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 Language
135 Reference} is available online in PDF format.
139 Python is used for XML2ly and is used for building 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 LilyPond 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.
266 This section describes style guidelines for LilyPond
273 * Naming conventions::
282 @subsection Languages
284 C++ and Python are preferred. Python code should use PEP 8.
288 @subsection Filenames
290 Definitions of classes that are only accessed via pointers (*) or
291 references (&) shall not be included as include files.
297 ".cc" Implementation files
298 ".icc" Inline definition files
299 ".tcc" non inline Template defs
303 (setq auto-mode-alist
304 (append '(("\\.make$" . makefile-mode)
305 ("\\.cc$" . c++-mode)
306 ("\\.icc$" . c++-mode)
307 ("\\.tcc$" . c++-mode)
308 ("\\.hh$" . c++-mode)
309 ("\\.pod$" . text-mode)
314 The class Class_name is coded in @q{class-name.*}
318 @subsection Indentation
320 Standard GNU coding style is used. In emacs:
323 (add-hook 'c++-mode-hook
324 '(lambda() (c-set-style "gnu")
328 If you like using font-lock, you can also add this to your
332 (setq font-lock-maximum-decoration t)
333 (setq c++-font-lock-keywords-3
335 c++-font-lock-keywords-3
336 '(("\\b\\(a-zA-Z_?+_\\)\\b" 1 font-lock-variable-name-face) ("\\b\\(A-Z?+a-z_?+\\)\\b" 1 font-lock-type-face))
340 Some source files may not currently have proper indenting. If this
341 is the case, it is desirable to fix the improper indenting when the
342 file is modified, with the hope of continually improving the code.
345 @subheading Indenting files with fixcc.py
347 LilyPond provides a python script that will correct the indentation
351 scripts/auxiliar/fixcc.py lily/my-test-file.cc
354 Be sure you replace @code{my-test-file.cc} with the name of the file
357 If you are editing a file that contains an ADD_TRANSLATOR or ADD_INTERFACE
358 macro, the fixcc.py script will move the final parenthesis up one line
359 from where it should be. Please check the end of the file before
360 you run fixcc.py, and then put the final parenthesis and semicolon
361 back on a line by themselves.
364 @subheading Indenting files with emacs in script mode
366 @c email to wl@gnu.org when I get here.
368 @warning{this is pending some confirmation on -devel. July 2009 -gp}
370 Command-line script to format stuff with emacs:
374 emacs $1 -batch --eval '(indent-region (point-min) (point-max) nil)' -f save-buffer
377 (that's all on one line)
379 Save it as a shell script, then run on the file(s) you modified.
382 @subheading Indenting with vim
384 Although emacs indentation is the LilyPond standard, acceptable
385 indentation can usually be accomplished with vim. Some hints for
397 filetype plugin indent on
399 set ignorecase smartcase
402 set statusline=%F%m%r%h%w\ %{&ff}\ %Y\ [ASCII=\%03.3b]\ [HEX=\%02.2B]\ %04l,%04v\ %p%%\ [LEN=%L]
405 " Remove trailing whitespace on write
406 autocmd BufWritePre * :%s/\s\+$//e
409 With this .vimrc, files can be reindented automatically by highlighting
410 the lines to be indented in visual mode (use V to enter visual mode)
413 A scheme.vim file will help improve the indentation. This one
414 was suggested by Patrick McCarty. It should be saved in
415 ~/.vim/after/syntax/scheme.vim.
418 " Additional Guile-specific 'forms'
419 syn keyword schemeSyntax define-public define*-public
420 syn keyword schemeSyntax define* lambda* let-keywords*
421 syn keyword schemeSyntax defmacro defmacro* define-macro
422 syn keyword schemeSyntax defmacro-public defmacro*-public
423 syn keyword schemeSyntax use-modules define-module
424 syn keyword schemeSyntax define-method define-class
426 " Additional LilyPond-specific 'forms'
427 syn keyword schemeSyntax define-markup-command define-markup-list-command
428 syn keyword schemeSyntax define-safe-public define-music-function
429 syn keyword schemeSyntax def-grace-function
431 " All of the above should influence indenting too
432 set lw+=define-public,define*-public
433 set lw+=define*,lambda*,let-keywords*
434 set lw+=defmacro,defmacro*,define-macro
435 set lw+=defmacro-public,defmacro*-public
436 set lw+=use-modules,define-module
437 set lw+=define-method,define-class
438 set lw+=define-markup-command,define-markup-list-command
439 set lw+=define-safe-public,define-music-function
440 set lw+=def-grace-function
442 " These forms should not influence indenting
446 " Try to highlight all ly: procedures
447 syn match schemeFunc "ly:[^) ]\+"
451 @node Naming conventions
452 @subsection Naming Conventions
454 Naming conventions have been established for LilyPond
457 @subheading Classes and Types
459 Classes begin with an uppercase letter, and words
460 in class names are separated with @code{_}:
468 Member variable names end with an underscore:
476 Macro names should be written in uppercase completely,
477 with words separated by @code{_}:
483 @subheading Variables
485 Variable names should be complete words, rather than abbreviations.
486 For example, it is preferred to use @code{thickness} rather than
487 @code{th} or @code{t}.
489 Multi-word variable names in C++ should have the words separated
490 by the underscore character (@q{_}):
493 cxx_multiword_variable
496 Multi-word variable names in Scheme should have the words separated
500 scheme-multiword-variable
504 @subsection Broken code
506 Do not write broken code. This includes hardwired dependencies,
507 hardwired constants, slow algorithms and obvious limitations. If
508 you can not avoid it, mark the place clearly, and add a comment
509 explaining shortcomings of the code.
511 Ideally, the comment marking the shortcoming would include
512 TODO, so that it is marked for future fixing.
514 We reject broken-in-advance on principle.
518 @subsection Code comments
520 Comments may not be needed if descriptive variable names are used
521 in the code and the logic is straightforward. However, if the
522 logic is difficult to follow, and particularly if non-obvious
523 code has been included to resolve a bug, a comment describing
524 the logic and/or the need for the non-obvious code should be included.
526 There are instances where the current code could be commented better.
527 If significant time is required to understand the code as part of
528 preparing a patch, it would be wise to add comments reflecting your
529 understanding to make future work easier.
532 @node Handling errors
533 @subsection Handling errors
535 As a general rule, you should always try to continue computations,
536 even if there is some kind of error. When the program stops, it
537 is often very hard for a user to pinpoint what part of the input
538 causes an error. Finding the culprit is much easier if there is
539 some viewable output.
541 So functions and methods do not return errorcodes, they never
542 crash, but report a programming_error and try to carry on.
544 Error and warning messages need to be localized.
548 @subsection Localization
550 This document provides some guidelines to help programmers write
552 messages. To help translations, user messages must follow
553 uniform conventions. Follow these rules when coding for LilyPond.
554 Hopefully, this can be replaced by general GNU guidelines in the
555 future. Even better would be to have an English (en_BR, en_AM)
556 guide helping programmers writing consistent messages for all GNU
559 Non-preferred messages are marked with `+'. By convention,
560 ungrammatical examples are marked with `*'. However, such ungrammatical
561 examples may still be preferred.
566 Every message to the user should be localized (and thus be marked
567 for localization). This includes warning and error messages.
570 Do not localize/gettextify:
574 `programming_error ()'s
577 `programming_warning ()'s
583 output strings (PostScript, TeX, etc.)
588 Messages to be localized must be encapsulated in `_ (STRING)' or
589 `_f (FORMAT, ...)'. E.g.:
592 warning (_ ("need music in a score"));
593 error (_f ("cannot open file: `%s'", file_name));
596 In some rare cases you may need to call `gettext ()' by hand. This
597 happens when you pre-define (a list of) string constants for later
598 use. In that case, you'll probably also need to mark these string
599 constants for translation, using `_i (STRING)'. The `_i' macro is
600 a no-op, it only serves as a marker for `xgettext'.
603 char const* messages[] = @{
604 _i ("enable debugging output"),
605 _i ("ignore lilypond version"),
612 puts (gettext (messages i));
616 See also `flower/getopt-long.cc' and `lily/main.cc'.
619 Do not use leading or trailing whitespace in messages. If you need
620 whitespace to be printed, prepend or append it to the translated
624 message ("Calculating line breaks..." + " ");
628 Error or warning messages displayed with a file name and line
629 number never start with a capital, eg,
632 foo.ly: 12: not a duration: 3
635 Messages containing a final verb, or a gerund (`-ing'-form) always
636 start with a capital. Other (simpler) messages start with a
642 Not declaring: `foo'.
646 Avoid abbreviations or short forms, use `cannot' and `do not'
647 rather than `can't' or `don't'
648 To avoid having a number of different messages for the same
649 situation, well will use quoting like this `"message: `%s'"' for all
650 strings. Numbers are not quoted:
653 _f ("cannot open file: `%s'", name_str)
654 _f ("cannot find character number: %d", i)
658 Think about translation issues. In a lot of cases, it is better to
659 translate a whole message. English grammar must not be imposed on the
660 translator. So, instead of
663 stem at + moment.str () + does not fit in beam
669 _f ("stem at %s does not fit in beam", moment.str ())
673 Split up multi-sentence messages, whenever possible. Instead of
676 warning (_f ("out of tune! Can't find: `%s'", "Key_engraver"));
677 warning (_f ("cannot find font `%s', loading default", font_name));
683 warning (_ ("out of tune:"));
684 warning (_f ("cannot find: `%s', "Key_engraver"));
685 warning (_f ("cannot find font: `%s', font_name));
686 warning (_f ("Loading default font"));
690 If you must have multiple-sentence messages, use full punctuation.
691 Use two spaces after end of sentence punctuation. No punctuation
692 (esp. period) is used at the end of simple messages.
695 _f ("Non-matching braces in text `%s', adding braces", text)
696 _ ("Debug output disabled. Compiled with NPRINT.")
697 _f ("Huh? Not a Request: `%s'. Ignoring.", request)
701 Do not modularize too much; words frequently cannot be translated
702 without context. It is probably safe to treat most occurrences of
703 words like stem, beam, crescendo as separately translatable words.
706 When translating, it is preferable to put interesting information
707 at the end of the message, rather than embedded in the middle.
708 This especially applies to frequently used messages, even if this
709 would mean sacrificing a bit of eloquency. This holds for original
710 messages too, of course.
713 en: cannot open: `foo.ly'
714 + nl: kan `foo.ly' niet openen (1)
715 kan niet openen: `foo.ly'* (2)
716 niet te openen: `foo.ly'* (3)
720 The first nl message, although grammatically and stylistically
721 correct, is not friendly for parsing by humans (even if they speak
722 dutch). I guess we would prefer something like (2) or (3).
725 Do not run make po/po-update with GNU gettext < 0.10.35
731 @node Debugging LilyPond
732 @section Debugging LilyPond
734 The most commonly used tool for debugging LilyPond is the GNU
735 debugger gdb. The gdb tool is used for investigating and debugging
736 core Lilypond code written in C++. Another tool is available for
737 debugging Scheme code using the Guile debugger. This section
738 describes how to use both gdb and the Guile Debugger.
741 * Debugging overview::
742 * Debugging C++ code::
743 * Debugging Scheme code::
746 @node Debugging overview
747 @subsection Debugging overview
749 Using a debugger simplifies troubleshooting in at least two ways.
751 First, breakpoints can be set to pause execution at any desired point.
752 Then, when execution has paused, debugger commands can be issued to
753 explore the values of various variables or to execute functions.
755 Second, the debugger can display a stack trace, which shows the
756 sequence in which functions have been called and the arguments
757 passed to the called functions.
759 @node Debugging C++ code
760 @subsection Debugging C++ code
762 The GNU debugger, gdb, is the principal tool for debugging C++ code.
764 @subheading Compiling LilyPond for use with gdb
766 In order to use gdb with LilyPond, it is necessary to compile
767 LilyPond with debugging information. This is accomplished by running
768 the following commands in the main LilyPond source directory.
771 ./configure --disable-optimising
775 This will create a version of LilyPond containing debugging
776 information that will allow the debugger to tie the source code
777 to the compiled code.
779 You should not do @var{make install} if you want to use a debugger
780 with LilyPond. The @var{make install} command will strip debugging
781 information from the LilyPond binary.
783 @subheading Typical gdb usage
785 Once you have compiled the Lilypond image with the necessary
786 debugging information it will have been written to a location in a
787 subfolder of your current working directory:
793 This is important as you will need to let gdb know where to find the
794 image containing the symbol tables. You can invoke gdb from the
795 command line using the following:
801 This loads the LilyPond symbol tables into gdb. Then, to run
802 LilyPond on @code{test.ly} under the debugger, enter the following:
811 As an alternative to running gdb at the command line you may try
812 a graphical interface to gdb such as ddd:
818 You can also use sets of standard gdb commands stored in a .gdbinit
819 file (see next section).
821 @subheading Typical .gdbinit files
823 The behavior of gdb can be readily customized through the use of a
824 @var{.gdbinit} file. A @var{.gdbinit} file is a file named
825 @var{.gdbinit} (notice the @qq{.} at the beginning of the file name)
826 that is placed in a user's home directory.
828 The @var{.gdbinit} file below is from Han-Wen. It sets breakpoints
829 for all errors and defines functions for displaying scheme objects
830 (ps), grobs (pgrob), and parsed music expressions (pmusic).
833 file lily/out/lilypond
835 b Grob::programming_error
838 print ly_display_scm($arg0)
841 print ly_display_scm($arg0->self_scm_)
842 print ly_display_scm($arg0->mutable_property_alist_)
843 print ly_display_scm($arg0->immutable_property_alist_)
844 print ly_display_scm($arg0->object_alist_)
847 print ly_display_scm($arg0->self_scm_)
848 print ly_display_scm($arg0->mutable_property_alist_)
849 print ly_display_scm($arg0->immutable_property_alist_)
853 @node Debugging Scheme code
854 @subsection Debugging Scheme code
856 Scheme code can be developed using the Guile command line
857 interpreter @code{top-repl}. You can either investigate
858 interactively using just Guile or you can use the debugging
859 tools available within Guile.
861 @subheading Using Guile interactively with LilyPond
863 In order to experiment with Scheme programming in the LilyPond
864 environment, it is necessary to have a Guile interpreter that
865 has all the LilyPond modules loaded. This requires the following
868 First, define a Scheme symbol for the active module in the .ly file:
871 #(module-define! (resolve-module '(guile-user))
872 'lilypond-module (current-module))
875 Now place a Scheme function in the .ly file that gives an
876 interactive Guile prompt:
882 When the .ly file is compiled, this causes the compilation to be
883 interrupted and an interactive guile prompt to appear. Once the
884 guile prompt appears, the LilyPond active module must be set as the
885 current guile module:
888 guile> (set-current-module lilypond-module)
891 You can demonstrate these commands are operating properly by typing the name
892 of a LilyPond public scheme function to check it has been defined:
895 guile> fret-diagram-verbose-markup
896 #<procedure fret-diagram-verbose-markup (layout props marking-list)>
899 If the LilyPond module has not been correctly loaded, an error
900 message will be generated:
903 guile> fret-diagram-verbose-markup
904 ERROR: Unbound variable: fret-diagram-verbose-markup
905 ABORT: (unbound-variable)
908 Once the module is properly loaded, any valid LilyPond Scheme
909 expression can be entered at the interactive prompt.
911 After the investigation is complete, the interactive guile
912 interpreter can be exited:
918 The compilation of the .ly file will then continue.
920 @subheading Using the Guile debugger
922 To set breakpoints and/or enable tracing in Scheme functions, put
925 \include "guile-debugger.ly"
928 in your input file after any scheme procedures you have defined in
929 that file. This will invoke the Guile command-line after having set
930 up the environment for the debug command-line. When your input file
931 is processed, a guile prompt will be displayed. You may now enter
932 commands to set up breakpoints and enable tracing by the Guile debugger.
934 @subheading Using breakpoints
936 At the guile prompt, you can set breakpoints with
937 the @code{set-break!} procedure:
940 guile> (set-break! my-scheme-procedure)
943 Once you have set the desired breakpoints, you exit the guile repl frame
950 Then, when one of the scheme routines for which you have set
951 breakpoints is entered, guile will interrupt execution in a debug
952 frame. At this point you will have access to Guile debugging
953 commands. For a listing of these commands, type:
959 Alternatively you may code the breakpoints in your Lilypond source
960 file using a command such as:
963 #(set-break! my-scheme-procedure)
966 immediately after the @code{\include} statement. In this case the
967 breakpoint will be set straight after you enter the @code{(quit)}
968 command at the guile prompt.
970 Embedding breakpoint commands like this is particularly useful if
971 you want to look at how the Scheme procedures in the @var{.scm}
972 files supplied with LilyPond work. To do this, edit the file in
973 the relevant directory to add this line near the top:
976 (use-modules (scm guile-debugger))
979 Now you can set a breakpoint after the procedure you are interested
980 in has been declared. For example, if you are working on routines
981 called by @var{print-book-with} in @var{lily-library.scm}:
984 (define (print-book-with parser book process-procedure)
985 (let* ((paper (ly:parser-lookup parser '$defaultpaper))
986 (layout (ly:parser-lookup parser '$defaultlayout))
987 (outfile-name (get-outfile-name parser)))
988 (process-procedure book paper layout outfile-name)))
990 (define-public (print-book-with-defaults parser book)
991 (print-book-with parser book ly:book-process))
993 (define-public (print-book-with-defaults-as-systems parser book)
994 (print-book-with parser book ly:book-process-to-systems))
998 At this point in the code you could add this to set a breakpoint at
1002 (set-break! print-book-with)
1005 @subheading Tracing procedure calls and evaluator steps
1007 Two forms of trace are available:
1010 (set-trace-call! my-scheme-procedure)
1016 (set-trace-subtree! my-scheme-procedure)
1019 @code{set-trace-call!} causes Scheme to log a line to the standard
1020 output to show when the procedure is called and when it exits.
1022 @code{set-trace-subtree!} traces every step the Scheme evaluator
1023 performs in evaluating the procedure.
1025 @node Tracing object relationships
1026 @section Tracing object relationships
1028 Understanding the LilyPond source often boils down to figuring out what
1029 is happening to the Grobs. Where (and why) are they being created,
1030 modified and destroyed? Tracing Lily through a debugger in order to
1031 identify these relationships can be time-consuming and tedious.
1033 In order to simplify this process, a facility has been added to
1034 display the grobs that are created and the properties that are set
1035 and modified. Although it can be complex to get set up, once set up
1036 it easily provides detailed information about the life of grobs
1037 in the form of a network graph.
1039 Each of the steps necessary to use the graphviz utility
1044 @item Installing graphviz
1046 In order to create the graph of the object relationships, it is
1047 first necessary to install Graphviz. graphviz is available for a
1048 number of different platforms:
1051 @uref{http://www.graphviz.org/Download..php}
1054 @item Modifying config.make
1056 In order for the Graphviz tool to work, config.make must be modified.
1057 It is probably a good idea to first save a copy of config.make under
1058 a different name. Then, edit config.make by removing every occurrence
1061 @item Rebuilding LilyPond
1063 The executable code of LilyPond must be rebuilt from scratch:
1066 make -C lily clean && make -C lily
1069 @item Create a graphviz-compatible @file{.ly} file
1071 In order to use the graphviz utility, the @file{.ly} file must include
1072 @file{ly/@/graphviz@/-init@/.ly}, and should then specify the
1073 grobs and symbols that should be tracked. An example of this
1074 is found in @file{input/regression/graphviz.ly}.
1076 @item Run lilypond with output sent to a log file
1078 The Graphviz data is sent to stderr by lilypond, so it is
1079 necessary to redirect stderr to a logfile:
1082 lilypond graphviz.ly 2> graphviz.log
1085 @item Edit the logfile
1087 The logfile has standard lilypond output, as well as the Graphviz
1088 output data. Delete everything from the beginning of the file
1089 up to but not including the first occurrence of @code{digraph}.
1091 @item Process the logfile with @code{dot}
1093 The directed graph is created from the log file with the program
1097 dot -Tpdf graphviz.log > graphviz.pdf
1102 The pdf file can then be viewed with any pdf viewer.
1104 When compiled without @code{-DNDEBUG}, lilypond may run slower
1105 than normal. The original configuration can be restored by either
1106 renaming the saved copy of @code{config.make} or rerunning
1107 @code{configure}. Then rebuild lilypond with
1110 make -C lily clean && make -C lily
1114 @node Adding or modifying features
1115 @section Adding or modifying features
1117 When a new feature is to be added to LilyPond, it is necessary to
1118 ensure that the feature is properly integrated to maintain
1119 its long-term support. This section describes the steps necessary
1120 for feature addition and modification.
1125 * Write regression tests::
1126 * Write convert-ly rule::
1127 * Automatically update auxiliary information::
1128 * Manually update auxiliary information::
1129 * Edit changes.tely::
1130 * Verify successful build::
1131 * Verify regression tests::
1132 * Post patch for comments::
1134 * Closing the issues::
1137 @node Write the code
1138 @subsection Write the code
1140 You should probably create a new git branch for writing the code, as that
1141 will separate it from the master branch and allow you to continue
1142 to work on small projects related to master.
1144 Please be sure to follow the rules for programming style discussed
1145 earlier in this chapter.
1148 @node Write regression tests
1149 @subsection Write regression tests
1151 In order to demonstrate that the code works properly, you will
1152 need to write one or more regression tests. These tests are
1153 typically .ly files that are found in input/regression.
1155 Regression tests should be as brief as possible to demonstrate the
1156 functionality of the code.
1158 Regression tests should generally cover one issue per test. Several
1159 short, single-issue regression tests are preferred to a single, long,
1160 multiple-issue regression test.
1162 Use existing regression tests as templates to demonstrate the type of
1163 header information that should be included in a regression test.
1166 @node Write convert-ly rule
1167 @subsection Write convert-ly rule
1169 If the modification changes the input syntax, a convert-ly rule
1170 should be written to automatically update input files from older
1173 convert-ly rules are found in python/convertrules.py
1175 If possible, the convert-ly rule should allow automatic updating
1176 of the file. In some cases, this will not be possible, so the
1177 rule will simply point out to the user that the feature needs
1180 @subsubheading Updating version numbers
1182 If a development release occurs between you writing your patch and
1183 having it approved+pushed, you will need to update the version
1184 numbers in your tree. This can be done with:
1187 scripts/auxiliar/update-patch-version old.version.number new.version.number
1190 It will change all files in git, so use with caution and examine
1194 @node Automatically update auxiliary information
1195 @subsection Automatically update auxiliary information
1197 convert-ly should be used to update the documentation, the snippets,
1198 and the regression tests. This not only makes the necessary syntax
1199 changes, it also tests the convert-ly rules.
1201 The automatic updating is a three step process. First, be sure you
1202 are in the top-level source directory. Then, for the
1206 find Documentation/ -name '*.itely' | xargs convert-ly -e --from @qq{@var{X.Y.Z}}
1210 where @var{X.Y.Z} is the version number of the last released development
1213 Next, for the snippets, do:
1216 find Documentation/snippets/ -name '*.ly' | xargs convert-ly -e --from @qq{@var{X.Y.Z}}
1219 Finally, for the regression tests, do:
1222 find input/regression/ -name '*.ly' | xargs convert-ly -e --from @qq{@var{X.Y.Z}}
1227 @node Manually update auxiliary information
1228 @subsection Manually update auxiliary information
1230 Where the convert-ly rule is not able to automatically update the inline
1231 lilypond code in the documentation (i.e. if a NOT_SMART rule is used), the
1232 documentation must be manually updated. The inline snippets that require
1233 changing must be changed in the English version of the docs and all
1234 translated versions. If the inline code is not changed in the
1235 translated documentation, the old snippets will show up in the
1236 English version of the documentation.
1238 Where the convert-ly rule is not able to automatically update snippets
1239 in Documentation/snippets/, those snippets must be manually updated.
1240 Those snippets should be copied to Documentation/snippets/new. The
1241 comments at the top of the snippet describing its automatic generation
1242 should be removed. All translated texidoc strings should be removed.
1243 The comment @qq{% begin verbatim} should be removed. The syntax of
1244 the snippet should then be manually edited.
1246 Where snippets in Documentation/snippets are made obsolete, the snippet
1247 should be copied to Documentation/snippets/new. The comments and
1248 texidoc strings should be removed as described above. Then the body
1249 of the snippet should be changed to:
1253 This snippet is deprecated as of version X.Y.Z and
1254 will be removed from the documentation.
1259 where X.Y.Z is the version number for which the convert-ly rule was
1262 Update the snippet files by running:
1265 scripts/auxiliar/makelsr.py
1268 Where the convert-ly rule is not able to automatically update regression
1269 tests, the regression tests in input/regression should be manually
1272 Although it is not required, it is helpful if the developer
1273 can write relevant material for inclusion in the Notation
1274 Reference. If the developer does not feel qualified to write
1275 the documentation, a documentation editor will be able to
1276 write it from the regression tests. The text that is added to
1277 or removed from the documentation should be changed only in
1278 the English version.
1281 @node Edit changes.tely
1282 @subsection Edit changes.tely
1284 An entry should be added to Documentation/changes.tely to describe
1285 the feature changes to be implemented. This is especially important
1286 for changes that change input file syntax.
1288 Hints for changes.tely entries are given at the top of the file.
1290 New entries in changes.tely go at the top of the file.
1292 The changes.tely entry should be written to show how the new change
1293 improves LilyPond, if possible.
1296 @node Verify successful build
1297 @subsection Verify successful build
1299 When the changes have been made, successful completion must be
1307 When these commands complete without error, the patch is
1308 considered to function successfully.
1310 Developers on Windows who are unable to build LilyPond should
1311 get help from a Linux or OSX developer to do the make tests.
1314 @node Verify regression tests
1315 @subsection Verify regression tests
1317 In order to avoid breaking LilyPond, it is important to verify that
1318 the regression tests succeed, and that no unwanted changes are
1319 introduced into the output. This process is described in
1320 @ref{Identifying code regressions}.
1322 @subheading Typical developer's edit/compile/test cycle
1324 TODO: is @code{[-j@var{X} CPU_COUNT=@var{X}]} useful for
1325 @code{test-baseline}, @code{check}, @code{clean},
1326 @code{test-redo}? Neil Puttock says it is useful for
1327 everything but @code{clean}, which is disk-limited.
1328 Need to check formally.
1337 make [-j@var{X} CPU_COUNT=@var{X}] check
1341 Edit/compile/test cycle:
1344 @emph{## edit source files, then...}
1346 make clean @emph{## only if needed (see below)}
1347 make [-j@var{X}] @emph{## only if needed (see below)}
1348 make test-redo @emph{## redo files differing from baseline}
1349 make [-j@var{X} CPU_COUNT=@var{X}] check @emph{## CPU_COUNT here?}
1360 If you modify any source files that have to be compiled (such as
1361 @file{.cc} or @file{.hh} files in @file{flower/} or @file{lily/}),
1362 then you must run @command{make} before @command{make test-redo},
1363 so @command{make} can compile the modified files and relink all
1364 the object files. If you only modify files which are interpreted,
1365 like those in the @file{scm/} and @file{ly/} directories, then
1366 @command{make} is not needed before @command{make test-redo}.
1368 TODO: Fix the following paragraph. You can do @command{rm mf/out/*}
1369 instead of make clean, and you can probably do
1370 @command{make -C mf/ clean} as well, but I haven't checked it -- cds
1372 Also, if you modify any font definitions in the @file{mf/}
1373 directory then you must run @command{make clean} and
1374 @command{make} before running @command{make test-redo}. This will
1375 recompile everything, whether modified or not, and takes a lot
1378 Running @command{make@tie{}check} will leave an HTML page
1379 @file{out/@/test@/-results/@/index@/.html}. This page shows all the
1380 important differences that your change introduced, whether in the
1381 layout, MIDI, performance or error reporting.
1386 @node Post patch for comments
1387 @subsection Post patch for comments
1389 See @ref{Uploading a patch for review}.
1393 @subsection Push patch
1395 Once all the comments have been addressed, the patch can be pushed.
1397 If the author has push privileges, the author will push the patch.
1398 Otherwise, a developer with push privileges will push the patch.
1401 @node Closing the issues
1402 @subsection Closing the issues
1404 Once the patch has been pushed, all the relevant issues should be
1407 On Rietveld, the author should log in an close the issue either by
1408 using the @q{Edit Issue} link, or by clicking the circled x icon
1409 to the left of the issue name.
1411 If the changes were in response to a feature request on the Google
1412 issue tracker for LilyPond, the author should change the status to
1413 Fixed and a tag @q{fixed_x_y_z} should be added, where the patch was
1414 fixed in version x.y.z. If
1415 the author does not have privileges to change the status, an email
1416 should be sent to bug-lilypond requesting the BugMeister to change
1420 @node Iterator tutorial
1421 @section Iterator tutorial
1423 TODO -- this is a placeholder for a tutorial on iterators
1425 Iterators are routines written in C++ that process music expressions
1426 and sent the music events to the appropriate engravers and/or
1430 @node Engraver tutorial
1431 @section Engraver tutorial
1433 Engravers are C++ classes that catch music events and
1434 create the appropriate grobs for display on the page. Though the
1435 majority of engravers are responsible for the creation of a single grob,
1436 in some cases (e.g. @code{New_fingering_engraver}), several different grobs
1439 Engravers listen for events and acknowledge grobs. Events are passed to
1440 the engraver in time-step order during the iteration phase. Grobs are
1441 made available to the engraver when they are created by other engravers
1442 during the iteration phase.
1446 * Useful methods for information processing::
1447 * Translation process::
1448 * Preventing garbage collection for SCM member variables::
1449 * Listening to music events::
1450 * Acknowledging grobs::
1451 * Engraver declaration/documentation::
1454 @node Useful methods for information processing
1455 @subsection Useful methods for information processing
1457 An engraver inherits the following public methods from the Translator
1458 base class, which can be used to process listened events and acknowledged
1462 @item @code{virtual void initialize ()}
1463 @item @code{void start_translation_timestep ()}
1464 @item @code{void process_music ()}
1465 @item @code{void process_acknowledged ()}
1466 @item @code{void stop_translation_timestep ()}
1467 @item @code{virtual void finalize ()}
1470 These methods are listed in order of translation time, with
1471 @code{initialize ()} and @code{finalize ()} bookending the whole
1472 process. @code{initialize ()} can be used for one-time initialization
1473 of context properties before translation starts, whereas
1474 @code{finalize ()} is often used to tie up loose ends at the end of
1475 translation: for example, an unterminated spanner might be completed
1476 automatically or reported with a warning message.
1479 @node Translation process
1480 @subsection Translation process
1482 At each timestep in the music, translation proceeds by calling the
1483 following methods in turn:
1485 @code{start_translation_timestep ()} is called before any user
1486 information enters the translators, i.e., no property operations
1487 (\set, \override, etc.) or events have been processed yet.
1489 @code{process_music ()} and @code{process_acknowledged ()} are called
1490 after all events in the current time step have been heard, or all
1491 grobs in the current time step have been acknowledged. The latter
1492 tends to be used exclusively with engravers which only acknowledge
1493 grobs, whereas the former is the default method for main processing
1496 @code{stop_translation_timestep ()} is called after all user
1497 information has been processed prior to beginning the translation for
1501 @node Preventing garbage collection for SCM member variables
1502 @subsection Preventing garbage collection for SCM member variables
1504 In certain cases, an engraver might need to ensure private Scheme
1505 variables (with type SCM) do not get swept away by Guile's garbage
1506 collector: for example, a cache of the previous key signature which
1507 must persist between timesteps. The method
1508 @code{virtual derived_mark () const} can be used in such cases:
1511 Engraver_name::derived_mark ()
1513 scm_gc_mark (private_scm_member_)
1518 @node Listening to music events
1519 @subsection Listening to music events
1521 External interfaces to the engraver are implemented by protected
1522 macros including one or more of the following:
1525 @item @code{DECLARE_TRANSLATOR_LISTENER (event_name)}
1526 @item @code{IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)}
1530 where @var{event_name} is the type of event required to provide the
1531 input the engraver needs and @var{Engraver_name} is the name of the
1534 Following declaration of a listener, the method is implemented as follows:
1537 IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)
1539 Engraver_name::listen_event_name (Stream event *event)
1541 ...body of listener method...
1546 @node Acknowledging grobs
1547 @subsection Acknowledging grobs
1549 Some engravers also need information from grobs as they are created
1550 and as they terminate. The mechanism and methods to obtain this
1551 information are set up by the macros:
1554 @item @code{DECLARE_ACKNOWLEDGER (grob_interface)}
1555 @item @code{DECLARE_END_ACKNOWLEDGER (grob_interface)}
1558 where @var{grob_interface} is an interface supported by the
1559 grob(s) which should be acknowledged. For example, the following
1560 code would declare acknowledgers for a @code{NoteHead} grob (via the
1561 @code{note-head-interface}) and any grobs which support the
1562 @code{side-position-interface}:
1565 @code{DECLARE_ACKNOWLEDGER (note_head)}
1566 @code{DECLARE_ACKNOWLEDGER (side_position)}
1569 The @code{DECLARE_END_ACKNOWLEDGER ()} macro sets up a spanner-specific
1570 acknowledger which will be called whenever a spanner ends.
1572 Following declaration of an acknowledger, the method is coded as follows:
1576 Engraver_name::acknowledge_interface_name (Grob_info info)
1578 ...body of acknowledger method...
1583 @node Engraver declaration/documentation
1584 @subsection Engraver declaration/documentation
1586 An engraver must have a public macro
1589 @item @code{TRANSLATOR_DECLARATIONS (Engraver_name)}
1593 where @code{Engraver_name} is the name of the engraver. This
1594 defines the common variables and methods used by every engraver.
1596 At the end of the engraver file, one or both of the following
1597 macros are generally called to document the engraver in the
1598 Internals Reference:
1601 @item @code{ADD_ACKNOWLEDGER (Engraver_name, grob_interface)}
1602 @item @code{ADD_TRANSLATOR (Engraver_name, Engraver_doc,
1603 Engraver_creates, Engraver_reads, Engraver_writes)}
1607 where @code{Engraver_name} is the name of the engraver, @code{grob_interface}
1608 is the name of the interface that will be acknowledged,
1609 @code{Engraver_doc} is a docstring for the engraver,
1610 @code{Engraver_creates} is the set of grobs created by the engraver,
1611 @code{Engraver_reads} is the set of properties read by the engraver,
1612 and @code{Engraver_writes} is the set of properties written by
1615 The @code{ADD_ACKNOWLEDGER} and @code{ADD_TRANSLATOR} macros use a
1616 non-standard indentation system. Each interface, grob, read property,
1617 and write property is on its own line, and the closing parenthesis
1618 and semicolon for the macro all occupy a separate line beneath the final
1619 interface or write property. See existing engraver files for more
1623 @node Callback tutorial
1624 @section Callback tutorial
1626 TODO -- This is a placeholder for a tutorial on callback functions.
1628 @node LilyPond scoping
1629 @section LilyPond scoping
1631 The Lilypond language has a concept of scoping, i.e. you can do
1637 (display (+ foo 2)))
1640 @noindent with @code{\paper}, @code{\midi} and @code{\header} being
1641 nested scope inside the @file{.ly} file-level scope. @w{@code{foo = 1}}
1642 is translated in to a scheme variable definition.
1644 This implemented using modules, with each scope being an anonymous
1645 module that imports its enclosing scope's module.
1647 Lilypond's core, loaded from @file{.scm} files, is usually placed in the
1648 @code{lily} module, outside the @file{.ly} level. In the case of
1655 we want to reuse the built-in definitions, without changes effected in
1656 user-level @file{a.ly} leaking into the processing of @file{b.ly}.
1658 The user-accessible definition commands have to take care to avoid
1659 memory leaks that could occur when running multiple files. All
1660 information belonging to user-defined commands and markups is stored in
1661 a manner that allows it to be garbage-collected when the module is
1662 dispersed, either by being stored module-locally, or in weak hash
1665 @node LilyPond miscellany
1666 @section LilyPond miscellany
1668 This is a place to dump information that may be of use to developers
1669 but doesn't yet have a proper home. Ideally, the length of this section
1670 would become zero as items are moved to other homes.
1674 * Spacing algorithms::
1675 * Info from Han-Wen email::
1676 * Music functions and GUILE debugging::
1679 @node Spacing algorithms
1680 @subsection Spacing algorithms
1682 Here is information from an email exchange about spacing algorithms.
1684 On Thu, 2010-02-04 at 15:33 -0500, Boris Shingarov wrote:
1685 I am experimenting with some modifications to the line breaking code,
1686 and I am stuck trying to understand how some of it works. So far my
1687 understanding is that Simple_spacer operates on a vector of Grobs, and
1688 it is a well-known Constrained-QP problem (rods = constraints, springs
1689 = quadratic function to minimize). What I don't understand is, if the
1690 spacer operates at the level of Grobs, which are built at an earlier
1691 stage in the pipeline, how are the changes necessitated by differences
1692 in line breaking, taken into account? in other words, if I take the
1693 last measure of a line and place it on the next line, it is not just a
1694 matter of literally moving that graphic to where the start of the next
1695 line is, but I also need to draw a clef, key signature, and possibly
1696 other fundamental things -- but at that stage in the rendering
1697 pipeline, is it not too late??
1699 Joe Neeman answered:
1701 We create lots of extra grobs (eg. a BarNumber at every bar line) but
1702 most of them are not drawn. See the break-visibility property in
1706 @node Info from Han-Wen email
1707 @subsection Info from Han-Wen email
1709 In 2004, Douglas Linhardt decided to try starting a document that would
1710 explain LilyPond architecture and design principles. The material below
1711 is extracted from that email, which can be found at
1712 @uref{http://thread.gmane.org/gmane.comp.gnu.lilypond.devel/2992}.
1713 The headings reflect questions from Doug or comments from Han-Wen;
1714 the body text are Han-Wen's answers.
1716 @subheading Figuring out how things work.
1718 I must admit that when I want to know how a program works, I use grep
1719 and emacs and dive into the source code. The comments and the code
1720 itself are usually more revealing than technical documents.
1722 @subheading What's a grob, and how is one used?
1724 Graphical object - they are created from within engravers, either as
1725 Spanners (derived class) -slurs, beams- or Items (also a derived
1726 class) -notes, clefs, etc.
1728 There are two other derived classes System (derived from Spanner,
1729 containing a "line of music") and Paper_column (derived from Item, it
1730 contains all items that happen at the same moment). They are separate
1731 classes because they play a special role in the linebreaking process.
1733 @subheading What's a smob, and how is one used?
1735 A C(++) object that is encapsulated so it can be used as a Scheme
1736 object. See GUILE info, "19.3 Defining New Types (Smobs)"
1738 @@subheading When is each C++ class constructed and used
1745 In the parser.yy see the macro calls MAKE_MUSIC_BY_NAME().
1750 Constructed during "interpreting" phase.
1755 Executive branch of Contexts, plugins that create grobs, usually one
1756 engraver per grob type. Created together with context.
1766 These are not C++ classes per se. The idea of a Grob interface hasn't
1767 crystallized well. ATM, an interface is a symbol, with a bunch of grob
1768 properties. They are not objects that are created or destroyed.
1773 Objects that walk through different music classes, and deliver events
1774 in a synchronized way, so that notes that play together are processed
1775 at the same moment and (as a result) end up on the same horizontal position.
1777 Created during interpreting phase.
1779 BTW, the entry point for interpreting is ly:run-translator
1780 (ly_run_translator on the C++ side)
1784 @subheading Can you get to Context properties from a Music object?
1786 You can create music object with a Scheme function that reads context
1787 properties (the \applycontext syntax). However, that function is
1788 executed during Interpreting, so you can not really get Context
1789 properties from Music objects, since music objects are not directly
1790 connected to Contexts. That connection is made by the Music_iterators
1792 @subheading Can you get to Music properties from a Context object?
1794 Yes, if you are given the music object within a Context
1795 object. Normally, the music objects enter Contexts in synchronized
1796 fashion, and the synchronization is done by Music_iterators.
1798 @subheading What is the relationship between C++ classes and Scheme objects?
1800 Smobs are C++ objects in Scheme. Scheme objects (lists, functions) are
1801 manipulated from C++ as well using the GUILE C function interface
1804 @subheading How do Scheme procedures get called from C++ functions?
1806 scm_call_*, where * is an integer from 0 to 4.
1807 Also scm_c_eval_string (), scm_eval ()
1809 @subheading How do C++ functions get called from Scheme procedures?
1811 Export a C++ function to Scheme with LY_DEFINE.
1813 @subheading What is the flow of control in the program?
1815 Good question. Things used to be clear-cut, but we have Scheme
1816 and SMOBs now, which means that interactions do not follow a very
1817 rigid format anymore. See below for an overview, though.
1819 @subheading Does the parser make Scheme procedure calls or C++ function calls?
1821 Both. And the Scheme calls can call C++ and vice versa. It's nested,
1822 with the SCM datatype as lubrication between the interactions
1824 (I think the word "lubrication" describes the process better than the
1825 traditional word "glue")
1827 @subheading How do the front-end and back-end get started?
1829 Front-end: a file is parsed, the rest follows from that. Specifically,
1831 Parsing leads to a Music + Music_output_def object (see parser.yy,
1832 definition of toplevel_expression )
1834 A Music + Music_output_def object leads to a Global_context object (see
1835 ly_run_translator ())
1837 During interpreting, Global_context + Music leads to a bunch of
1838 Contexts (see Global_translator::run_iterator_on_me ()).
1840 After interpreting, Global_context contains a Score_context (which
1841 contains staves, lyrics etc.) as a child. Score_context::get_output ()
1842 spews a Music_output object (either a Paper_score object for notation
1843 or Performance object for MIDI).
1845 The Music_output object is the entry point for the backend (see
1846 ly_render_output ()).
1848 The main steps of the backend itself are in
1853 paper-score.cc , Paper_score::process_
1856 system.cc , System::get_lines()
1859 The step, where things go from grobs to output, is in
1860 System::get_line(): each grob delivers a Stencil (a Device
1861 independent output description), which is interpreted by our
1862 outputting backends (scm/output-tex.scm and scm/output-ps.scm)
1863 to produce TeX and PS.
1867 Interactions between grobs and putting things into .tex and .ps files
1868 have gotten a little more complex lately. Jan has implemented
1869 page-breaking, so now the backend also involves Paper_book,
1870 Paper_lines and other things. This area is still heavily in flux, and
1871 perhaps not something you should want to look at.
1873 @subheading How do the front-end and back-end communicate?
1875 There is no communication from backend to front-end. From front-end to
1876 backend is simply the program flow: music + definitions gives
1877 contexts, contexts yield output, after processing, output is written
1880 @subheading Where is the functionality associated with KEYWORDs?
1882 See my-lily-lexer.cc (keywords, there aren't that many) and ly/*.ly
1883 (most of the other backslashed \words are identifiers)
1885 @subheading What Contexts/Properties/Music/etc. are available when they are processed?
1887 What do you mean exactly with this question?
1889 See ly/engraver-init.ly for contexts, see scm/define-*.scm for other
1892 @subheading How do you decide if something is a Music, Context, or Grob property?
1893 Why is part-combine-status a Music property when it seems (IMO)
1894 to be related to the Staff context?
1896 The Music_iterators and Context communicate through two channels
1898 Music_iterators can set and read context properties, idem for
1899 Engravers and Contexts
1901 Music_iterators can send "synthetic" music events (which aren't in
1902 the input) to a context. These are caught by Engravers. This is
1903 mostly a one way communication channel.
1905 part-combine-status is part of such a synthetic event, used by
1906 Part_combine_iterator to communicate with Part_combine_engraver.
1909 @subheading Deciding between context and music properties
1911 I'm adding a property to affect how \autochange works. It seems to
1912 me that it should be a context property, but the Scheme autochange
1913 procedure has a Music argument. Does this mean I should use
1916 \autochange is one of these extra strange beasts: it requires
1917 look-ahead to decide when to change staves. This is achieved by
1918 running the interpreting step twice (see scm/part-combiner.scm , at
1919 the bottom), and storing the result of the first step (where to switch
1920 staves) in a Music property. Since you want to influence that
1921 where-to-switch list, your must affect the code in
1922 make-autochange-music (scm/part-combiner.scm). That code is called
1923 directly from the parser and there are no official "parsing
1924 properties" yet, so there is no generic way to tune \autochange. We
1925 would have to invent something new for this, or add a separate
1929 \autochange #around-central-C ..music..
1933 where around-central-C is some function that is called from
1934 make-autochange-music.
1936 @subheading More on context and music properties
1938 From Neil Puttock, in response to a question about transposition:
1940 Context properties (using \set & \unset) are tied to engravers: they
1941 provide information relevant to the generation of graphical objects.
1943 Since transposition occurs at the music interpretation stage, it has
1944 no direct connection with engravers: the pitch of a note is fixed
1945 before a notehead is created. Consider the following minimal snippet:
1951 This generates (simplified) a NoteEvent, with its pitch and duration
1952 as event properties,
1958 (ly:make-duration 2 0 1 1)
1960 (ly:make-pitch 0 0 0)
1963 which the Note_heads_engraver hears. It passes this information on to
1964 the NoteHead grob it creates from the event, so the head's correct
1965 position and duration-log can be determined once it's ready for
1968 If we transpose the snippet,
1971 \transpose c d @{ c' @}
1974 the pitch is changed before it reaches the engraver (in fact, it
1975 happens just after the parsing stage with the creation of a
1976 TransposedMusic music object):
1982 (ly:make-duration 2 0 1 1)
1984 (ly:make-pitch 0 1 0)
1987 You can see an example of a music property relevant to transposition:
1991 \transpose c d @{ c'2 \withMusicProperty #'untransposable ##t c' @}
1994 -> the second c' remains untransposed.
1996 Take a look at lily/music.cc to see where the transposition takes place.
1999 @subheading How do I tell about the execution environment?
2001 I get lost figuring out what environment the code I'm looking at is in when it
2002 executes. I found both the C++ and Scheme autochange code. Then I was trying
2003 to figure out where the code got called from. I finally figured out that the
2004 Scheme procedure was called before the C++ iterator code, but it took me a
2005 while to figure that out, and I still didn't know who did the calling in the
2006 first place. I only know a little bit about Flex and Bison, so reading those
2007 files helped only a little bit.
2009 @emph{Han-Wen:} GDB can be of help here. Set a breakpoint in C++, and run. When you
2010 hit the breakpoint, do a backtrace. You can inspect Scheme objects
2011 along the way by doing
2014 p ly_display_scm(obj)
2017 this will display OBJ through GUILE.
2019 @node Music functions and GUILE debugging
2020 @subsection Music functions and GUILE debugging
2022 Ian Hulin was trying to do some debugging in music functions, and
2023 came up with the following question
2026 I'm working on the Guile Debugger Stuff, and would like to try
2027 debugging a music function definition such as:
2030 conditionalMark = #(define-music-function (parser location) ()
2031 #@{ \tag #'instrumental-part @{\mark \default@} #@} )
2034 It appears conditionalMark does not get set up as an
2035 equivalent of a Scheme
2038 (define conditionalMark = define-music-function(parser location () ...
2042 although something gets defined because Scheme apparently recognizes
2045 #(set-break! conditionalMark)
2049 later on in the file without signalling any Guile errors.
2051 However the breakpoint trap is never encountered as
2052 define-music-function passed things on to ly:make-music-function,
2053 which is really C++ code ly_make_music_function, so Guile never
2054 finds out about the breakpoint.
2056 Han-Wen answered as follows:
2058 You can see the definition by doing
2061 #(display conditionalMark)
2065 inside the .ly file.
2067 The breakpoint failing may have to do with the call sequence. See
2068 parser.yy, run_music_function(). The function is called directly from
2069 C++, without going through the GUILE evaluator, so I think that is why
2070 there is no debugger trap.