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 @file{@/*@/-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 @file{@/*@/-engraver@/.cc}
62 and @file{@/*@/-performer@/.cc}.
63 Much of the work of translating is handled by Scheme functions,
64 which is one of the keys to LilyPond's exceptional flexibility.
66 @sourceimage{architecture-diagram,,,png}
69 @node LilyPond programming languages
70 @section LilyPond programming languages
72 Programming in LilyPond is done in a variety of programming languages. Each
73 language is used for a specific purpose or purposes. This section describes
74 the languages used and provides links to reference manuals and tutorials for
75 the relevant language.
79 The core functionality of LilyPond is implemented in C++.
81 C++ is so ubiquitous that it is difficult to identify either a reference
82 manual or a tutorial. Programmers unfamiliar with C++ will need to spend some
83 time to learn the language before attempting to modify the C++ code.
85 The C++ code calls Scheme/GUILE through the GUILE interface, which is
87 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html, GUILE
92 The LilyPond lexer is implemented in Flex, an implementation of the Unix lex
93 lexical analyser generator. Resources for Flex can be found
94 @uref{http://flex.sourceforge.net/, here}.
98 The LilyPond parser is implemented in Bison, a GNU parser generator. The
99 Bison homepage is found at @uref{http://www.gnu.org/software/bison/,
100 gnu.org}. The manual (which includes both a reference and tutorial) is
101 @uref{http://www.gnu.org/software/bison/manual/index.html, available} in a
106 GNU Make is used to control the compiling process and to build the
107 documentation and the website. GNU Make documentation is available at
108 @uref{http://www.gnu.org/software/make/manual/, the GNU website}.
110 @subsection GUILE or Scheme
112 GUILE is the dialect of Scheme that is used as LilyPond's extension language.
113 Many extensions to LilyPond are written entirely in GUILE. The
114 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html,
115 GUILE Reference Manual} is available online.
117 @uref{http://mitpress.mit.edu/sicp/full-text/book/book.html, Structure and
118 Interpretation of Computer Programs}, a popular textbook used to teach
119 programming in Scheme is available in its entirety online.
121 An introduction to Guile/Scheme as used in LilyPond can be found in the
122 @rextend{Scheme tutorial}.
126 MetaFont is used to create the music fonts used by LilyPond. A MetaFont
127 tutorial is available at @uref{http://metafont.tutorial.free.fr/, the
128 METAFONT tutorial page}.
130 @subsection PostScript
132 PostScript is used to generate graphical output. A brief PostScript tutorial
133 is @uref{http://local.wasp.uwa.edu.au/~pbourke/dataformats/postscript/,
134 available online}. The
135 @uref{http://www.adobe.com/devnet/postscript/pdfs/PLRM.pdf, PostScript Language
136 Reference} is available online in PDF format.
140 Python is used for XML2ly and is used for building the documentation and the
143 Python documentation is available at @uref{http://www.python.org/doc/,
146 @node Programming without compiling
147 @section Programming without compiling
149 Much of the development work in LilyPond takes place by changing @file{@/*@/.ly} or
150 @file{@/*@/.scm} files. These changes can be made without compiling LilyPond. Such
151 changes are described in this section.
154 @subsection Modifying distribution files
156 Much of LilyPond is written in Scheme or LilyPond input files. These
157 files are interpreted when the program is run, rather than being compiled
158 when the program is built, and are present in all LilyPond distributions.
159 You will find @file{@/.ly} files in the @file{ly/@/} directory and the Scheme files in the
160 @file{scm/@/} directory. Both Scheme files and @file{@/.ly} files can be modified and
161 saved with any text editor. It's probably wise to make a backup copy of
162 your files before you modify them, although you can reinstall if the
163 files become corrupted.
165 Once you've modified the files, you can test the changes just by running
166 LilyPond on some input file. It's a good idea to create a file that
167 demonstrates the feature you're trying to add. This file will eventually
168 become a regression test and will be part of the LilyPond distribution.
170 @subsection Desired file formatting
172 Files that are part of the LilyPond distribution have Unix-style line
173 endings (LF), rather than DOS (CR+LF) or MacOS 9 and earlier (CR). Make
174 sure you use the necessary tools to ensure that Unix-style line endings are
175 preserved in the patches you create.
177 Tab characters should not be included in files for distribution. All
178 indentation should be done with spaces. Most editors have settings to
179 allow the setting of tab stops and ensuring that no tab characters are
180 included in the file.
182 Scheme files and LilyPond files should be written according to standard
183 style guidelines. Scheme file guidelines can be found at
184 @uref{http://community.schemewiki.org/?scheme-style}. Following these
185 guidelines will make your code easier to read. Both you and others that
186 work on your code will be glad you followed these guidelines.
188 For LilyPond files, you should follow the guidelines for LilyPond snippets
189 in the documentation. You can find these guidelines at
190 @ref{Texinfo introduction and usage policy}.
192 @node Finding functions
193 @section Finding functions
195 When making changes or fixing bugs in LilyPond, one of the initial
196 challenges is finding out where in the code tree the functions to
197 be modified live. With nearly 3000 files in the source tree,
198 trial-and-error searching is generally ineffective. This section
199 describes a process for finding interesting code.
201 @subsection Using the ROADMAP
203 The file ROADMAP is located in the main directory of the lilypond source.
204 ROADMAP lists all of the directories in the LilyPond source tree, along
205 with a brief description of the kind of files found in each directory.
206 This can be a very helpful tool for deciding which directories to search
207 when looking for a function.
210 @subsection Using grep to search
212 Having identified a likely subdirectory to search, the grep utility can
213 be used to search for a function name. The format of the grep command is
216 grep -i functionName subdirectory/*
219 This command will search all the contents of the directory subdirectory/
220 and display every line in any of the files that contains
221 functionName. The @code{-i} option makes @command{grep} ignore
222 case -- this can be very useful if you are not yet familiar with
223 our capitalization conventions.
225 The most likely directories to grep for function names are @file{scm/@/} for
226 scheme files, ly/ for lilypond input (@file{@/*@/.ly}) files, and @file{lily/@/} for C++
230 @subsection Using git grep to search
232 If you have used git to obtain the source, you have access to a
233 powerful tool to search for functions. The command:
236 git grep functionName
239 will search through all of the files that are present in the git
240 repository looking for functionName. It also presents the results
241 of the search using @code{less}, so the results are displayed one page
244 @subsection Searching on the git repository at Savannah
246 You can also use the equivalent of git grep on the Savannah server.
251 Go to http://git.sv.gnu.org/gitweb/?p=lilypond.git
254 In the pulldown box that says commit, select grep.
257 Type functionName in the search box, and hit enter/return
261 This will initiate a search of the remote git repository.
267 This section describes style guidelines for LilyPond
274 * Naming conventions::
283 @subsection Languages
285 C++ and Python are preferred. Python code should use PEP 8.
289 @subsection Filenames
291 Definitions of classes that are only accessed via pointers (*) or
292 references (&) shall not be included as include files.
298 ".cc" Implementation files
299 ".icc" Inline definition files
300 ".tcc" non inline Template defs
304 (setq auto-mode-alist
305 (append '(("\\.make$" . makefile-mode)
306 ("\\.cc$" . c++-mode)
307 ("\\.icc$" . c++-mode)
308 ("\\.tcc$" . c++-mode)
309 ("\\.hh$" . c++-mode)
310 ("\\.pod$" . text-mode)
315 The class Class_name is coded in @q{class-name.*}
319 @subsection Indentation
321 Standard GNU coding style is used. In emacs:
324 (add-hook 'c++-mode-hook
325 '(lambda() (c-set-style "gnu")
329 If you like using font-lock, you can also add this to your
333 (setq font-lock-maximum-decoration t)
334 (setq c++-font-lock-keywords-3
336 c++-font-lock-keywords-3
337 '(("\\b\\(a-zA-Z_?+_\\)\\b" 1 font-lock-variable-name-face) ("\\b\\(A-Z?+a-z_?+\\)\\b" 1 font-lock-type-face))
341 Some source files may not currently have proper indenting. If this
342 is the case, it is desirable to fix the improper indenting when the
343 file is modified, with the hope of continually improving the code.
346 @subheading Indenting files with fixcc.py
348 LilyPond provides a python script that will correct the indentation
352 scripts/auxiliar/fixcc.py lily/my-test-file.cc
355 Be sure you replace @file{my@/-test@/-file@/.cc} with the name of the file
358 If you are editing a file that contains an ADD_TRANSLATOR or ADD_INTERFACE
359 macro, the fixcc.py script will move the final parenthesis up one line
360 from where it should be. Please check the end of the file before
361 you run fixcc.py, and then put the final parenthesis and semicolon
362 back on a line by themselves.
365 @subheading Indenting files with emacs in script mode
367 @c email to wl@gnu.org when I get here.
369 @warning{this is pending some confirmation on -devel. July 2009 -gp}
371 Command-line script to format stuff with emacs:
375 emacs $1 -batch --eval '(indent-region (point-min) (point-max) nil)' -f save-buffer
378 (that's all on one line)
380 Save it as a shell script, then run on the file(s) you modified.
383 @subheading Indenting with vim
385 Although emacs indentation is the LilyPond standard, acceptable
386 indentation can usually be accomplished with vim. Some hints for
398 filetype plugin indent on
400 set ignorecase smartcase
403 set statusline=%F%m%r%h%w\ %{&ff}\ %Y\ [ASCII=\%03.3b]\ [HEX=\%02.2B]\ %04l,%04v\ %p%%\ [LEN=%L]
406 " Remove trailing whitespace on write
407 autocmd BufWritePre * :%s/\s\+$//e
410 With this .vimrc, files can be reindented automatically by highlighting
411 the lines to be indented in visual mode (use V to enter visual mode)
414 A scheme.vim file will help improve the indentation. This one
415 was suggested by Patrick McCarty. It should be saved in
416 ~/.vim/after/syntax/scheme.vim.
419 " Additional Guile-specific 'forms'
420 syn keyword schemeSyntax define-public define*-public
421 syn keyword schemeSyntax define* lambda* let-keywords*
422 syn keyword schemeSyntax defmacro defmacro* define-macro
423 syn keyword schemeSyntax defmacro-public defmacro*-public
424 syn keyword schemeSyntax use-modules define-module
425 syn keyword schemeSyntax define-method define-class
427 " Additional LilyPond-specific 'forms'
428 syn keyword schemeSyntax define-markup-command define-markup-list-command
429 syn keyword schemeSyntax define-safe-public define-music-function
430 syn keyword schemeSyntax def-grace-function
432 " All of the above should influence indenting too
433 set lw+=define-public,define*-public
434 set lw+=define*,lambda*,let-keywords*
435 set lw+=defmacro,defmacro*,define-macro
436 set lw+=defmacro-public,defmacro*-public
437 set lw+=use-modules,define-module
438 set lw+=define-method,define-class
439 set lw+=define-markup-command,define-markup-list-command
440 set lw+=define-safe-public,define-music-function
441 set lw+=def-grace-function
443 " These forms should not influence indenting
447 " Try to highlight all ly: procedures
448 syn match schemeFunc "ly:[^) ]\+"
452 @node Naming conventions
453 @subsection Naming Conventions
455 Naming conventions have been established for LilyPond
458 @subheading Classes and Types
460 Classes begin with an uppercase letter, and words
461 in class names are separated with @code{_}:
469 Member variable names end with an underscore:
477 Macro names should be written in uppercase completely,
478 with words separated by @code{_}:
484 @subheading Variables
486 Variable names should be complete words, rather than abbreviations.
487 For example, it is preferred to use @code{thickness} rather than
488 @code{th} or @code{t}.
490 Multi-word variable names in C++ should have the words separated
491 by the underscore character (@q{_}):
494 cxx_multiword_variable
497 Multi-word variable names in Scheme should have the words separated
501 scheme-multiword-variable
505 @subsection Broken code
507 Do not write broken code. This includes hardwired dependencies,
508 hardwired constants, slow algorithms and obvious limitations. If
509 you can not avoid it, mark the place clearly, and add a comment
510 explaining shortcomings of the code.
512 Ideally, the comment marking the shortcoming would include
513 TODO, so that it is marked for future fixing.
515 We reject broken-in-advance on principle.
519 @subsection Code comments
521 Comments may not be needed if descriptive variable names are used
522 in the code and the logic is straightforward. However, if the
523 logic is difficult to follow, and particularly if non-obvious
524 code has been included to resolve a bug, a comment describing
525 the logic and/or the need for the non-obvious code should be included.
527 There are instances where the current code could be commented better.
528 If significant time is required to understand the code as part of
529 preparing a patch, it would be wise to add comments reflecting your
530 understanding to make future work easier.
533 @node Handling errors
534 @subsection Handling errors
536 As a general rule, you should always try to continue computations,
537 even if there is some kind of error. When the program stops, it
538 is often very hard for a user to pinpoint what part of the input
539 causes an error. Finding the culprit is much easier if there is
540 some viewable output.
542 So functions and methods do not return errorcodes, they never
543 crash, but report a programming_error and try to carry on.
545 Error and warning messages need to be localized.
549 @subsection Localization
551 This document provides some guidelines to help programmers write
553 messages. To help translations, user messages must follow
554 uniform conventions. Follow these rules when coding for LilyPond.
555 Hopefully, this can be replaced by general GNU guidelines in the
556 future. Even better would be to have an English (en_BR, en_AM)
557 guide helping programmers writing consistent messages for all GNU
560 Non-preferred messages are marked with `+'. By convention,
561 ungrammatical examples are marked with `*'. However, such ungrammatical
562 examples may still be preferred.
567 Every message to the user should be localized (and thus be marked
568 for localization). This includes warning and error messages.
571 Do not localize/gettextify:
575 `programming_error ()'s
578 `programming_warning ()'s
584 output strings (PostScript, TeX, etc.)
589 Messages to be localized must be encapsulated in `_ (STRING)' or
590 `_f (FORMAT, ...)'. E.g.:
593 warning (_ ("need music in a score"));
594 error (_f ("cannot open file: `%s'", file_name));
597 In some rare cases you may need to call `gettext ()' by hand. This
598 happens when you pre-define (a list of) string constants for later
599 use. In that case, you'll probably also need to mark these string
600 constants for translation, using `_i (STRING)'. The `_i' macro is
601 a no-op, it only serves as a marker for `xgettext'.
604 char const* messages[] = @{
605 _i ("enable debugging output"),
606 _i ("ignore lilypond version"),
613 puts (gettext (messages i));
617 See also @file{flower/@/getopt@/-long@/.cc} and @file{lily/@/main@/.cc}.
620 Do not use leading or trailing whitespace in messages. If you need
621 whitespace to be printed, prepend or append it to the translated
625 message ("Calculating line breaks..." + " ");
629 Error or warning messages displayed with a file name and line
630 number never start with a capital, eg,
633 foo.ly: 12: not a duration: 3
636 Messages containing a final verb, or a gerund (`-ing'-form) always
637 start with a capital. Other (simpler) messages start with a
643 Not declaring: `foo'.
647 Avoid abbreviations or short forms, use `cannot' and `do not'
648 rather than `can't' or `don't'
649 To avoid having a number of different messages for the same
650 situation, well will use quoting like this `"message: `%s'"' for all
651 strings. Numbers are not quoted:
654 _f ("cannot open file: `%s'", name_str)
655 _f ("cannot find character number: %d", i)
659 Think about translation issues. In a lot of cases, it is better to
660 translate a whole message. English grammar must not be imposed on the
661 translator. So, instead of
664 stem at + moment.str () + does not fit in beam
670 _f ("stem at %s does not fit in beam", moment.str ())
674 Split up multi-sentence messages, whenever possible. Instead of
677 warning (_f ("out of tune! Can't find: `%s'", "Key_engraver"));
678 warning (_f ("cannot find font `%s', loading default", font_name));
684 warning (_ ("out of tune:"));
685 warning (_f ("cannot find: `%s', "Key_engraver"));
686 warning (_f ("cannot find font: `%s', font_name));
687 warning (_f ("Loading default font"));
691 If you must have multiple-sentence messages, use full punctuation.
692 Use two spaces after end of sentence punctuation. No punctuation
693 (esp. period) is used at the end of simple messages.
696 _f ("Non-matching braces in text `%s', adding braces", text)
697 _ ("Debug output disabled. Compiled with NPRINT.")
698 _f ("Huh? Not a Request: `%s'. Ignoring.", request)
702 Do not modularize too much; words frequently cannot be translated
703 without context. It is probably safe to treat most occurrences of
704 words like stem, beam, crescendo as separately translatable words.
707 When translating, it is preferable to put interesting information
708 at the end of the message, rather than embedded in the middle.
709 This especially applies to frequently used messages, even if this
710 would mean sacrificing a bit of eloquency. This holds for original
711 messages too, of course.
714 en: cannot open: `foo.ly'
715 + nl: kan `foo.ly' niet openen (1)
716 kan niet openen: `foo.ly'* (2)
717 niet te openen: `foo.ly'* (3)
721 The first nl message, although grammatically and stylistically
722 correct, is not friendly for parsing by humans (even if they speak
723 dutch). I guess we would prefer something like (2) or (3).
726 Do not run make po/po-update with GNU gettext < 0.10.35
732 @node Debugging LilyPond
733 @section Debugging LilyPond
735 The most commonly used tool for debugging LilyPond is the GNU
736 debugger gdb. The gdb tool is used for investigating and debugging
737 core Lilypond code written in C++. Another tool is available for
738 debugging Scheme code using the Guile debugger. This section
739 describes how to use both gdb and the Guile Debugger.
742 * Debugging overview::
743 * Debugging C++ code::
744 * Debugging Scheme code::
747 @node Debugging overview
748 @subsection Debugging overview
750 Using a debugger simplifies troubleshooting in at least two ways.
752 First, breakpoints can be set to pause execution at any desired point.
753 Then, when execution has paused, debugger commands can be issued to
754 explore the values of various variables or to execute functions.
756 Second, the debugger can display a stack trace, which shows the
757 sequence in which functions have been called and the arguments
758 passed to the called functions.
760 @node Debugging C++ code
761 @subsection Debugging C++ code
763 The GNU debugger, gdb, is the principal tool for debugging C++ code.
765 @subheading Compiling LilyPond for use with gdb
767 In order to use gdb with LilyPond, it is necessary to compile
768 LilyPond with debugging information. This is accomplished by running
769 the following commands in the main LilyPond source directory.
772 ./configure --disable-optimising
776 This will create a version of LilyPond containing debugging
777 information that will allow the debugger to tie the source code
778 to the compiled code.
780 You should not do @var{make install} if you want to use a debugger
781 with LilyPond. The @var{make install} command will strip debugging
782 information from the LilyPond binary.
784 @subheading Typical gdb usage
786 Once you have compiled the Lilypond image with the necessary
787 debugging information it will have been written to a location in a
788 subfolder of your current working directory:
794 This is important as you will need to let gdb know where to find the
795 image containing the symbol tables. You can invoke gdb from the
796 command line using the following:
802 This loads the LilyPond symbol tables into gdb. Then, to run
803 LilyPond on @file{test@/.ly} under the debugger, enter the following:
812 As an alternative to running gdb at the command line you may try
813 a graphical interface to gdb such as ddd:
819 You can also use sets of standard gdb commands stored in a .gdbinit
820 file (see next section).
822 @subheading Typical .gdbinit files
824 The behavior of gdb can be readily customized through the use of a
825 @var{.gdbinit} file. A @var{.gdbinit} file is a file named
826 @var{.gdbinit} (notice the @qq{.} at the beginning of the file name)
827 that is placed in a user's home directory.
829 The @var{.gdbinit} file below is from Han-Wen. It sets breakpoints
830 for all errors and defines functions for displaying scheme objects
831 (ps), grobs (pgrob), and parsed music expressions (pmusic).
834 file lily/out/lilypond
836 b Grob::programming_error
839 print ly_display_scm($arg0)
842 print ly_display_scm($arg0->self_scm_)
843 print ly_display_scm($arg0->mutable_property_alist_)
844 print ly_display_scm($arg0->immutable_property_alist_)
845 print ly_display_scm($arg0->object_alist_)
848 print ly_display_scm($arg0->self_scm_)
849 print ly_display_scm($arg0->mutable_property_alist_)
850 print ly_display_scm($arg0->immutable_property_alist_)
854 @node Debugging Scheme code
855 @subsection Debugging Scheme code
857 Scheme code can be developed using the Guile command line
858 interpreter @code{top-repl}. You can either investigate
859 interactively using just Guile or you can use the debugging
860 tools available within Guile.
862 @subheading Using Guile interactively with LilyPond
864 In order to experiment with Scheme programming in the LilyPond
865 environment, it is necessary to have a Guile interpreter that
866 has all the LilyPond modules loaded. This requires the following
869 First, define a Scheme symbol for the active module in the @file{@/.ly} file:
872 #(module-define! (resolve-module '(guile-user))
873 'lilypond-module (current-module))
876 Now place a Scheme function in the @file{@/.ly} file that gives an
877 interactive Guile prompt:
883 When the @file{@/.ly} file is compiled, this causes the compilation to be
884 interrupted and an interactive guile prompt to appear. Once the
885 guile prompt appears, the LilyPond active module must be set as the
886 current guile module:
889 guile> (set-current-module lilypond-module)
892 You can demonstrate these commands are operating properly by typing the name
893 of a LilyPond public scheme function to check it has been defined:
896 guile> fret-diagram-verbose-markup
897 #<procedure fret-diagram-verbose-markup (layout props marking-list)>
900 If the LilyPond module has not been correctly loaded, an error
901 message will be generated:
904 guile> fret-diagram-verbose-markup
905 ERROR: Unbound variable: fret-diagram-verbose-markup
906 ABORT: (unbound-variable)
909 Once the module is properly loaded, any valid LilyPond Scheme
910 expression can be entered at the interactive prompt.
912 After the investigation is complete, the interactive guile
913 interpreter can be exited:
919 The compilation of the @file{@/.ly} file will then continue.
921 @subheading Using the Guile debugger
923 To set breakpoints and/or enable tracing in Scheme functions, put
926 \include "guile-debugger.ly"
929 in your input file after any scheme procedures you have defined in
930 that file. This will invoke the Guile command-line after having set
931 up the environment for the debug command-line. When your input file
932 is processed, a guile prompt will be displayed. You may now enter
933 commands to set up breakpoints and enable tracing by the Guile debugger.
935 @subheading Using breakpoints
937 At the guile prompt, you can set breakpoints with
938 the @code{set-break!} procedure:
941 guile> (set-break! my-scheme-procedure)
944 Once you have set the desired breakpoints, you exit the guile repl frame
951 Then, when one of the scheme routines for which you have set
952 breakpoints is entered, guile will interrupt execution in a debug
953 frame. At this point you will have access to Guile debugging
954 commands. For a listing of these commands, type:
960 Alternatively you may code the breakpoints in your Lilypond source
961 file using a command such as:
964 #(set-break! my-scheme-procedure)
967 immediately after the @code{\include} statement. In this case the
968 breakpoint will be set straight after you enter the @code{(quit)}
969 command at the guile prompt.
971 Embedding breakpoint commands like this is particularly useful if
972 you want to look at how the Scheme procedures in the @file{@/.scm}
973 files supplied with LilyPond work. To do this, edit the file in
974 the relevant directory to add this line near the top:
977 (use-modules (scm guile-debugger))
980 Now you can set a breakpoint after the procedure you are interested
981 in has been declared. For example, if you are working on routines
982 called by @var{print-book-with} in @file{lily@/-library@/.scm}:
985 (define (print-book-with parser book process-procedure)
986 (let* ((paper (ly:parser-lookup parser '$defaultpaper))
987 (layout (ly:parser-lookup parser '$defaultlayout))
988 (outfile-name (get-outfile-name parser)))
989 (process-procedure book paper layout outfile-name)))
991 (define-public (print-book-with-defaults parser book)
992 (print-book-with parser book ly:book-process))
994 (define-public (print-book-with-defaults-as-systems parser book)
995 (print-book-with parser book ly:book-process-to-systems))
999 At this point in the code you could add this to set a breakpoint at
1003 (set-break! print-book-with)
1006 @subheading Tracing procedure calls and evaluator steps
1008 Two forms of trace are available:
1011 (set-trace-call! my-scheme-procedure)
1017 (set-trace-subtree! my-scheme-procedure)
1020 @code{set-trace-call!} causes Scheme to log a line to the standard
1021 output to show when the procedure is called and when it exits.
1023 @code{set-trace-subtree!} traces every step the Scheme evaluator
1024 performs in evaluating the procedure.
1026 @node Tracing object relationships
1027 @section Tracing object relationships
1029 Understanding the LilyPond source often boils down to figuring out what
1030 is happening to the Grobs. Where (and why) are they being created,
1031 modified and destroyed? Tracing Lily through a debugger in order to
1032 identify these relationships can be time-consuming and tedious.
1034 In order to simplify this process, a facility has been added to
1035 display the grobs that are created and the properties that are set
1036 and modified. Although it can be complex to get set up, once set up
1037 it easily provides detailed information about the life of grobs
1038 in the form of a network graph.
1040 Each of the steps necessary to use the graphviz utility
1045 @item Installing graphviz
1047 In order to create the graph of the object relationships, it is
1048 first necessary to install Graphviz. graphviz is available for a
1049 number of different platforms:
1052 @uref{http://www.graphviz.org/Download..php}
1055 @item Modifying config.make
1057 In order for the Graphviz tool to work, config.make must be modified.
1058 It is probably a good idea to first save a copy of config.make under
1059 a different name. Then, edit config.make by removing every occurrence
1062 @item Rebuilding LilyPond
1064 The executable code of LilyPond must be rebuilt from scratch:
1067 make -C lily clean && make -C lily
1070 @item Create a graphviz-compatible @file{@/.ly} file
1072 In order to use the graphviz utility, the @file{@/.ly} file must include
1073 @file{ly/@/graphviz@/-init@/.ly}, and should then specify the
1074 grobs and symbols that should be tracked. An example of this
1075 is found in @file{input/@/regression/@/graphviz@/.ly}.
1077 @item Run lilypond with output sent to a log file
1079 The Graphviz data is sent to stderr by lilypond, so it is
1080 necessary to redirect stderr to a logfile:
1083 lilypond graphviz.ly 2> graphviz.log
1086 @item Edit the logfile
1088 The logfile has standard lilypond output, as well as the Graphviz
1089 output data. Delete everything from the beginning of the file
1090 up to but not including the first occurrence of @code{digraph}.
1092 @item Process the logfile with @code{dot}
1094 The directed graph is created from the log file with the program
1098 dot -Tpdf graphviz.log > graphviz.pdf
1103 The pdf file can then be viewed with any pdf viewer.
1105 When compiled without @code{-DNDEBUG}, lilypond may run slower
1106 than normal. The original configuration can be restored by either
1107 renaming the saved copy of @code{config.make} or rerunning
1108 @code{configure}. Then rebuild lilypond with
1111 make -C lily clean && make -C lily
1115 @node Adding or modifying features
1116 @section Adding or modifying features
1118 When a new feature is to be added to LilyPond, it is necessary to
1119 ensure that the feature is properly integrated to maintain
1120 its long-term support. This section describes the steps necessary
1121 for feature addition and modification.
1126 * Write regression tests::
1127 * Write convert-ly rule::
1128 * Automatically update documentation::
1129 * Manually update documentation::
1130 * Edit changes.tely::
1131 * Verify successful build::
1132 * Verify regression tests::
1133 * Post patch for comments::
1135 * Closing the issues::
1138 @node Write the code
1139 @subsection Write the code
1141 You should probably create a new git branch for writing the code, as that
1142 will separate it from the master branch and allow you to continue
1143 to work on small projects related to master.
1145 Please be sure to follow the rules for programming style discussed
1146 earlier in this chapter.
1149 @node Write regression tests
1150 @subsection Write regression tests
1152 In order to demonstrate that the code works properly, you will
1153 need to write one or more regression tests. These tests are
1154 typically @file{@/.ly} files that are found in @file{input/@/regression}.
1156 Regression tests should be as brief as possible to demonstrate the
1157 functionality of the code.
1159 Regression tests should generally cover one issue per test. Several
1160 short, single-issue regression tests are preferred to a single, long,
1161 multiple-issue regression test.
1163 Use existing regression tests as templates to demonstrate the type of
1164 header information that should be included in a regression test.
1167 @node Write convert-ly rule
1168 @subsection Write convert-ly rule
1170 If the modification changes the input syntax, a convert-ly rule
1171 should be written to automatically update input files from older
1174 convert-ly rules are found in python/convertrules.py
1176 If possible, the convert-ly rule should allow automatic updating
1177 of the file. In some cases, this will not be possible, so the
1178 rule will simply point out to the user that the feature needs
1181 @subsubheading Updating version numbers
1183 If a development release occurs between you writing your patch and
1184 having it approved+pushed, you will need to update the version
1185 numbers in your tree. This can be done with:
1188 scripts/auxiliar/update-patch-version old.version.number new.version.number
1191 It will change all files in git, so use with caution and examine
1195 @node Automatically update documentation
1196 @subsection Automatically update documentation
1198 @command{convert-ly} should be used to update the documentation,
1199 the snippets, and the regression tests. This not only makes the
1200 necessary syntax changes, it also tests the @command{convert-ly}
1203 The automatic updating is performed by moving to the top-level
1204 source directory, then running:
1207 scripts/auxiliar/update-with-convert-ly.sh
1210 If you did an out-of-tree build, pass in the relative path:
1213 BUILD_DIR=../build-lilypond/ scripts/auxiliar/update-with-convert-ly.sh
1217 @node Manually update documentation
1218 @subsection Manually update documentation
1220 Where the convert-ly rule is not able to automatically update the inline
1221 lilypond code in the documentation (i.e. if a NOT_SMART rule is used), the
1222 documentation must be manually updated. The inline snippets that require
1223 changing must be changed in the English version of the docs and all
1224 translated versions. If the inline code is not changed in the
1225 translated documentation, the old snippets will show up in the
1226 English version of the documentation.
1228 Where the convert-ly rule is not able to automatically update snippets
1229 in Documentation/snippets/, those snippets must be manually updated.
1230 Those snippets should be copied to Documentation/snippets/new. The
1231 comments at the top of the snippet describing its automatic generation
1232 should be removed. All translated texidoc strings should be removed.
1233 The comment @qq{% begin verbatim} should be removed. The syntax of
1234 the snippet should then be manually edited.
1236 Where snippets in Documentation/snippets are made obsolete, the snippet
1237 should be copied to Documentation/snippets/new. The comments and
1238 texidoc strings should be removed as described above. Then the body
1239 of the snippet should be changed to:
1243 This snippet is deprecated as of version X.Y.Z and
1244 will be removed from the documentation.
1249 where X.Y.Z is the version number for which the convert-ly rule was
1252 Update the snippet files by running:
1255 scripts/auxiliar/makelsr.py
1258 Where the convert-ly rule is not able to automatically update regression
1259 tests, the regression tests in input/regression should be manually
1262 Although it is not required, it is helpful if the developer
1263 can write relevant material for inclusion in the Notation
1264 Reference. If the developer does not feel qualified to write
1265 the documentation, a documentation editor will be able to
1266 write it from the regression tests. The text that is added to
1267 or removed from the documentation should be changed only in
1268 the English version.
1271 @node Edit changes.tely
1272 @subsection Edit changes.tely
1274 An entry should be added to Documentation/changes.tely to describe
1275 the feature changes to be implemented. This is especially important
1276 for changes that change input file syntax.
1278 Hints for changes.tely entries are given at the top of the file.
1280 New entries in changes.tely go at the top of the file.
1282 The changes.tely entry should be written to show how the new change
1283 improves LilyPond, if possible.
1286 @node Verify successful build
1287 @subsection Verify successful build
1289 When the changes have been made, successful completion must be
1297 When these commands complete without error, the patch is
1298 considered to function successfully.
1300 Developers on Windows who are unable to build LilyPond should
1301 get help from a Linux or OSX developer to do the make tests.
1304 @node Verify regression tests
1305 @subsection Verify regression tests
1307 In order to avoid breaking LilyPond, it is important to verify that
1308 the regression tests succeed, and that no unwanted changes are
1309 introduced into the output. This process is described in
1310 @ref{Identifying code regressions}.
1312 @subheading Typical developer's edit/compile/test cycle
1314 TODO: is @code{[-j@var{X} CPU_COUNT=@var{X}]} useful for
1315 @code{test-baseline}, @code{check}, @code{clean},
1316 @code{test-redo}? Neil Puttock says it is useful for
1317 everything but @code{clean}, which is disk-limited.
1318 Need to check formally.
1327 make [-j@var{X} CPU_COUNT=@var{X}] check
1331 Edit/compile/test cycle:
1334 @emph{## edit source files, then...}
1336 make clean @emph{## only if needed (see below)}
1337 make [-j@var{X}] @emph{## only if needed (see below)}
1338 make test-redo @emph{## redo files differing from baseline}
1339 make [-j@var{X} CPU_COUNT=@var{X}] check @emph{## CPU_COUNT here?}
1350 If you modify any source files that have to be compiled (such as
1351 @file{@/.cc} or @file{@/.hh} files in @file{flower/@/} or @file{lily/@/}),
1352 then you must run @command{make} before @command{make test-redo},
1353 so @command{make} can compile the modified files and relink all
1354 the object files. If you only modify files which are interpreted,
1355 like those in the @file{scm/@/} and @file{ly/@/} directories, then
1356 @command{make} is not needed before @command{make test-redo}.
1358 TODO: Fix the following paragraph. You can do @command{rm mf/out/*}
1359 instead of make clean, and you can probably do
1360 @command{make -C mf/ clean} as well, but I haven't checked it -- cds
1362 Also, if you modify any font definitions in the @file{mf/@/}
1363 directory then you must run @command{make clean} and
1364 @command{make} before running @command{make test-redo}. This will
1365 recompile everything, whether modified or not, and takes a lot
1368 Running @command{make@tie{}check} will leave an HTML page
1369 @file{out/@/test@/-results/@/index@/.html}. This page shows all the
1370 important differences that your change introduced, whether in the
1371 layout, MIDI, performance or error reporting.
1376 @node Post patch for comments
1377 @subsection Post patch for comments
1379 See @ref{Uploading a patch for review}.
1383 @subsection Push patch
1385 Once all the comments have been addressed, the patch can be pushed.
1387 If the author has push privileges, the author will push the patch.
1388 Otherwise, a developer with push privileges will push the patch.
1391 @node Closing the issues
1392 @subsection Closing the issues
1394 Once the patch has been pushed, all the relevant issues should be
1397 On Rietveld, the author should log in an close the issue either by
1398 using the @q{Edit Issue} link, or by clicking the circled x icon
1399 to the left of the issue name.
1401 If the changes were in response to a feature request on the Google
1402 issue tracker for LilyPond, the author should change the status to
1403 Fixed and a tag @q{fixed_x_y_z} should be added, where the patch was
1404 fixed in version x.y.z. If
1405 the author does not have privileges to change the status, an email
1406 should be sent to bug-lilypond requesting the BugMeister to change
1410 @node Iterator tutorial
1411 @section Iterator tutorial
1413 TODO -- this is a placeholder for a tutorial on iterators
1415 Iterators are routines written in C++ that process music expressions
1416 and sent the music events to the appropriate engravers and/or
1420 @node Engraver tutorial
1421 @section Engraver tutorial
1423 Engravers are C++ classes that catch music events and
1424 create the appropriate grobs for display on the page. Though the
1425 majority of engravers are responsible for the creation of a single grob,
1426 in some cases (e.g. @code{New_fingering_engraver}), several different grobs
1429 Engravers listen for events and acknowledge grobs. Events are passed to
1430 the engraver in time-step order during the iteration phase. Grobs are
1431 made available to the engraver when they are created by other engravers
1432 during the iteration phase.
1436 * Useful methods for information processing::
1437 * Translation process::
1438 * Preventing garbage collection for SCM member variables::
1439 * Listening to music events::
1440 * Acknowledging grobs::
1441 * Engraver declaration/documentation::
1444 @node Useful methods for information processing
1445 @subsection Useful methods for information processing
1447 An engraver inherits the following public methods from the Translator
1448 base class, which can be used to process listened events and acknowledged
1452 @item @code{virtual void initialize ()}
1453 @item @code{void start_translation_timestep ()}
1454 @item @code{void process_music ()}
1455 @item @code{void process_acknowledged ()}
1456 @item @code{void stop_translation_timestep ()}
1457 @item @code{virtual void finalize ()}
1460 These methods are listed in order of translation time, with
1461 @code{initialize ()} and @code{finalize ()} bookending the whole
1462 process. @code{initialize ()} can be used for one-time initialization
1463 of context properties before translation starts, whereas
1464 @code{finalize ()} is often used to tie up loose ends at the end of
1465 translation: for example, an unterminated spanner might be completed
1466 automatically or reported with a warning message.
1469 @node Translation process
1470 @subsection Translation process
1472 At each timestep in the music, translation proceeds by calling the
1473 following methods in turn:
1475 @code{start_translation_timestep ()} is called before any user
1476 information enters the translators, i.e., no property operations
1477 (\set, \override, etc.) or events have been processed yet.
1479 @code{process_music ()} and @code{process_acknowledged ()} are called
1480 after all events in the current time step have been heard, or all
1481 grobs in the current time step have been acknowledged. The latter
1482 tends to be used exclusively with engravers which only acknowledge
1483 grobs, whereas the former is the default method for main processing
1486 @code{stop_translation_timestep ()} is called after all user
1487 information has been processed prior to beginning the translation for
1491 @node Preventing garbage collection for SCM member variables
1492 @subsection Preventing garbage collection for SCM member variables
1494 In certain cases, an engraver might need to ensure private Scheme
1495 variables (with type SCM) do not get swept away by Guile's garbage
1496 collector: for example, a cache of the previous key signature which
1497 must persist between timesteps. The method
1498 @code{virtual derived_mark () const} can be used in such cases:
1501 Engraver_name::derived_mark ()
1503 scm_gc_mark (private_scm_member_)
1508 @node Listening to music events
1509 @subsection Listening to music events
1511 External interfaces to the engraver are implemented by protected
1512 macros including one or more of the following:
1515 @item @code{DECLARE_TRANSLATOR_LISTENER (event_name)}
1516 @item @code{IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)}
1520 where @var{event_name} is the type of event required to provide the
1521 input the engraver needs and @var{Engraver_name} is the name of the
1524 Following declaration of a listener, the method is implemented as follows:
1527 IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)
1529 Engraver_name::listen_event_name (Stream event *event)
1531 ...body of listener method...
1536 @node Acknowledging grobs
1537 @subsection Acknowledging grobs
1539 Some engravers also need information from grobs as they are created
1540 and as they terminate. The mechanism and methods to obtain this
1541 information are set up by the macros:
1544 @item @code{DECLARE_ACKNOWLEDGER (grob_interface)}
1545 @item @code{DECLARE_END_ACKNOWLEDGER (grob_interface)}
1548 where @var{grob_interface} is an interface supported by the
1549 grob(s) which should be acknowledged. For example, the following
1550 code would declare acknowledgers for a @code{NoteHead} grob (via the
1551 @code{note-head-interface}) and any grobs which support the
1552 @code{side-position-interface}:
1555 @code{DECLARE_ACKNOWLEDGER (note_head)}
1556 @code{DECLARE_ACKNOWLEDGER (side_position)}
1559 The @code{DECLARE_END_ACKNOWLEDGER ()} macro sets up a spanner-specific
1560 acknowledger which will be called whenever a spanner ends.
1562 Following declaration of an acknowledger, the method is coded as follows:
1566 Engraver_name::acknowledge_interface_name (Grob_info info)
1568 ...body of acknowledger method...
1573 @node Engraver declaration/documentation
1574 @subsection Engraver declaration/documentation
1576 An engraver must have a public macro
1579 @item @code{TRANSLATOR_DECLARATIONS (Engraver_name)}
1583 where @code{Engraver_name} is the name of the engraver. This
1584 defines the common variables and methods used by every engraver.
1586 At the end of the engraver file, one or both of the following
1587 macros are generally called to document the engraver in the
1588 Internals Reference:
1591 @item @code{ADD_ACKNOWLEDGER (Engraver_name, grob_interface)}
1592 @item @code{ADD_TRANSLATOR (Engraver_name, Engraver_doc,
1593 Engraver_creates, Engraver_reads, Engraver_writes)}
1597 where @code{Engraver_name} is the name of the engraver, @code{grob_interface}
1598 is the name of the interface that will be acknowledged,
1599 @code{Engraver_doc} is a docstring for the engraver,
1600 @code{Engraver_creates} is the set of grobs created by the engraver,
1601 @code{Engraver_reads} is the set of properties read by the engraver,
1602 and @code{Engraver_writes} is the set of properties written by
1605 The @code{ADD_ACKNOWLEDGER} and @code{ADD_TRANSLATOR} macros use a
1606 non-standard indentation system. Each interface, grob, read property,
1607 and write property is on its own line, and the closing parenthesis
1608 and semicolon for the macro all occupy a separate line beneath the final
1609 interface or write property. See existing engraver files for more
1613 @node Callback tutorial
1614 @section Callback tutorial
1616 TODO -- This is a placeholder for a tutorial on callback functions.
1618 @node LilyPond scoping
1619 @section LilyPond scoping
1621 The Lilypond language has a concept of scoping, i.e. you can do
1627 (display (+ foo 2)))
1630 @noindent with @code{\paper}, @code{\midi} and @code{\header} being
1631 nested scope inside the @file{@/.ly} file-level scope. @w{@code{foo = 1}}
1632 is translated in to a scheme variable definition.
1634 This implemented using modules, with each scope being an anonymous
1635 module that imports its enclosing scope's module.
1637 Lilypond's core, loaded from @file{@/.scm} files, is usually placed in the
1638 @code{lily} module, outside the @file{@/.ly} level. In the case of
1645 we want to reuse the built-in definitions, without changes effected in
1646 user-level @file{a@/.ly} leaking into the processing of @file{b@/.ly}.
1648 The user-accessible definition commands have to take care to avoid
1649 memory leaks that could occur when running multiple files. All
1650 information belonging to user-defined commands and markups is stored in
1651 a manner that allows it to be garbage-collected when the module is
1652 dispersed, either by being stored module-locally, or in weak hash
1655 @node LilyPond miscellany
1656 @section LilyPond miscellany
1658 This is a place to dump information that may be of use to developers
1659 but doesn't yet have a proper home. Ideally, the length of this section
1660 would become zero as items are moved to other homes.
1664 * Spacing algorithms::
1665 * Info from Han-Wen email::
1666 * Music functions and GUILE debugging::
1669 @node Spacing algorithms
1670 @subsection Spacing algorithms
1672 Here is information from an email exchange about spacing algorithms.
1674 On Thu, 2010-02-04 at 15:33 -0500, Boris Shingarov wrote:
1675 I am experimenting with some modifications to the line breaking code,
1676 and I am stuck trying to understand how some of it works. So far my
1677 understanding is that Simple_spacer operates on a vector of Grobs, and
1678 it is a well-known Constrained-QP problem (rods = constraints, springs
1679 = quadratic function to minimize). What I don't understand is, if the
1680 spacer operates at the level of Grobs, which are built at an earlier
1681 stage in the pipeline, how are the changes necessitated by differences
1682 in line breaking, taken into account? in other words, if I take the
1683 last measure of a line and place it on the next line, it is not just a
1684 matter of literally moving that graphic to where the start of the next
1685 line is, but I also need to draw a clef, key signature, and possibly
1686 other fundamental things -- but at that stage in the rendering
1687 pipeline, is it not too late??
1689 Joe Neeman answered:
1691 We create lots of extra grobs (eg. a BarNumber at every bar line) but
1692 most of them are not drawn. See the break-visibility property in
1696 @node Info from Han-Wen email
1697 @subsection Info from Han-Wen email
1699 In 2004, Douglas Linhardt decided to try starting a document that would
1700 explain LilyPond architecture and design principles. The material below
1701 is extracted from that email, which can be found at
1702 @uref{http://thread.gmane.org/gmane.comp.gnu.lilypond.devel/2992}.
1703 The headings reflect questions from Doug or comments from Han-Wen;
1704 the body text are Han-Wen's answers.
1706 @subheading Figuring out how things work.
1708 I must admit that when I want to know how a program works, I use grep
1709 and emacs and dive into the source code. The comments and the code
1710 itself are usually more revealing than technical documents.
1712 @subheading What's a grob, and how is one used?
1714 Graphical object - they are created from within engravers, either as
1715 Spanners (derived class) -slurs, beams- or Items (also a derived
1716 class) -notes, clefs, etc.
1718 There are two other derived classes System (derived from Spanner,
1719 containing a "line of music") and Paper_column (derived from Item, it
1720 contains all items that happen at the same moment). They are separate
1721 classes because they play a special role in the linebreaking process.
1723 @subheading What's a smob, and how is one used?
1725 A C(++) object that is encapsulated so it can be used as a Scheme
1726 object. See GUILE info, "19.3 Defining New Types (Smobs)"
1728 @@subheading When is each C++ class constructed and used
1735 In the parser.yy see the macro calls MAKE_MUSIC_BY_NAME().
1740 Constructed during "interpreting" phase.
1745 Executive branch of Contexts, plugins that create grobs, usually one
1746 engraver per grob type. Created together with context.
1756 These are not C++ classes per se. The idea of a Grob interface hasn't
1757 crystallized well. ATM, an interface is a symbol, with a bunch of grob
1758 properties. They are not objects that are created or destroyed.
1763 Objects that walk through different music classes, and deliver events
1764 in a synchronized way, so that notes that play together are processed
1765 at the same moment and (as a result) end up on the same horizontal position.
1767 Created during interpreting phase.
1769 BTW, the entry point for interpreting is ly:run-translator
1770 (ly_run_translator on the C++ side)
1774 @subheading Can you get to Context properties from a Music object?
1776 You can create music object with a Scheme function that reads context
1777 properties (the \applycontext syntax). However, that function is
1778 executed during Interpreting, so you can not really get Context
1779 properties from Music objects, since music objects are not directly
1780 connected to Contexts. That connection is made by the Music_iterators
1782 @subheading Can you get to Music properties from a Context object?
1784 Yes, if you are given the music object within a Context
1785 object. Normally, the music objects enter Contexts in synchronized
1786 fashion, and the synchronization is done by Music_iterators.
1788 @subheading What is the relationship between C++ classes and Scheme objects?
1790 Smobs are C++ objects in Scheme. Scheme objects (lists, functions) are
1791 manipulated from C++ as well using the GUILE C function interface
1794 @subheading How do Scheme procedures get called from C++ functions?
1796 scm_call_*, where * is an integer from 0 to 4.
1797 Also scm_c_eval_string (), scm_eval ()
1799 @subheading How do C++ functions get called from Scheme procedures?
1801 Export a C++ function to Scheme with LY_DEFINE.
1803 @subheading What is the flow of control in the program?
1805 Good question. Things used to be clear-cut, but we have Scheme
1806 and SMOBs now, which means that interactions do not follow a very
1807 rigid format anymore. See below for an overview, though.
1809 @subheading Does the parser make Scheme procedure calls or C++ function calls?
1811 Both. And the Scheme calls can call C++ and vice versa. It's nested,
1812 with the SCM datatype as lubrication between the interactions
1814 (I think the word "lubrication" describes the process better than the
1815 traditional word "glue")
1817 @subheading How do the front-end and back-end get started?
1819 Front-end: a file is parsed, the rest follows from that. Specifically,
1821 Parsing leads to a Music + Music_output_def object (see parser.yy,
1822 definition of toplevel_expression )
1824 A Music + Music_output_def object leads to a Global_context object (see
1825 ly_run_translator ())
1827 During interpreting, Global_context + Music leads to a bunch of
1828 Contexts (see Global_translator::run_iterator_on_me ()).
1830 After interpreting, Global_context contains a Score_context (which
1831 contains staves, lyrics etc.) as a child. Score_context::get_output ()
1832 spews a Music_output object (either a Paper_score object for notation
1833 or Performance object for MIDI).
1835 The Music_output object is the entry point for the backend (see
1836 ly_render_output ()).
1838 The main steps of the backend itself are in
1843 @file{paper@/-score@/.cc} , Paper_score::process_
1846 @file{system@/.cc} , System::get_lines()
1849 The step, where things go from grobs to output, is in
1850 System::get_line(): each grob delivers a Stencil (a Device
1851 independent output description), which is interpreted by our
1852 outputting backends (@file{scm/@/output@/-tex@/.scm} and
1853 @file{scm/@/output@/-ps@/.scm}) to produce TeX and PS.
1857 Interactions between grobs and putting things into .tex and .ps files
1858 have gotten a little more complex lately. Jan has implemented
1859 page-breaking, so now the backend also involves Paper_book,
1860 Paper_lines and other things. This area is still heavily in flux, and
1861 perhaps not something you should want to look at.
1863 @subheading How do the front-end and back-end communicate?
1865 There is no communication from backend to front-end. From front-end to
1866 backend is simply the program flow: music + definitions gives
1867 contexts, contexts yield output, after processing, output is written
1870 @subheading Where is the functionality associated with KEYWORDs?
1872 See @file{my@/-lily@/-lexer@/.cc} (keywords, there aren't that many)
1873 and @file{ly/@/@/*@/.ly} (most of the other backslashed @code{@/\words} are identifiers)
1875 @subheading What Contexts/Properties/Music/etc. are available when they are processed?
1877 What do you mean exactly with this question?
1879 See @file{ly/@/engraver@/-init@/.ly} for contexts,
1880 see @file{scm/@/define@/-@/*@/.scm} for other objects.
1882 @subheading How do you decide if something is a Music, Context, or Grob property?
1883 Why is part-combine-status a Music property when it seems (IMO)
1884 to be related to the Staff context?
1886 The Music_iterators and Context communicate through two channels
1888 Music_iterators can set and read context properties, idem for
1889 Engravers and Contexts
1891 Music_iterators can send "synthetic" music events (which aren't in
1892 the input) to a context. These are caught by Engravers. This is
1893 mostly a one way communication channel.
1895 part-combine-status is part of such a synthetic event, used by
1896 Part_combine_iterator to communicate with Part_combine_engraver.
1899 @subheading Deciding between context and music properties
1901 I'm adding a property to affect how \autochange works. It seems to
1902 me that it should be a context property, but the Scheme autochange
1903 procedure has a Music argument. Does this mean I should use
1906 \autochange is one of these extra strange beasts: it requires
1907 look-ahead to decide when to change staves. This is achieved by
1908 running the interpreting step twice (see
1909 @file{scm/@/part@/-combiner@/.scm} , at the bottom), and
1910 storing the result of the first step (where to switch
1911 staves) in a Music property. Since you want to influence that
1912 where-to-switch list, your must affect the code in
1913 make-autochange-music (@file{scm/@/part@/-combiner@/.scm}).
1914 That code is called directly from the parser and there are no
1915 official "parsing properties" yet, so there is no generic way
1916 to tune \autochange. We would have to invent something new
1917 for this, or add a separate argument,
1920 \autochange #around-central-C ..music..
1924 where around-central-C is some function that is called from
1925 make-autochange-music.
1927 @subheading More on context and music properties
1929 From Neil Puttock, in response to a question about transposition:
1931 Context properties (using \set & \unset) are tied to engravers: they
1932 provide information relevant to the generation of graphical objects.
1934 Since transposition occurs at the music interpretation stage, it has
1935 no direct connection with engravers: the pitch of a note is fixed
1936 before a notehead is created. Consider the following minimal snippet:
1942 This generates (simplified) a NoteEvent, with its pitch and duration
1943 as event properties,
1949 (ly:make-duration 2 0 1 1)
1951 (ly:make-pitch 0 0 0)
1954 which the Note_heads_engraver hears. It passes this information on to
1955 the NoteHead grob it creates from the event, so the head's correct
1956 position and duration-log can be determined once it's ready for
1959 If we transpose the snippet,
1962 \transpose c d @{ c' @}
1965 the pitch is changed before it reaches the engraver (in fact, it
1966 happens just after the parsing stage with the creation of a
1967 TransposedMusic music object):
1973 (ly:make-duration 2 0 1 1)
1975 (ly:make-pitch 0 1 0)
1978 You can see an example of a music property relevant to transposition:
1982 \transpose c d @{ c'2 \withMusicProperty #'untransposable ##t c' @}
1985 -> the second c' remains untransposed.
1987 Take a look at @file{lily/@/music@/.cc} to see where the transposition takes place.
1990 @subheading How do I tell about the execution environment?
1992 I get lost figuring out what environment the code I'm looking at is in when it
1993 executes. I found both the C++ and Scheme autochange code. Then I was trying
1994 to figure out where the code got called from. I finally figured out that the
1995 Scheme procedure was called before the C++ iterator code, but it took me a
1996 while to figure that out, and I still didn't know who did the calling in the
1997 first place. I only know a little bit about Flex and Bison, so reading those
1998 files helped only a little bit.
2000 @emph{Han-Wen:} GDB can be of help here. Set a breakpoint in C++, and run. When you
2001 hit the breakpoint, do a backtrace. You can inspect Scheme objects
2002 along the way by doing
2005 p ly_display_scm(obj)
2008 this will display OBJ through GUILE.
2010 @node Music functions and GUILE debugging
2011 @subsection Music functions and GUILE debugging
2013 Ian Hulin was trying to do some debugging in music functions, and
2014 came up with the following question
2017 I'm working on the Guile Debugger Stuff, and would like to try
2018 debugging a music function definition such as:
2021 conditionalMark = #(define-music-function (parser location) ()
2022 #@{ \tag #'instrumental-part @{\mark \default@} #@} )
2025 It appears conditionalMark does not get set up as an
2026 equivalent of a Scheme
2029 (define conditionalMark = define-music-function(parser location () ...
2033 although something gets defined because Scheme apparently recognizes
2036 #(set-break! conditionalMark)
2040 later on in the file without signalling any Guile errors.
2042 However the breakpoint trap is never encountered as
2043 define-music-function passed things on to ly:make-music-function,
2044 which is really C++ code ly_make_music_function, so Guile never
2045 finds out about the breakpoint.
2047 Han-Wen answered as follows:
2049 You can see the definition by doing
2052 #(display conditionalMark)
2056 inside the @file{@/.ly} file.
2058 The breakpoint failing may have to do with the call sequence. See
2059 @file{parser@/.yy}, run_music_function(). The function is called directly from
2060 C++, without going through the GUILE evaluator, so I think that is why
2061 there is no debugger trap.