 dot -Tpdf graphviz.log > graphviz.pdf
1079 The pdf file can then be viewed with any pdf viewer.
1081 When compiled without @code{-DNDEBUG}, lilypond may run slower
1082 than normal. The original configuration can be restored by either
1083 renaming the saved copy of @code{config.make} or rerunning
1084 @code{configure}. Then rebuild lilypond with
1087 make -C lily clean && make -C lily
1091 @node Adding or modifying features
1092 @section Adding or modifying features
1094 When a new feature is to be added to LilyPond, it is necessary to
1095 ensure that the feature is properly integrated to maintain
1096 its long-term support. This section describes the steps necessary
1097 for feature addition and modification.
1099 @subsection Write the code
1101 You should probably create a new git branch for writing the code, as that
1102 will separate it from the master branch and allow you to continue
1103 to work on small projects related to master.
1105 Please be sure to follow the rules for programming style discussed
1106 earlier in this chapter.
1108 @subsection Write regression tests
1110 In order to demonstrate that the code works properly, you will
1111 need to write one or more regression tests. These tests are
1112 typically .ly files that are found in input/regression.
1114 Regression tests should be as brief as possible to demonstrate the
1115 functionality of the code.
1117 Regression tests should generally cover one issue per test. Several
1118 short, single-issue regression tests are preferred to a single, long,
1119 multiple-issue regression test.
1121 Use existing regression tests as templates to demonstrate the type of
1122 header information that should be included in a regression test.
1124 @subsection Write convert-ly rule
1126 If the modification changes the input syntax, a convert-ly rule
1127 should be written to automatically update input files from older
1130 convert-ly rules are found in python/convertrules.py
1132 If possible, the convert-ly rule should allow automatic updating
1133 of the file. In some cases, this will not be possible, so the
1134 rule will simply point out to the user that the feature needs
1137 @subsection Automatically update documentation, snippets, and regtests
1139 convert-ly should be used to update the documentation, the snippets,
1140 and the regression tests. This not only makes the necessary syntax
1141 changes, it also tests the convert-ly rules.
1143 The automatic updating is a three step process. First, be sure you
1144 are in the top-level source directory. Then, for the
1148 find Documentation/ -name '*.itely' | xargs convert-ly -e --from @qq{@var{X.Y.Z}}
1152 where @var{X.Y.Z} is the version number of the last released development
1155 Next, for the snippets, do:
1158 find Documentation/snippets/ -name '*.ly' | xargs convert-ly -e --from @qq{@var{X.Y.Z}}
1161 Finally, for the regression tests, do:
1164 find input/regression/ -name '*.ly' | xargs convert-ly -e --from @qq{@var{X.Y.Z}}
1168 @subsection Manually update documentation, snippets, and regtests
1170 Where the convert-ly rule is not able to automatically update the inline
1171 lilypond code in the documentation (i.e. if a NOT_SMART rule is used), the
1172 documentation must be manually updated. The inline snippets that require
1173 changing must be changed in the English version of the docs and all
1174 translated versions. If the inline code is not changed in the
1175 translated documentation, the old snippets will show up in the
1176 English version of the documentation.
1178 Where the convert-ly rule is not able to automatically update snippets
1179 in Documentation/snippets/, those snippets must be manually updated.
1180 Those snippets should be copied to Documentation/snippets/new. The
1181 comments at the top of the snippet describing its automatice generation
1182 should be removed. All translated texidoc strings should be removed.
1183 The comment @qq{% begin verbatim} should be removed. The syntax of
1184 the snippet should then be manually edited.
1186 Where snippets in Documentation/snippets are made obsolete, the snippet
1187 should be copied to Documentation/snippets/new. The comments and
1188 texidoc strings should be removed as described above. Then the body
1189 of the snippet should be changed to:
1193 This snippet is deprecated as of version X.Y.Z and
1194 will be removed from the documentation.
1199 where X.Y.Z is the version number for which the convert-ly rule was
1202 Update the snippet files by running:
1205 scripts/auxiliar/makelsr.py
1208 Where the convert-ly rule is not able to automatically update regression
1209 tests, the regression tests in input/regression should be manually
1212 Although it is not required, it is helpful if the developer
1213 can write relevant material for inclusion in the Notation
1214 Reference. If the developer does not feel qualified to write
1215 the documentation, a documentation editor will be able to
1216 write it from the regression tests. The text that is added to
1217 or removed from the documentation should be changed only in
1218 the English version.
1220 @subsection Edit changes.tely
1222 An entry should be added to Documentation/changes.tely to describe
1223 the feature changes to be implemented. This is especially important
1224 for changes that change input file syntax.
1226 Hints for changes.tely entries are given at the top of the file.
1228 New entries in changes.tely go at the top of the file.
1230 The changes.tely entry should be written to show how the new change
1231 improves LilyPond, if possible.
1233 @subsection Verify successful build
1235 When the changes have been made, successful completion must be
1243 When these commands complete without error, the patch is
1244 considered to function successfully.
1246 Developers on Windows who are unable to build LilyPond should
1247 get help from a Linux or OSX developer to do the make tests.
1249 @subsection Verify regression test
1251 In order to avoid breaking LilyPond, it is important to verify that
1252 the regression tests all succeed. This process is described in
1253 @ref{Regression tests}.
1255 @subsection Post patch for comments
1257 For any change other than a minor change, a patch set should be
1258 posted on @uref{http://codereview.appspot.com/, Rietveld} for comment.
1259 This requires the use of an external package, git-cl, and an email
1262 git-cl is installed by:
1265 git clone git://neugierig.org/git-cl.git
1268 Then, add the git-cl directory to your PATH, or create a
1269 symbolic link to the git-cl and upload.py in one of your
1270 PATH directories (like usr/bin). git-cl is then
1271 configured by entering the command
1278 in the LilyPond git directory and answering the questions that
1279 are asked. If you do not understand the question answer with just
1282 The patch set is posted to Rietveld as follows. Ensure your changes
1283 are committed in a separate branch, which should differ from the
1284 reference branch to be used by just the changes to be uploaded.
1285 If the reference branch is to be origin/master, ensure this is
1286 up-to-date. If necessary, use git rebase to rebase the branch
1287 containing the changes to the head of origin/master. Finally,
1288 check out branch with the changes and enter the command:
1291 git cl upload <reference SHA1 ID>
1295 where <reference SHA1 ID> is the SHA1 ID of the commit to be used
1296 as a reference source for the patch. Generally, this will be the
1297 SHA1 ID of origin/master, and in that case the command
1300 git cl upload origin/master
1306 After prompting for your Google email address and password, the
1307 patch set will be posted to Rietveld.
1309 You should then announce the patch by sending
1310 an email to lilypond-devel, with a subject line
1311 starting with PATCH:, asking for comments on the patch.
1313 As revisions are made in response to comments, successive patch sets
1314 for the same issue can be uploaded by reissuing the git-cl command
1315 with the modified branch checked out.
1317 @subsection Push patch
1319 Once all the comments have been addressed, the patch can be pushed.
1321 If the author has push privileges, the author will push the patch.
1322 Otherwise, a developer with push privileges will push the patch.
1324 @subsection Closing the issues
1326 Once the patch has been pushed, all the relevant issues should be
1329 On Rietveld, the author should log in an close the issue either by
1330 using the @q{Edit Issue} link, or by clicking the circled x icon
1331 to the left of the issue name.
1333 If the changes were in response to a feature request on the Google
1334 issue tracker for LilyPond, the author should change the status to
1335 Fixed and a tag @q{fixed_x_y_z} should be added, where the patch was
1336 fixed in version x.y.z. If
1337 the author does not have privileges to change the status, an email
1338 should be sent to bug-lilypond requesting the BugMeister to change
1341 @node Iterator tutorial
1342 @section Iterator tutorial
1344 TODO -- this is a placeholder for a tutorial on iterators
1346 Iterators are routines written in C++ that process music expressions
1347 and sent the music events to the appropriate engravers and/or
1350 @node Engraver tutorial
1351 @section Engraver tutorial
1353 Engravers are C++ classes that catch music events and
1354 create the appropriate grobs for display on the page. Though the
1355 majority of engravers are responsible for the creation of a single grob,
1356 in some cases (e.g. @code{New_fingering_engraver}), several different grobs
1359 Engravers listen for events and acknowledge grobs. Events are passed to
1360 the engraver in time-step order during the iteration phase. Grobs are
1361 made available to the engraver when they are created by other engravers
1362 during the iteration phase.
1364 @subsection Useful methods for information processing
1366 An engraver inherits the following public methods from the Translator
1367 base class, which can be used to process listened events and acknowledged
1371 @item @code{virtual void initialize ()}
1372 @item @code{void start_translation_timestep ()}
1373 @item @code{void process_music ()}
1374 @item @code{void process_acknowledged ()}
1375 @item @code{void stop_translation_timestep ()}
1376 @item @code{virtual void finalize ()}
1379 These methods are listed in order of translation time, with
1380 @code{initialize ()} and @code{finalize ()} bookending the whole
1381 process. @code{initialize ()} can be used for one-time initialization
1382 of context properties before translation starts, whereas
1383 @code{finalize ()} is often used to tie up loose ends at the end of
1384 translation: for example, an unterminated spanner might be completed
1385 automatically or reported with a warning message.
1387 @subsection Translation process
1389 At each timestep in the music, translation proceeds by calling the
1390 following methods in turn:
1392 @code{start_translation_timestep ()} is called before any user
1393 information enters the translators, i.e., no property operations
1394 (\set, \override, etc.) or events have been processed yet.
1396 @code{process_music ()} and @code{process_acknowledged ()} are called
1397 after all events in the current time step have been heard, or all
1398 grobs in the current time step have been acknowledged. The latter
1399 tends to be used exclusively with engravers which only acknowledge
1400 grobs, whereas the former is the default method for main processing
1403 @code{stop_translation_timestep ()} is called after all user
1404 information has been processed prior to beginning the translation for
1407 @subsection Preventing garbage collection for SCM member variables
1409 In certain cases, an engraver might need to ensure private Scheme
1410 variables (with type SCM) do not get swept away by Guile's garbage
1411 collector: for example, a cache of the previous key signature which
1412 must persist between timesteps. The method
1413 @code{virtual derived_mark () const} can be used in such cases:
1416 Engraver_name::derived_mark ()
1418 scm_gc_mark (private_scm_member_)
1423 @subsection Listening to music events
1425 External interfaces to the engraver are implemented by protected
1426 macros including one or more of the following:
1429 @item @code{DECLARE_TRANSLATOR_LISTENER (event_name)}
1430 @item @code{IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)}
1434 where @var{event_name} is the type of event required to provide the
1435 input the engraver needs and @var{Engraver_name} is the name of the
1438 Following declaration of a listener, the method is implemented as follows:
1441 IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)
1443 Engraver_name::listen_event_name (Stream event *event)
1445 ...body of listener method...
1449 @subsection Acknowledging grobs
1451 Some engravers also need information from grobs as they are created
1452 and as they terminate. The mechanism and methods to obtain this
1453 information are set up by the macros:
1456 @item @code{DECLARE_ACKNOWLEDGER (grob_interface)}
1457 @item @code{DECLARE_END_ACKNOWLEDGER (grob_interface)}
1460 where @var{grob_interface} is an interface supported by the
1461 grob(s) which should be acknowledged. For example, the following
1462 code would declare acknowledgers for a @code{NoteHead} grob (via the
1463 @code{note-head-interface}) and any grobs which support the
1464 @code{side-position-interface}:
1467 @code{DECLARE_ACKNOWLEDGER (note_head)}
1468 @code{DECLARE_ACKNOWLEDGER (side_position)}
1471 The @code{DECLARE_END_ACKNOWLEDGER ()} macro sets up a spanner-specific
1472 acknowledger which will be called whenever a spanner ends.
1474 Following declaration of an acknowledger, the method is coded as follows:
1478 Engraver_name::acknowledge_interface_name (Grob_info info)
1480 ...body of acknowledger method...
1484 @subsection Engraver declaration/documentation
1486 An engraver must have a public macro
1489 @item @code{TRANSLATOR_DECLARATIONS (Engraver_name)}
1493 where @code{Engraver_name} is the name of the engraver. This
1494 defines the common variables and methods used by every engraver.
1496 At the end of the engraver file, one or both of the following
1497 macros are generally called to document the engraver in the
1498 Internals Reference:
1501 @item @code{ADD_ACKNOWLEDGER (Engraver_name, grob_interface)}
1502 @item @code{ADD_TRANSLATOR (Engraver_name, Engraver_doc,
1503 Engraver_creates, Engraver_reads, Engraver_writes)}
1507 where @code{Engraver_name} is the name of the engraver, @code{grob_interface}
1508 is the name of the interface that will be acknowledged,
1509 @code{Engraver_doc} is a docstring for the engraver,
1510 @code{Engraver_creates} is the set of grobs created by the engraver,
1511 @code{Engraver_reads} is the set of properties read by the engraver,
1512 and @code{Engraver_writes} is the set of properties written by
1515 @node Callback tutorial
1516 @section Callback tutorial
1518 TODO -- This is a placeholder for a tutorial on callback functions.
1520 @node LilyPond scoping
1521 @section LilyPond scoping
1523 The Lilypond language has a concept of scoping, ie you can do
1529 (display (+ foo 2)))
1532 @noindent with @code{\paper}, @code{\midi} and @code{\header} being
1533 nested scope inside the @file{.ly} file-level scope. @w{@code{foo = 1}}
1534 is translated in to a scheme variable definition.
1536 This implemented using modules, with each scope being an anonymous
1537 module that imports its enclosing scope's module.
1539 Lilypond's core, loaded from @file{.scm} files, is usually placed in the
1540 @code{lily} module, outside the @file{.ly} level. In the case of
1547 we want to reuse the built-in definitions, without changes effected in
1548 user-level @file{a.ly} leaking into the processing of @file{b.ly}.
1550 The user-accessible definition commands have to take care to avoid
1551 memory leaks that could occur when running multiple files. All
1552 information belonging to user-defined commands and markups is stored in
1553 a manner that allows it to be garbage-collected when the module is
1554 dispersed, either by being stored module-locally, or in weak hash
1557 @node LilyPond miscellany
1558 @section LilyPond miscellany
1560 This is a place to dump information that may be of use to developers
1561 but doesn't yet have a proper home. Ideally, the length of this section
1562 would become zero as items are moved to other homes.
1564 @subsection Spacing algorithms
1566 Here is information from an email exchange about spacing algorithms.
1568 On Thu, 2010-02-04 at 15:33 -0500, Boris Shingarov wrote:
1569 I am experimenting with some modifications to the line breaking code,
1570 and I am stuck trying to understand how some of it works. So far my
1571 understanding is that Simple_spacer operates on a vector of Grobs, and
1572 it is a well-known Constrained-QP problem (rods = constraints, springs
1573 = quadratic function to minimize). What I don't understand is, if the
1574 spacer operates at the level of Grobs, which are built at an earlier
1575 stage in the pipeline, how are the changes necessitated by differences
1576 in line breaking, taken into account? in other words, if I take the
1577 last measure of a line and place it on the next line, it is not just a
1578 matter of literally moving that graphic to where the start of the next
1579 line is, but I also need to draw a clef, key signature, and possibly
1580 other fundamental things -- but at that stage in the rendering
1581 pipeline, is it not too late??
1583 Joe Neeman answered:
1585 We create lots of extra grobs (eg. a BarNumber at every bar line) but
1586 most of them are not drawn. See the break-visibility property in
1589 @subsection Info from Han-Wen Email
1591 In 2004, Douglas Linhardt decided to try starting a document that would
1592 explain LilyPond architecture and design principles. The material below
1593 is extracted from that email, which can be found at
1594 @uref{http://thread.gmane.org/gmane.comp.gnu.lilypond.devel/2992}.
1595 The headings reflect questions from Doug or comments from Han-Wen;
1596 the body text are Han-Wen's answers.
1598 @unnumberedsubsubsec Figuring out how things work.
1600 I must admit that when I want to know how a program works, I use grep
1601 and emacs and dive into the source code. The comments and the code
1602 itself are usually more revealing than technical documents.
1604 @unnumberedsubsubsec What's a grob, and how is one used?
1606 Graphical object - they are created from within engravers, either as
1607 Spanners (derived class) -slurs, beams- or Items (also a derived
1608 class) -notes, clefs, etc.
1610 There are two other derived classes System (derived from Spanner,
1611 contaning a "line of music") and Paper_column (derived from Item, it
1612 contains all items that happen at the same moment). They are separate
1613 classes because they play a special role in the linebreaking process.
1615 @unnumberedsubsubsec What's a smob, and how is one used?
1617 A C(++) object that is encapsulated so it can be used as a Scheme
1618 object. See GUILE info, "19.3 Defining New Types (Smobs)"
1620 @unnumberedsubsubsec When is each C++ class constructed and used
1627 In the parser.yy see the macro calls MAKE_MUSIC_BY_NAME().
1632 Constructed during "interpreting" phase.
1637 Executive branch of Contexts, plugins that create grobs, usually one
1638 engraver per grob type. Created together with context.
1648 These are not C++ classes per se. The idea of a Grob interface hasn't
1649 crystallized well. ATM, an interface is a symbol, with a bunch of grob
1650 properties. They are not objects that are created or destroyed.
1655 Objects that walk through different music classes, and deliver events
1656 in a synchronized way, so that notes that play together are processed
1657 at the same moment and (as a result) end up on the same horizontal position.
1659 Created during interpreting phase.
1661 BTW, the entry point for interpreting is ly:run-translator
1662 (ly_run_translator on the C++ side)
1666 @unnumberedsubsubsec Can you get to Context properties from a Music object?
1668 You can create music object with a Scheme function that reads context
1669 properties (the \applycontext syntax). However, that function is
1670 executed during Interpreting, so you can not really get Context
1671 properties from Music objects, since music objects are not directly
1672 connected to Contexts. That connection is made by the Music_iterators
1674 @unnumberedsubsubsec Can you get to Music properties from a Context object?
1676 Yes, if you are given the music object within a Context
1677 object. Normally, the music objects enter Contexts in synchronized
1678 fashion, and the synchronization is done by Music_iterators.
1680 @unnumberedsubsubsec What is the relationship between C++ classes and Scheme objects?
1682 Smobs are C++ objects in Scheme. Scheme objects (lists, functions) are
1683 manipulated from C++ as well using the GUILE C function interface
1686 @unnumberedsubsubsec How do Scheme procedures get called from C++ functions?
1688 scm_call_*, where * is an integer from 0 to 4.
1689 Also scm_c_eval_string (), scm_eval ()
1691 @unnumberedsubsubsec How do C++ functions get called from Scheme procedures?
1693 Export a C++ function to Scheme with LY_DEFINE.
1695 @unnumberedsubsubsec What is the flow of control in the program?
1697 Good question. Things used to be clear-cut, but we have Scheme
1698 and SMOBs now, which means that interactions do not follow a very
1699 rigid format anymore. See below for an overview, though.
1701 @unnumberedsubsubsec Does the parser make Scheme procedure calls or C++ function calls?
1703 Both. And the Scheme calls can call C++ and vice versa. It's nested,
1704 with the SCM datatype as lubrication between the interactions
1706 (I think the word "lubrication" describes the process better than the
1707 traditional word "glue")
1709 @unnumberedsubsubsec How do the front-end and back-end get started?
1711 Front-end: a file is parsed, the rest follows from that. Specifically,
1713 Parsing leads to a Music + Music_output_def object (see parser.yy,
1714 definition of toplevel_expression )
1716 A Music + Music_output_def object leads to a Global_context object (see
1717 ly_run_translator ())
1719 During interpreting, Global_context + Music leads to a bunch of
1720 Contexts. (see Global_translator::run_iterator_on_me () )
1722 After interpreting, Global_context contains a Score_context (which
1723 contains staves, lyrics etc.) as a child. Score_context::get_output ()
1724 spews a Music_output object (either a Paper_score object for notation
1725 or Performance object for MIDI).
1727 The Music_output object is the entry point for the backend. (see
1728 ly_render_output () )
1730 The main steps of the backend itself are in
1735 paper-score.cc , Paper_score::process_
1738 system.cc , System::get_lines()
1741 The step, where things go from grobs to output, is in
1742 System::get_line(): each grob delivers a Stencil (a Device
1743 independent output description), which is interpreted by our
1744 outputting backends (scm/output-tex.scm and scm/output-ps.scm)
1745 to produce TeX and PS.
1749 Interactions between grobs and putting things into .tex and .ps files
1750 have gotten a little more complex lately. Jan has implemented
1751 page-breaking, so now the backend also involves Paper_book,
1752 Paper_lines and other things. This area is still heavily in flux, and
1753 perhaps not something you should want to look at.
1755 @unnumberedsubsubsec How do the front-end and back-end communicate?
1757 There is no communication from backend to front-end. From front-end to
1758 backend is simply the program flow: music + definitions gives
1759 contexts, contexts yield output, after processing, output is written
1762 @unnumberedsubsubsec Where is the functionality associated with KEYWORDs?
1764 See my-lily-lexer.cc (keywords, there aren't that many) and ly/*.ly
1765 (most of the other backslashed \words are identifiers)
1767 @unnumberedsubsubsec What Contexts/Properties/Music/etc. are available when they are processed?
1769 What do you mean exactly with this question?
1771 See ly/engraver-init.ly for contexts, see scm/define-*.scm for other
1774 @unnumberedsubsubsec How do you decide if something is a Music, Context, or Grob property?
1775 Why is part-combine-status a Music property when it seems (IMO)
1776 to be related to the Staff context?
1778 The Music_iterators and Context communicate through two channels
1780 Music_iterators can set and read context properties, idem for
1781 Engravers and Contexts
1783 Music_iterators can send "synthetic" music events (which aren't in
1784 the input) to a context. These are caught by Engravers. This is
1785 mostly a one way communication channel.
1787 part-combine-status is part of such a synthetic event, used by
1788 Part_combine_iterator to communicate with Part_combine_engraver.
1791 @unnumberedsubsubsec I'm adding a property to affect how \autochange works. It seems to
1792 me that it should be a context property, but the Scheme autochange
1793 procecure has a Music argument. Does this mean I should use
1796 \autochange is one of these extra strange beasts: it requires
1797 look-ahead to decide when to change staves. This is achieved by
1798 running the interpreting step twice (see scm/part-combiner.scm , at
1799 the bottom), and storing the result of the first step (where to switch
1800 staves) in a Music property. Since you want to influence that
1801 where-to-switch list, your must affect the code in
1802 make-autochange-music (scm/part-combiner.scm). That code is called
1803 directly from the parser and there are no official "parsing
1804 properties" yet, so there is no generic way to tune \autochange. We
1805 would have to invent something new for this, or add a separate
1809 \autochange #around-central-C ..music..
1813 where around-central-C is some function that is called from
1814 make-autochange-music.
1816 @unnumberedsubsubsec I get lost figuring out what environment the code I'm looking at is in when it executes.
1817 I found both the C++ and Scheme autochange code. Then I was
1818 trying to figure out where the code got called from. I finally figured out that
1819 the Scheme procedure was called before the C++ iterator code, but it took me a
1820 while to figure that out, and I still didn't know who did the calling in the
1821 first place. I only know a little bit about Flex and Bison, so reading those
1822 files helped only a little bit.
1824 @emph{Han-Wen:} GDB can be of help here. Set a breakpoint in C++, and run. When you
1825 hit the breakpoint, do a backtrace. You can inspect Scheme objects
1826 along the way by doing
1829 p ly_display_scm(obj)
1832 this will display OBJ through GUILE.
1834 @subsection Music functions and GUILE debugging
1836 Ian Hulin was trying to do some debugging in music functions, and
1837 came up with the following question
1840 I'm working on the Guile Debugger Stuff, and would like to try
1841 debugging a music function definition such as:
1844 conditionalMark = #(define-music-function (parser location) ()
1845 #@{ \tag #'instrumental-part @{\mark \default@} #@} )
1848 It appears conditionalMark does not get set up as an
1849 equivalent of a Scheme
1852 (define conditionalMark = define-music-function(parser location () ...
1856 although something gets defined because Scheme apparently recognizes
1859 #(set-break! conditionalMark)
1863 later on in the file without signalling any Guile errors.
1865 However the breakpoint trap is never encountered as
1866 define-music-function passed things on to ly:make-music-function,
1867 which is really C++ code ly_make_music_function, so Guile never
1868 finds out about the breakpoint.
1870 Han-Wen answered as follows:
1872 You can see the defintion by doing
1875 #(display conditionalMark)
1879 inside the .ly file.
1881 The breakpoint failing may have to do with the call sequence. See
1882 parser.yy, run_music_function(). The function is called directly from
1883 C++, without going through the GUILE evaluator, so I think that is why
1884 there is no debugger trap.