1 @c -*- coding: utf-8; mode: texinfo; -*-
3 @chapter Programming work
6 * Overview of LilyPond architecture::
7 * LilyPond programming languages::
8 * Programming without compiling::
11 * Warnings Errors Progress and Debug Output::
12 * Debugging LilyPond::
13 * Tracing object relationships::
14 * Adding or modifying features::
19 * LilyPond miscellany::
22 @node Overview of LilyPond architecture
23 @section Overview of LilyPond architecture
25 LilyPond processes the input file into graphical and musical output in a
26 number of stages. This process, along with the types of routines that
27 accomplish the various stages of the process, is described in this section. A
28 more complete description of the LilyPond architecture and internal program
29 execution is found in Erik Sandberg's
30 @uref{http://lilypond.org/web/images/thesis-erik-sandberg.pdf, master's
33 The first stage of LilyPond processing is @emph{parsing}. In the parsing
34 process, music expressions in LilyPond input format are converted to music
35 expressions in Scheme format. In Scheme format, a music expression is a list
36 in tree form, with nodes that indicate the relationships between various music
37 events. The LilyPond parser is written in Bison.
39 The second stage of LilyPond processing is @emph{iterating}. Iterating
40 assigns each music event to a context, which is the environment in which the
41 music will be finally engraved. The context is responsible for all further
42 processing of the music. It is during the iteration stage that contexts are
43 created as necessary to ensure that every note has a Voice type context (e.g.
44 Voice, TabVoice, DrumVoice, CueVoice, MensuralVoice, VaticanaVoice,
45 GregorianTranscriptionVoice), that the Voice type contexts exist in
46 appropriate Staff type contexts, and that parallel Staff type contexts exist
47 in StaffGroup type contexts. In addition, during the iteration stage each
48 music event is assigned a moment, or a time in the music when the event
51 Each type of music event has an associated iterator. Iterators are defined in
52 @file{*-iterator.cc}. During iteration, an
53 event's iterator is called to deliver that music event to the appropriate
56 The final stage of LilyPond processing is @emph{translation}. During
57 translation, music events are prepared for graphical or midi output. The
58 translation step is accomplished by the polymorphic base class Translator
59 through its two derived classes: Engraver (for graphical output) and
60 Performer (for midi output).
62 Translators are defined in C++ files named @file{*-engraver.cc}
63 and @file{*-performer.cc}.
64 Much of the work of translating is handled by Scheme functions,
65 which is one of the keys to LilyPond's exceptional flexibility.
67 @sourceimage{architecture-diagram,,,png}
70 @node LilyPond programming languages
71 @section LilyPond programming languages
73 Programming in LilyPond is done in a variety of programming languages. Each
74 language is used for a specific purpose or purposes. This section describes
75 the languages used and provides links to reference manuals and tutorials for
76 the relevant language.
80 The core functionality of LilyPond is implemented in C++.
82 C++ is so ubiquitous that it is difficult to identify either a reference
83 manual or a tutorial. Programmers unfamiliar with C++ will need to spend some
84 time to learn the language before attempting to modify the C++ code.
86 The C++ code calls Scheme/GUILE through the GUILE interface, which is
88 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html, GUILE
93 The LilyPond lexer is implemented in Flex, an implementation of the Unix lex
94 lexical analyser generator. Resources for Flex can be found
95 @uref{http://flex.sourceforge.net/, here}.
99 The LilyPond parser is implemented in Bison, a GNU parser generator. The
100 Bison homepage is found at @uref{http://www.gnu.org/software/bison/,
101 gnu.org}. The manual (which includes both a reference and tutorial) is
102 @uref{http://www.gnu.org/software/bison/manual/index.html, available} in a
107 GNU Make is used to control the compiling process and to build the
108 documentation and the website. GNU Make documentation is available at
109 @uref{http://www.gnu.org/software/make/manual/, the GNU website}.
111 @subsection GUILE or Scheme
113 GUILE is the dialect of Scheme that is used as LilyPond's extension language.
114 Many extensions to LilyPond are written entirely in GUILE. The
115 @uref{http://www.gnu.org/software/guile/manual/html_node/index.html,
116 GUILE Reference Manual} is available online.
118 @uref{http://mitpress.mit.edu/sicp/full-text/book/book.html, Structure and
119 Interpretation of Computer Programs}, a popular textbook used to teach
120 programming in Scheme is available in its entirety online.
122 An introduction to Guile/Scheme as used in LilyPond can be found in the
123 @rextend{Scheme tutorial}.
127 MetaFont is used to create the music fonts used by LilyPond. A MetaFont
128 tutorial is available at @uref{http://metafont.tutorial.free.fr/, the
129 METAFONT tutorial page}.
131 @subsection PostScript
133 PostScript is used to generate graphical output. A brief PostScript tutorial
134 is @uref{http://local.wasp.uwa.edu.au/~pbourke/dataformats/postscript/,
135 available online}. The
136 @uref{http://www.adobe.com/devnet/postscript/pdfs/PLRM.pdf, PostScript Language
137 Reference} is available online in PDF format.
141 Python is used for XML2ly and is used for building the documentation and the
144 Python documentation is available at @uref{http://www.python.org/doc/,
147 @node Programming without compiling
148 @section Programming without compiling
150 Much of the development work in LilyPond takes place by changing @file{*.ly} or
151 @file{*.scm} files. These changes can be made without compiling LilyPond. Such
152 changes are described in this section.
155 @subsection Modifying distribution files
157 Much of LilyPond is written in Scheme or LilyPond input files. These
158 files are interpreted when the program is run, rather than being compiled
159 when the program is built, and are present in all LilyPond distributions.
160 You will find @file{.ly} files in the @file{ly/} directory and the Scheme files in the
161 @file{scm/} directory. Both Scheme files and @file{.ly} files can be modified and
162 saved with any text editor. It's probably wise to make a backup copy of
163 your files before you modify them, although you can reinstall if the
164 files become corrupted.
166 Once you've modified the files, you can test the changes just by running
167 LilyPond on some input file. It's a good idea to create a file that
168 demonstrates the feature you're trying to add. This file will eventually
169 become a regression test and will be part of the LilyPond distribution.
171 @subsection Desired file formatting
173 Files that are part of the LilyPond distribution have Unix-style line
174 endings (LF), rather than DOS (CR+LF) or MacOS 9 and earlier (CR). Make
175 sure you use the necessary tools to ensure that Unix-style line endings are
176 preserved in the patches you create.
178 Tab characters should not be included in files for distribution. All
179 indentation should be done with spaces. Most editors have settings to
180 allow the setting of tab stops and ensuring that no tab characters are
181 included in the file.
183 Scheme files and LilyPond files should be written according to standard
184 style guidelines. Scheme file guidelines can be found at
185 @uref{http://community.schemewiki.org/?scheme-style}. Following these
186 guidelines will make your code easier to read. Both you and others that
187 work on your code will be glad you followed these guidelines.
189 For LilyPond files, you should follow the guidelines for LilyPond snippets
190 in the documentation. You can find these guidelines at
191 @ref{Texinfo introduction and usage policy}.
193 @node Finding functions
194 @section Finding functions
196 When making changes or fixing bugs in LilyPond, one of the initial
197 challenges is finding out where in the code tree the functions to
198 be modified live. With nearly 3000 files in the source tree,
199 trial-and-error searching is generally ineffective. This section
200 describes a process for finding interesting code.
202 @subsection Using the ROADMAP
204 The file ROADMAP is located in the main directory of the lilypond source.
205 ROADMAP lists all of the directories in the LilyPond source tree, along
206 with a brief description of the kind of files found in each directory.
207 This can be a very helpful tool for deciding which directories to search
208 when looking for a function.
211 @subsection Using grep to search
213 Having identified a likely subdirectory to search, the grep utility can
214 be used to search for a function name. The format of the grep command is
217 grep -i functionName subdirectory/*
220 This command will search all the contents of the directory subdirectory/
221 and display every line in any of the files that contains
222 functionName. The @option{-i} option makes @command{grep} ignore
223 case -- this can be very useful if you are not yet familiar with
224 our capitalization conventions.
226 The most likely directories to grep for function names are @file{scm/} for
227 scheme files, ly/ for lilypond input (@file{*.ly}) files, and @file{lily/} for C++
231 @subsection Using git grep to search
233 If you have used git to obtain the source, you have access to a
234 powerful tool to search for functions. The command:
237 git grep functionName
240 will search through all of the files that are present in the git
241 repository looking for functionName. It also presents the results
242 of the search using @code{less}, so the results are displayed one page
245 @subsection Searching on the git repository at Savannah
247 You can also use the equivalent of git grep on the Savannah server.
252 Go to http://git.sv.gnu.org/gitweb/?p=lilypond.git
255 In the pulldown box that says commit, select grep.
258 Type functionName in the search box, and hit enter/return
262 This will initiate a search of the remote git repository.
268 This section describes style guidelines for LilyPond
275 * Naming conventions::
284 @subsection Languages
286 C++ and Python are preferred. Python code should use PEP 8.
290 @subsection Filenames
292 Definitions of classes that are only accessed via pointers (*) or
293 references (&) shall not be included as include files.
299 ".cc" Implementation files
300 ".icc" Inline definition files
301 ".tcc" non inline Template defs
305 (setq auto-mode-alist
306 (append '(("\\.make$" . makefile-mode)
307 ("\\.cc$" . c++-mode)
308 ("\\.icc$" . c++-mode)
309 ("\\.tcc$" . c++-mode)
310 ("\\.hh$" . c++-mode)
311 ("\\.pod$" . text-mode)
316 The class Class_name is coded in @q{class-name.*}
320 @subsection Indentation
322 Standard GNU coding style is used.
324 @subsubheading Indenting files with @code{fixcc.py} (recommended)
326 LilyPond provides a python script that will adjust the indentation
327 and spacing on a @code{.cc} or @code{.hh} file to very near the
331 scripts/auxiliar/fixcc.py FILENAME
334 This can be run on all files at once, but this is not recommended
335 for normal contributors or developers.
338 scripts/auxiliar/fixcc.py \
339 $(find flower lily -name '*cc' -o -name '*hh' | grep -v /out)
343 @subsubheading Indenting with emacs
345 The following hooks will produce indentation which is similar to
346 our official indentation as produced with @code{fixcc.py}.
349 (add-hook 'c++-mode-hook
352 (setq indent-tabs-mode nil))
355 If you like using font-lock, you can also add this to your
359 (setq font-lock-maximum-decoration t)
360 (setq c++-font-lock-keywords-3
362 c++-font-lock-keywords-3
363 '(("\\b\\(a-zA-Z_?+_\\)\\b" 1 font-lock-variable-name-face) ("\\b\\(A-Z?+a-z_?+\\)\\b" 1 font-lock-type-face))
368 @subheading Indenting with vim
370 Although emacs indentation is the GNU standard, acceptable
371 indentation can usually be accomplished with vim. Some hints for
383 filetype plugin indent on
385 set ignorecase smartcase
388 set statusline=%F%m%r%h%w\ %@{&ff@}\ %Y\ [ASCII=\%03.3b]\ [HEX=\%02.2B]\ %04l,%04v\ %p%%\ [LEN=%L]
391 " Remove trailing whitespace on write
392 autocmd BufWritePre * :%s/\s\+$//e
395 With this @file{.vimrc}, files can be reindented automatically by
396 highlighting the lines to be indented in visual mode (use V to
397 enter visual mode) and pressing @code{=}.
399 A @file{scheme.vim} file will help improve the indentation. This
400 one was suggested by Patrick McCarty. It should be saved in
401 @file{~/.vim/after/syntax/scheme.vim}.
404 " Additional Guile-specific 'forms'
405 syn keyword schemeSyntax define-public define*-public
406 syn keyword schemeSyntax define* lambda* let-keywords*
407 syn keyword schemeSyntax defmacro defmacro* define-macro
408 syn keyword schemeSyntax defmacro-public defmacro*-public
409 syn keyword schemeSyntax use-modules define-module
410 syn keyword schemeSyntax define-method define-class
412 " Additional LilyPond-specific 'forms'
413 syn keyword schemeSyntax define-markup-command define-markup-list-command
414 syn keyword schemeSyntax define-safe-public define-music-function
415 syn keyword schemeSyntax def-grace-function
417 " All of the above should influence indenting too
418 set lw+=define-public,define*-public
419 set lw+=define*,lambda*,let-keywords*
420 set lw+=defmacro,defmacro*,define-macro
421 set lw+=defmacro-public,defmacro*-public
422 set lw+=use-modules,define-module
423 set lw+=define-method,define-class
424 set lw+=define-markup-command,define-markup-list-command
425 set lw+=define-safe-public,define-music-function
426 set lw+=def-grace-function
428 " These forms should not influence indenting
432 " Try to highlight all ly: procedures
433 syn match schemeFunc "ly:[^) ]\+"
437 @node Naming conventions
438 @subsection Naming Conventions
440 Naming conventions have been established for LilyPond
443 @subheading Classes and Types
445 Classes begin with an uppercase letter, and words
446 in class names are separated with @code{_}:
454 Member variable names end with an underscore:
462 Macro names should be written in uppercase completely,
463 with words separated by @code{_}:
469 @subheading Variables
471 Variable names should be complete words, rather than abbreviations.
472 For example, it is preferred to use @code{thickness} rather than
473 @code{th} or @code{t}.
475 Multi-word variable names in C++ should have the words separated
476 by the underscore character (@q{_}):
479 cxx_multiword_variable
482 Multi-word variable names in Scheme should have the words separated
486 scheme-multiword-variable
490 @subsection Broken code
492 Do not write broken code. This includes hardwired dependencies,
493 hardwired constants, slow algorithms and obvious limitations. If
494 you can not avoid it, mark the place clearly, and add a comment
495 explaining shortcomings of the code.
497 Ideally, the comment marking the shortcoming would include
498 TODO, so that it is marked for future fixing.
500 We reject broken-in-advance on principle.
504 @subsection Code comments
506 Comments may not be needed if descriptive variable names are used
507 in the code and the logic is straightforward. However, if the
508 logic is difficult to follow, and particularly if non-obvious
509 code has been included to resolve a bug, a comment describing
510 the logic and/or the need for the non-obvious code should be included.
512 There are instances where the current code could be commented better.
513 If significant time is required to understand the code as part of
514 preparing a patch, it would be wise to add comments reflecting your
515 understanding to make future work easier.
518 @node Handling errors
519 @subsection Handling errors
521 As a general rule, you should always try to continue computations,
522 even if there is some kind of error. When the program stops, it
523 is often very hard for a user to pinpoint what part of the input
524 causes an error. Finding the culprit is much easier if there is
525 some viewable output.
527 So functions and methods do not return errorcodes, they never
528 crash, but report a programming_error and try to carry on.
530 Error and warning messages need to be localized.
534 @subsection Localization
536 This document provides some guidelines to help programmers write
538 messages. To help translations, user messages must follow
539 uniform conventions. Follow these rules when coding for LilyPond.
540 Hopefully, this can be replaced by general GNU guidelines in the
541 future. Even better would be to have an English (en_BR, en_AM)
542 guide helping programmers writing consistent messages for all GNU
545 Non-preferred messages are marked with `+'. By convention,
546 ungrammatical examples are marked with `*'. However, such ungrammatical
547 examples may still be preferred.
552 Every message to the user should be localized (and thus be marked
553 for localization). This includes warning and error messages.
556 Do not localize/gettextify:
560 `programming_error ()'s
563 `programming_warning ()'s
569 output strings (PostScript, TeX, etc.)
574 Messages to be localized must be encapsulated in `_ (STRING)' or
575 `_f (FORMAT, ...)'. E.g.:
578 warning (_ ("need music in a score"));
579 error (_f ("cannot open file: `%s'", file_name));
582 In some rare cases you may need to call `gettext ()' by hand. This
583 happens when you pre-define (a list of) string constants for later
584 use. In that case, you'll probably also need to mark these string
585 constants for translation, using `_i (STRING)'. The `_i' macro is
586 a no-op, it only serves as a marker for `xgettext'.
589 char const* messages[] = @{
590 _i ("enable debugging output"),
591 _i ("ignore lilypond version"),
598 puts (gettext (messages i));
602 See also @file{flower/getopt-long.cc} and @file{lily/main.cc}.
605 Do not use leading or trailing whitespace in messages. If you need
606 whitespace to be printed, prepend or append it to the translated
610 message ("Calculating line breaks..." + " ");
614 Error or warning messages displayed with a file name and line
615 number never start with a capital, eg,
618 foo.ly: 12: not a duration: 3
621 Messages containing a final verb, or a gerund (`-ing'-form) always
622 start with a capital. Other (simpler) messages start with a
628 Not declaring: `foo'.
632 Avoid abbreviations or short forms, use `cannot' and `do not'
633 rather than `can't' or `don't'
634 To avoid having a number of different messages for the same
635 situation, well will use quoting like this `"message: `%s'"' for all
636 strings. Numbers are not quoted:
639 _f ("cannot open file: `%s'", name_str)
640 _f ("cannot find character number: %d", i)
644 Think about translation issues. In a lot of cases, it is better to
645 translate a whole message. English grammar must not be imposed on the
646 translator. So, instead of
649 stem at + moment.str () + does not fit in beam
655 _f ("stem at %s does not fit in beam", moment.str ())
659 Split up multi-sentence messages, whenever possible. Instead of
662 warning (_f ("out of tune! Can't find: `%s'", "Key_engraver"));
663 warning (_f ("cannot find font `%s', loading default", font_name));
669 warning (_ ("out of tune:"));
670 warning (_f ("cannot find: `%s', "Key_engraver"));
671 warning (_f ("cannot find font: `%s', font_name));
672 warning (_f ("Loading default font"));
676 If you must have multiple-sentence messages, use full punctuation.
677 Use two spaces after end of sentence punctuation. No punctuation
678 (esp. period) is used at the end of simple messages.
681 _f ("Non-matching braces in text `%s', adding braces", text)
682 _ ("Debug output disabled. Compiled with NPRINT.")
683 _f ("Huh? Not a Request: `%s'. Ignoring.", request)
687 Do not modularize too much; words frequently cannot be translated
688 without context. It is probably safe to treat most occurrences of
689 words like stem, beam, crescendo as separately translatable words.
692 When translating, it is preferable to put interesting information
693 at the end of the message, rather than embedded in the middle.
694 This especially applies to frequently used messages, even if this
695 would mean sacrificing a bit of eloquency. This holds for original
696 messages too, of course.
699 en: cannot open: `foo.ly'
700 + nl: kan `foo.ly' niet openen (1)
701 kan niet openen: `foo.ly'* (2)
702 niet te openen: `foo.ly'* (3)
706 The first nl message, although grammatically and stylistically
707 correct, is not friendly for parsing by humans (even if they speak
708 dutch). I guess we would prefer something like (2) or (3).
711 Do not run make po/po-update with GNU gettext < 0.10.35
716 @node Warnings Errors Progress and Debug Output
717 @section Warnings, Errors, Progress and Debug Output
719 @unnumberedsubsec Available log levels
721 LilyPond has several loglevels, which specify how verbose the output on
722 the console should be:
724 @item NONE: No output at all, even on failure
725 @item ERROR: Only error messages
726 @item WARN: Only error messages and warnings
727 @item BASIC_PROGRESS: Warnings, errors and basic progress (success, etc.)
728 @item PROGRESS: Warnings, errors and full progress messages
729 @item INFO: Warnings, errors, progress and more detailed information
730 @item DEBUG: All messages, including vull debug messages (very verbose!)
733 The loglevel can either be set with the environment variable
734 @code{LILYPOND_LOGLEVEL} or on the command line with the @option{--loglevel=...}
737 @unnumberedsubsec Functions for debug and log output
739 LilyPond has two different types of error and log functions:
743 If a warning or error is caused by an identified position in the input file,
744 e.g. by a grob or by a music expression, the functions of the @code{Input}
745 class provide logging functionality that prints the position of the message
746 in addition to the message.
749 If a message can not be associated with a particular position in an input file,
750 e.g. the output file cannot be written, then the functions in the
751 @code{flower/include/warn.hh} file will provide logging functionality that
752 only prints out the message, but no location.
756 There are also Scheme functions to access all of these logging functions from
757 scheme. In addition, the Grob class contains some convenience wrappers for
758 even easier access to these functions.
760 The message and debug functions in @code{warn.hh} also have an optional
761 argument @code{newline}, which specifies whether the message should always
762 start on a new line or continue a previous message.
763 By default, @code{progress_indication} does NOT start on a new line, but rather
764 continue the previous output. All other functions by default start their
765 output on a new line.
767 @unnumberedsubsec All logging functions at a glance
769 Currently, there are no particular message functions for the INFO loglevel,
770 so it is basically identical to PROGRESS.
773 @multitable @columnfractions 0.16 0.42 0.42
775 @tab C++, no location
776 @tab C++ from input location
779 @tab @code{error ()}, @code{programming_error (msg)}, @code{non_fatal_error (msg)}
780 @tab @code{Input::error (msg)}, @code{Input::programming_error (msg)}
783 @tab @code{warning (msg)} @c WARN
784 @tab @code{Input::warning (msg)} @c WARN
787 @tab @code{successful (msg)}
791 @tab @code{progress_indication (msg)}, @code{message (msg)}
792 @tab @code{Input::message (msg)}
795 @tab @code{debug_output (msg)}
796 @tab @code{Input::debug_output (msg)}
802 @tab Scheme, music expression
805 @tab @code{Grob::programming_error (msg)}
809 @tab @code{Grob::warning (msg)}
810 @tab @code{(ly:music-warning music msg)}
818 @tab @code{(ly:music-message music msg)}
827 @tab Scheme, no location
828 @tab Scheme, input location
832 @tab @code{(ly:error msg args)}, @code{(ly:programming-error msg args)}
835 @tab @code{(ly:warning msg args)}
836 @tab @code{(ly:input-warning input msg args)}
839 @tab @code{(ly:success msg args)}
843 @tab (ly:progress msg args), (ly:message msg args)
844 @tab @code{(ly:input-message input msg args)}
847 @tab @code{(ly:debug msg args)}
856 @node Debugging LilyPond
857 @section Debugging LilyPond
859 The most commonly used tool for debugging LilyPond is the GNU
860 debugger gdb. The gdb tool is used for investigating and debugging
861 core Lilypond code written in C++. Another tool is available for
862 debugging Scheme code using the Guile debugger. This section
863 describes how to use both gdb and the Guile Debugger.
866 * Debugging overview::
867 * Debugging C++ code::
868 * Debugging Scheme code::
871 @node Debugging overview
872 @subsection Debugging overview
874 Using a debugger simplifies troubleshooting in at least two ways.
876 First, breakpoints can be set to pause execution at any desired point.
877 Then, when execution has paused, debugger commands can be issued to
878 explore the values of various variables or to execute functions.
880 Second, the debugger can display a stack trace, which shows the
881 sequence in which functions have been called and the arguments
882 passed to the called functions.
884 @node Debugging C++ code
885 @subsection Debugging C++ code
887 The GNU debugger, gdb, is the principal tool for debugging C++ code.
889 @subheading Compiling LilyPond for use with gdb
891 In order to use gdb with LilyPond, it is necessary to compile
892 LilyPond with debugging information. This is accomplished by running
893 the following commands in the main LilyPond source directory.
896 ./configure --disable-optimising
900 This will create a version of LilyPond containing debugging
901 information that will allow the debugger to tie the source code
902 to the compiled code.
904 You should not do @var{make install} if you want to use a debugger
905 with LilyPond. The @var{make install} command will strip debugging
906 information from the LilyPond binary.
908 @subheading Typical gdb usage
910 Once you have compiled the Lilypond image with the necessary
911 debugging information it will have been written to a location in a
912 subfolder of your current working directory:
918 This is important as you will need to let gdb know where to find the
919 image containing the symbol tables. You can invoke gdb from the
920 command line using the following:
926 This loads the LilyPond symbol tables into gdb. Then, to run
927 LilyPond on @file{test.ly} under the debugger, enter the following:
936 As an alternative to running gdb at the command line you may try
937 a graphical interface to gdb such as ddd:
943 You can also use sets of standard gdb commands stored in a .gdbinit
944 file (see next section).
946 @subheading Typical .gdbinit files
948 The behavior of gdb can be readily customized through the use of a
949 @var{.gdbinit} file. A @var{.gdbinit} file is a file named
950 @var{.gdbinit} (notice the @qq{.} at the beginning of the file name)
951 that is placed in a user's home directory.
953 The @var{.gdbinit} file below is from Han-Wen. It sets breakpoints
954 for all errors and defines functions for displaying scheme objects
955 (ps), grobs (pgrob), and parsed music expressions (pmusic).
958 file lily/out/lilypond
960 b Grob::programming_error
963 print ly_display_scm($arg0)
966 print ly_display_scm($arg0->self_scm_)
967 print ly_display_scm($arg0->mutable_property_alist_)
968 print ly_display_scm($arg0->immutable_property_alist_)
969 print ly_display_scm($arg0->object_alist_)
972 print ly_display_scm($arg0->self_scm_)
973 print ly_display_scm($arg0->mutable_property_alist_)
974 print ly_display_scm($arg0->immutable_property_alist_)
978 @node Debugging Scheme code
979 @subsection Debugging Scheme code
981 Scheme code can be developed using the Guile command line
982 interpreter @code{top-repl}. You can either investigate
983 interactively using just Guile or you can use the debugging
984 tools available within Guile.
986 @subheading Using Guile interactively with LilyPond
988 In order to experiment with Scheme programming in the LilyPond
989 environment, it is necessary to have a Guile interpreter that
990 has all the LilyPond modules loaded. This requires the following
993 First, define a Scheme symbol for the active module in the @file{.ly} file:
996 #(module-define! (resolve-module '(guile-user))
997 'lilypond-module (current-module))
1000 Now place a Scheme function in the @file{.ly} file that gives an
1001 interactive Guile prompt:
1007 When the @file{.ly} file is compiled, this causes the compilation to be
1008 interrupted and an interactive guile prompt to appear. Once the
1009 guile prompt appears, the LilyPond active module must be set as the
1010 current guile module:
1013 guile> (set-current-module lilypond-module)
1016 You can demonstrate these commands are operating properly by typing the name
1017 of a LilyPond public scheme function to check it has been defined:
1020 guile> fret-diagram-verbose-markup
1021 #<procedure fret-diagram-verbose-markup (layout props marking-list)>
1024 If the LilyPond module has not been correctly loaded, an error
1025 message will be generated:
1028 guile> fret-diagram-verbose-markup
1029 ERROR: Unbound variable: fret-diagram-verbose-markup
1030 ABORT: (unbound-variable)
1033 Once the module is properly loaded, any valid LilyPond Scheme
1034 expression can be entered at the interactive prompt.
1036 After the investigation is complete, the interactive guile
1037 interpreter can be exited:
1043 The compilation of the @file{.ly} file will then continue.
1045 @subheading Using the Guile debugger
1047 To set breakpoints and/or enable tracing in Scheme functions, put
1050 \include "guile-debugger.ly"
1053 in your input file after any scheme procedures you have defined in
1054 that file. This will invoke the Guile command-line after having set
1055 up the environment for the debug command-line. When your input file
1056 is processed, a guile prompt will be displayed. You may now enter
1057 commands to set up breakpoints and enable tracing by the Guile debugger.
1059 @subheading Using breakpoints
1061 At the guile prompt, you can set breakpoints with
1062 the @code{set-break!} procedure:
1065 guile> (set-break! my-scheme-procedure)
1068 Once you have set the desired breakpoints, you exit the guile repl frame
1075 Then, when one of the scheme routines for which you have set
1076 breakpoints is entered, guile will interrupt execution in a debug
1077 frame. At this point you will have access to Guile debugging
1078 commands. For a listing of these commands, type:
1084 Alternatively you may code the breakpoints in your Lilypond source
1085 file using a command such as:
1088 #(set-break! my-scheme-procedure)
1091 immediately after the @code{\include} statement. In this case the
1092 breakpoint will be set straight after you enter the @code{(quit)}
1093 command at the guile prompt.
1095 Embedding breakpoint commands like this is particularly useful if
1096 you want to look at how the Scheme procedures in the @file{.scm}
1097 files supplied with LilyPond work. To do this, edit the file in
1098 the relevant directory to add this line near the top:
1101 (use-modules (scm guile-debugger))
1104 Now you can set a breakpoint after the procedure you are interested
1105 in has been declared. For example, if you are working on routines
1106 called by @var{print-book-with} in @file{lily-library.scm}:
1109 (define (print-book-with parser book process-procedure)
1110 (let* ((paper (ly:parser-lookup parser '$defaultpaper))
1111 (layout (ly:parser-lookup parser '$defaultlayout))
1112 (outfile-name (get-outfile-name parser)))
1113 (process-procedure book paper layout outfile-name)))
1115 (define-public (print-book-with-defaults parser book)
1116 (print-book-with parser book ly:book-process))
1118 (define-public (print-book-with-defaults-as-systems parser book)
1119 (print-book-with parser book ly:book-process-to-systems))
1123 At this point in the code you could add this to set a breakpoint at
1127 (set-break! print-book-with)
1130 @subheading Tracing procedure calls and evaluator steps
1132 Two forms of trace are available:
1135 (set-trace-call! my-scheme-procedure)
1141 (set-trace-subtree! my-scheme-procedure)
1144 @code{set-trace-call!} causes Scheme to log a line to the standard
1145 output to show when the procedure is called and when it exits.
1147 @code{set-trace-subtree!} traces every step the Scheme evaluator
1148 performs in evaluating the procedure.
1150 @node Tracing object relationships
1151 @section Tracing object relationships
1153 Understanding the LilyPond source often boils down to figuring out what
1154 is happening to the Grobs. Where (and why) are they being created,
1155 modified and destroyed? Tracing Lily through a debugger in order to
1156 identify these relationships can be time-consuming and tedious.
1158 In order to simplify this process, a facility has been added to
1159 display the grobs that are created and the properties that are set
1160 and modified. Although it can be complex to get set up, once set up
1161 it easily provides detailed information about the life of grobs
1162 in the form of a network graph.
1164 Each of the steps necessary to use the graphviz utility
1169 @item Installing graphviz
1171 In order to create the graph of the object relationships, it is
1172 first necessary to install Graphviz. graphviz is available for a
1173 number of different platforms:
1176 @uref{http://www.graphviz.org/Download..php}
1179 @item Modifying config.make
1181 In order for the Graphviz tool to work, config.make must be modified.
1182 It is probably a good idea to first save a copy of config.make under
1183 a different name. Then, edit config.make by removing every occurrence
1184 of @option{-DNDEBUG}.
1186 @item Rebuilding LilyPond
1188 The executable code of LilyPond must be rebuilt from scratch:
1191 make -C lily clean && make -C lily
1194 @item Create a graphviz-compatible @file{.ly} file
1196 In order to use the graphviz utility, the @file{.ly} file must include
1197 @file{ly/graphviz-init.ly}, and should then specify the
1198 grobs and symbols that should be tracked. An example of this
1199 is found in @file{input/regression/graphviz.ly}.
1201 @item Run lilypond with output sent to a log file
1203 The Graphviz data is sent to stderr by lilypond, so it is
1204 necessary to redirect stderr to a logfile:
1207 lilypond graphviz.ly 2> graphviz.log
1210 @item Edit the logfile
1212 The logfile has standard lilypond output, as well as the Graphviz
1213 output data. Delete everything from the beginning of the file
1214 up to but not including the first occurrence of @code{digraph}.
1216 Also, delete the final liypond message about successs from the end
1219 @item Process the logfile with @code{dot}
1221 The directed graph is created from the log file with the program
1225 dot -Tpdf graphviz.log > graphviz.pdf
1230 The pdf file can then be viewed with any pdf viewer.
1232 When compiled without @option{-DNDEBUG}, lilypond may run slower
1233 than normal. The original configuration can be restored by either
1234 renaming the saved copy of @code{config.make} or rerunning
1235 @code{configure}. Then rebuild lilypond with
1238 make -C lily clean && make -C lily
1242 @node Adding or modifying features
1243 @section Adding or modifying features
1245 When a new feature is to be added to LilyPond, it is necessary to
1246 ensure that the feature is properly integrated to maintain
1247 its long-term support. This section describes the steps necessary
1248 for feature addition and modification.
1253 * Write regression tests::
1254 * Write convert-ly rule::
1255 * Automatically update documentation::
1256 * Manually update documentation::
1257 * Edit changes.tely::
1258 * Verify successful build::
1259 * Verify regression tests::
1260 * Post patch for comments::
1262 * Closing the issues::
1265 @node Write the code
1266 @subsection Write the code
1268 You should probably create a new git branch for writing the code, as that
1269 will separate it from the master branch and allow you to continue
1270 to work on small projects related to master.
1272 Please be sure to follow the rules for programming style discussed
1273 earlier in this chapter.
1276 @node Write regression tests
1277 @subsection Write regression tests
1279 In order to demonstrate that the code works properly, you will
1280 need to write one or more regression tests. These tests are
1281 typically @file{.ly} files that are found in @file{input/regression}.
1283 Regression tests should be as brief as possible to demonstrate the
1284 functionality of the code.
1286 Regression tests should generally cover one issue per test. Several
1287 short, single-issue regression tests are preferred to a single, long,
1288 multiple-issue regression test.
1290 Use existing regression tests as templates to demonstrate the type of
1291 header information that should be included in a regression test.
1294 @node Write convert-ly rule
1295 @subsection Write convert-ly rule
1297 If the modification changes the input syntax, a convert-ly rule
1298 should be written to automatically update input files from older
1301 convert-ly rules are found in python/convertrules.py
1303 If possible, the convert-ly rule should allow automatic updating
1304 of the file. In some cases, this will not be possible, so the
1305 rule will simply point out to the user that the feature needs
1308 @subsubheading Updating version numbers
1310 If a development release occurs between you writing your patch and
1311 having it approved+pushed, you will need to update the version
1312 numbers in your tree. This can be done with:
1315 scripts/auxiliar/update-patch-version old.version.number new.version.number
1318 It will change all files in git, so use with caution and examine
1322 @node Automatically update documentation
1323 @subsection Automatically update documentation
1325 @command{convert-ly} should be used to update the documentation,
1326 the snippets, and the regression tests. This not only makes the
1327 necessary syntax changes, it also tests the @command{convert-ly}
1330 The automatic updating is performed by moving to the top-level
1331 source directory, then running:
1334 scripts/auxiliar/update-with-convert-ly.sh
1337 If you did an out-of-tree build, pass in the relative path:
1340 BUILD_DIR=../build-lilypond/ scripts/auxiliar/update-with-convert-ly.sh
1344 @node Manually update documentation
1345 @subsection Manually update documentation
1347 Where the convert-ly rule is not able to automatically update the inline
1348 lilypond code in the documentation (i.e. if a NOT_SMART rule is used), the
1349 documentation must be manually updated. The inline snippets that require
1350 changing must be changed in the English version of the docs and all
1351 translated versions. If the inline code is not changed in the
1352 translated documentation, the old snippets will show up in the
1353 English version of the documentation.
1355 Where the convert-ly rule is not able to automatically update snippets
1356 in Documentation/snippets/, those snippets must be manually updated.
1357 Those snippets should be copied to Documentation/snippets/new. The
1358 comments at the top of the snippet describing its automatic generation
1359 should be removed. All translated texidoc strings should be removed.
1360 The comment @qq{% begin verbatim} should be removed. The syntax of
1361 the snippet should then be manually edited.
1363 Where snippets in Documentation/snippets are made obsolete, the snippet
1364 should be copied to Documentation/snippets/new. The comments and
1365 texidoc strings should be removed as described above. Then the body
1366 of the snippet should be changed to:
1370 This snippet is deprecated as of version X.Y.Z and
1371 will be removed from the documentation.
1376 where X.Y.Z is the version number for which the convert-ly rule was
1379 Update the snippet files by running:
1382 scripts/auxiliar/makelsr.py
1385 Where the convert-ly rule is not able to automatically update regression
1386 tests, the regression tests in input/regression should be manually
1389 Although it is not required, it is helpful if the developer
1390 can write relevant material for inclusion in the Notation
1391 Reference. If the developer does not feel qualified to write
1392 the documentation, a documentation editor will be able to
1393 write it from the regression tests. The text that is added to
1394 or removed from the documentation should be changed only in
1395 the English version.
1398 @node Edit changes.tely
1399 @subsection Edit changes.tely
1401 An entry should be added to Documentation/changes.tely to describe
1402 the feature changes to be implemented. This is especially important
1403 for changes that change input file syntax.
1405 Hints for changes.tely entries are given at the top of the file.
1407 New entries in changes.tely go at the top of the file.
1409 The changes.tely entry should be written to show how the new change
1410 improves LilyPond, if possible.
1413 @node Verify successful build
1414 @subsection Verify successful build
1416 When the changes have been made, successful completion must be
1424 When these commands complete without error, the patch is
1425 considered to function successfully.
1427 Developers on Windows who are unable to build LilyPond should
1428 get help from a Linux or OSX developer to do the make tests.
1431 @node Verify regression tests
1432 @subsection Verify regression tests
1434 In order to avoid breaking LilyPond, it is important to verify that
1435 the regression tests succeed, and that no unwanted changes are
1436 introduced into the output. This process is described in
1437 @ref{Regtest comparison}.
1439 @subheading Typical developer's edit/compile/test cycle
1441 TODO: is @code{[-j@var{X} CPU_COUNT=@var{X}]} useful for
1442 @code{test-baseline}, @code{check}, @code{clean},
1443 @code{test-redo}? Neil Puttock says it is useful for
1444 everything but @code{clean}, which is disk-limited.
1445 Need to check formally.
1454 make [-j@var{X} CPU_COUNT=@var{X}] check
1458 Edit/compile/test cycle:
1461 @emph{## edit source files, then...}
1463 make clean @emph{## only if needed (see below)}
1464 make [-j@var{X}] @emph{## only if needed (see below)}
1465 make test-redo @emph{## redo files differing from baseline}
1466 make [-j@var{X} CPU_COUNT=@var{X}] check @emph{## CPU_COUNT here?}
1477 If you modify any source files that have to be compiled (such as
1478 @file{.cc} or @file{.hh} files in @file{flower/} or @file{lily/}),
1479 then you must run @command{make} before @command{make test-redo},
1480 so @command{make} can compile the modified files and relink all
1481 the object files. If you only modify files which are interpreted,
1482 like those in the @file{scm/} and @file{ly/} directories, then
1483 @command{make} is not needed before @command{make test-redo}.
1485 TODO: Fix the following paragraph. You can do @command{rm mf/out/*}
1486 instead of make clean, and you can probably do
1487 @command{make -C mf/ clean} as well, but I haven't checked it -- cds
1489 Also, if you modify any font definitions in the @file{mf/}
1490 directory then you must run @command{make clean} and
1491 @command{make} before running @command{make test-redo}. This will
1492 recompile everything, whether modified or not, and takes a lot
1495 Running @command{make@tie{}check} will leave an HTML page
1496 @file{out/test-results/index.html}. This page shows all the
1497 important differences that your change introduced, whether in the
1498 layout, MIDI, performance or error reporting.
1503 @node Post patch for comments
1504 @subsection Post patch for comments
1506 See @ref{Uploading a patch for review}.
1510 @subsection Push patch
1512 Once all the comments have been addressed, the patch can be pushed.
1514 If the author has push privileges, the author will push the patch.
1515 Otherwise, a developer with push privileges will push the patch.
1518 @node Closing the issues
1519 @subsection Closing the issues
1521 Once the patch has been pushed, all the relevant issues should be
1524 On Rietveld, the author should log in an close the issue either by
1525 using the @q{Edit Issue} link, or by clicking the circled x icon
1526 to the left of the issue name.
1528 If the changes were in response to a feature request on the Google
1529 issue tracker for LilyPond, the author should change the status to
1530 Fixed and a tag @q{fixed_x_y_z} should be added, where the patch was
1531 fixed in version x.y.z. If
1532 the author does not have privileges to change the status, an email
1533 should be sent to bug-lilypond requesting the BugMeister to change
1537 @node Iterator tutorial
1538 @section Iterator tutorial
1540 TODO -- this is a placeholder for a tutorial on iterators
1542 Iterators are routines written in C++ that process music expressions
1543 and sent the music events to the appropriate engravers and/or
1547 @node Engraver tutorial
1548 @section Engraver tutorial
1550 Engravers are C++ classes that catch music events and
1551 create the appropriate grobs for display on the page. Though the
1552 majority of engravers are responsible for the creation of a single grob,
1553 in some cases (e.g. @code{New_fingering_engraver}), several different grobs
1556 Engravers listen for events and acknowledge grobs. Events are passed to
1557 the engraver in time-step order during the iteration phase. Grobs are
1558 made available to the engraver when they are created by other engravers
1559 during the iteration phase.
1563 * Useful methods for information processing::
1564 * Translation process::
1565 * Preventing garbage collection for SCM member variables::
1566 * Listening to music events::
1567 * Acknowledging grobs::
1568 * Engraver declaration/documentation::
1571 @node Useful methods for information processing
1572 @subsection Useful methods for information processing
1574 An engraver inherits the following public methods from the Translator
1575 base class, which can be used to process listened events and acknowledged
1579 @item @code{virtual void initialize ()}
1580 @item @code{void start_translation_timestep ()}
1581 @item @code{void process_music ()}
1582 @item @code{void process_acknowledged ()}
1583 @item @code{void stop_translation_timestep ()}
1584 @item @code{virtual void finalize ()}
1587 These methods are listed in order of translation time, with
1588 @code{initialize ()} and @code{finalize ()} bookending the whole
1589 process. @code{initialize ()} can be used for one-time initialization
1590 of context properties before translation starts, whereas
1591 @code{finalize ()} is often used to tie up loose ends at the end of
1592 translation: for example, an unterminated spanner might be completed
1593 automatically or reported with a warning message.
1596 @node Translation process
1597 @subsection Translation process
1599 At each timestep in the music, translation proceeds by calling the
1600 following methods in turn:
1602 @code{start_translation_timestep ()} is called before any user
1603 information enters the translators, i.e., no property operations
1604 (\set, \override, etc.) or events have been processed yet.
1606 @code{process_music ()} and @code{process_acknowledged ()} are called
1607 after all events in the current time step have been heard, or all
1608 grobs in the current time step have been acknowledged. The latter
1609 tends to be used exclusively with engravers which only acknowledge
1610 grobs, whereas the former is the default method for main processing
1613 @code{stop_translation_timestep ()} is called after all user
1614 information has been processed prior to beginning the translation for
1618 @node Preventing garbage collection for SCM member variables
1619 @subsection Preventing garbage collection for SCM member variables
1621 In certain cases, an engraver might need to ensure private Scheme
1622 variables (with type SCM) do not get swept away by Guile's garbage
1623 collector: for example, a cache of the previous key signature which
1624 must persist between timesteps. The method
1625 @code{virtual derived_mark () const} can be used in such cases:
1628 Engraver_name::derived_mark ()
1630 scm_gc_mark (private_scm_member_)
1635 @node Listening to music events
1636 @subsection Listening to music events
1638 External interfaces to the engraver are implemented by protected
1639 macros including one or more of the following:
1642 @item @code{DECLARE_TRANSLATOR_LISTENER (event_name)}
1643 @item @code{IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)}
1647 where @var{event_name} is the type of event required to provide the
1648 input the engraver needs and @var{Engraver_name} is the name of the
1651 Following declaration of a listener, the method is implemented as follows:
1654 IMPLEMENT_TRANSLATOR_LISTENER (Engraver_name, event_name)
1656 Engraver_name::listen_event_name (Stream event *event)
1658 ...body of listener method...
1663 @node Acknowledging grobs
1664 @subsection Acknowledging grobs
1666 Some engravers also need information from grobs as they are created
1667 and as they terminate. The mechanism and methods to obtain this
1668 information are set up by the macros:
1671 @item @code{DECLARE_ACKNOWLEDGER (grob_interface)}
1672 @item @code{DECLARE_END_ACKNOWLEDGER (grob_interface)}
1675 where @var{grob_interface} is an interface supported by the
1676 grob(s) which should be acknowledged. For example, the following
1677 code would declare acknowledgers for a @code{NoteHead} grob (via the
1678 @code{note-head-interface}) and any grobs which support the
1679 @code{side-position-interface}:
1682 @code{DECLARE_ACKNOWLEDGER (note_head)}
1683 @code{DECLARE_ACKNOWLEDGER (side_position)}
1686 The @code{DECLARE_END_ACKNOWLEDGER ()} macro sets up a spanner-specific
1687 acknowledger which will be called whenever a spanner ends.
1689 Following declaration of an acknowledger, the method is coded as follows:
1693 Engraver_name::acknowledge_interface_name (Grob_info info)
1695 ...body of acknowledger method...
1700 @node Engraver declaration/documentation
1701 @subsection Engraver declaration/documentation
1703 An engraver must have a public macro
1706 @item @code{TRANSLATOR_DECLARATIONS (Engraver_name)}
1710 where @code{Engraver_name} is the name of the engraver. This
1711 defines the common variables and methods used by every engraver.
1713 At the end of the engraver file, one or both of the following
1714 macros are generally called to document the engraver in the
1715 Internals Reference:
1718 @item @code{ADD_ACKNOWLEDGER (Engraver_name, grob_interface)}
1719 @item @code{ADD_TRANSLATOR (Engraver_name, Engraver_doc,
1720 Engraver_creates, Engraver_reads, Engraver_writes)}
1724 where @code{Engraver_name} is the name of the engraver, @code{grob_interface}
1725 is the name of the interface that will be acknowledged,
1726 @code{Engraver_doc} is a docstring for the engraver,
1727 @code{Engraver_creates} is the set of grobs created by the engraver,
1728 @code{Engraver_reads} is the set of properties read by the engraver,
1729 and @code{Engraver_writes} is the set of properties written by
1732 The @code{ADD_ACKNOWLEDGER} and @code{ADD_TRANSLATOR} macros use a
1733 non-standard indentation system. Each interface, grob, read property,
1734 and write property is on its own line, and the closing parenthesis
1735 and semicolon for the macro all occupy a separate line beneath the final
1736 interface or write property. See existing engraver files for more
1740 @node Callback tutorial
1741 @section Callback tutorial
1743 TODO -- This is a placeholder for a tutorial on callback functions.
1745 @node LilyPond scoping
1746 @section LilyPond scoping
1748 The Lilypond language has a concept of scoping, i.e. you can do
1754 (display (+ foo 2)))
1757 @noindent with @code{\paper}, @code{\midi} and @code{\header} being
1758 nested scope inside the @file{.ly} file-level scope. @w{@code{foo = 1}}
1759 is translated in to a scheme variable definition.
1761 This implemented using modules, with each scope being an anonymous
1762 module that imports its enclosing scope's module.
1764 Lilypond's core, loaded from @file{.scm} files, is usually placed in the
1765 @code{lily} module, outside the @file{.ly} level. In the case of
1772 we want to reuse the built-in definitions, without changes effected in
1773 user-level @file{a.ly} leaking into the processing of @file{b.ly}.
1775 The user-accessible definition commands have to take care to avoid
1776 memory leaks that could occur when running multiple files. All
1777 information belonging to user-defined commands and markups is stored in
1778 a manner that allows it to be garbage-collected when the module is
1779 dispersed, either by being stored module-locally, or in weak hash
1782 @node LilyPond miscellany
1783 @section LilyPond miscellany
1785 This is a place to dump information that may be of use to developers
1786 but doesn't yet have a proper home. Ideally, the length of this section
1787 would become zero as items are moved to other homes.
1791 * Spacing algorithms::
1792 * Info from Han-Wen email::
1793 * Music functions and GUILE debugging::
1796 @node Spacing algorithms
1797 @subsection Spacing algorithms
1799 Here is information from an email exchange about spacing algorithms.
1801 On Thu, 2010-02-04 at 15:33 -0500, Boris Shingarov wrote:
1802 I am experimenting with some modifications to the line breaking code,
1803 and I am stuck trying to understand how some of it works. So far my
1804 understanding is that Simple_spacer operates on a vector of Grobs, and
1805 it is a well-known Constrained-QP problem (rods = constraints, springs
1806 = quadratic function to minimize). What I don't understand is, if the
1807 spacer operates at the level of Grobs, which are built at an earlier
1808 stage in the pipeline, how are the changes necessitated by differences
1809 in line breaking, taken into account? in other words, if I take the
1810 last measure of a line and place it on the next line, it is not just a
1811 matter of literally moving that graphic to where the start of the next
1812 line is, but I also need to draw a clef, key signature, and possibly
1813 other fundamental things -- but at that stage in the rendering
1814 pipeline, is it not too late??
1816 Joe Neeman answered:
1818 We create lots of extra grobs (eg. a BarNumber at every bar line) but
1819 most of them are not drawn. See the break-visibility property in
1822 Here is another e-mail exchange. Janek Warchoł asked for a starting point
1823 to fixing 1301 (change clef colliding with notes). Neil Puttock replied:
1825 The clef is on a loose column (it floats before the head), so the
1826 first place I'd look would be lily/spacing-loose-columns.cc (and
1827 possibly lily/spacing-determine-loose-columns.cc).
1828 I'd guess the problem is the way loose columns are spaced between
1829 other columns: in this snippet, the columns for the quaver and tuplet
1830 minim are so close together that the clef's column gets dumped on top
1831 of the quaver (since it's loose, it doesn't influence the spacing).
1833 @node Info from Han-Wen email
1834 @subsection Info from Han-Wen email
1836 In 2004, Douglas Linhardt decided to try starting a document that would
1837 explain LilyPond architecture and design principles. The material below
1838 is extracted from that email, which can be found at
1839 @uref{http://thread.gmane.org/gmane.comp.gnu.lilypond.devel/2992}.
1840 The headings reflect questions from Doug or comments from Han-Wen;
1841 the body text are Han-Wen's answers.
1843 @subheading Figuring out how things work.
1845 I must admit that when I want to know how a program works, I use grep
1846 and emacs and dive into the source code. The comments and the code
1847 itself are usually more revealing than technical documents.
1849 @subheading What's a grob, and how is one used?
1851 Graphical object - they are created from within engravers, either as
1852 Spanners (derived class) -slurs, beams- or Items (also a derived
1853 class) -notes, clefs, etc.
1855 There are two other derived classes System (derived from Spanner,
1856 containing a "line of music") and Paper_column (derived from Item, it
1857 contains all items that happen at the same moment). They are separate
1858 classes because they play a special role in the linebreaking process.
1860 @subheading What's a smob, and how is one used?
1862 A C(++) object that is encapsulated so it can be used as a Scheme
1863 object. See GUILE info, "19.3 Defining New Types (Smobs)"
1865 @@subheading When is each C++ class constructed and used
1872 In the parser.yy see the macro calls MAKE_MUSIC_BY_NAME().
1877 Constructed during "interpreting" phase.
1882 Executive branch of Contexts, plugins that create grobs, usually one
1883 engraver per grob type. Created together with context.
1893 These are not C++ classes per se. The idea of a Grob interface hasn't
1894 crystallized well. ATM, an interface is a symbol, with a bunch of grob
1895 properties. They are not objects that are created or destroyed.
1900 Objects that walk through different music classes, and deliver events
1901 in a synchronized way, so that notes that play together are processed
1902 at the same moment and (as a result) end up on the same horizontal position.
1904 Created during interpreting phase.
1906 BTW, the entry point for interpreting is ly:run-translator
1907 (ly_run_translator on the C++ side)
1911 @subheading Can you get to Context properties from a Music object?
1913 You can create music object with a Scheme function that reads context
1914 properties (the \applycontext syntax). However, that function is
1915 executed during Interpreting, so you can not really get Context
1916 properties from Music objects, since music objects are not directly
1917 connected to Contexts. That connection is made by the Music_iterators
1919 @subheading Can you get to Music properties from a Context object?
1921 Yes, if you are given the music object within a Context
1922 object. Normally, the music objects enter Contexts in synchronized
1923 fashion, and the synchronization is done by Music_iterators.
1925 @subheading What is the relationship between C++ classes and Scheme objects?
1927 Smobs are C++ objects in Scheme. Scheme objects (lists, functions) are
1928 manipulated from C++ as well using the GUILE C function interface
1931 @subheading How do Scheme procedures get called from C++ functions?
1933 scm_call_*, where * is an integer from 0 to 4.
1934 Also scm_c_eval_string (), scm_eval ()
1936 @subheading How do C++ functions get called from Scheme procedures?
1938 Export a C++ function to Scheme with LY_DEFINE.
1940 @subheading What is the flow of control in the program?
1942 Good question. Things used to be clear-cut, but we have Scheme
1943 and SMOBs now, which means that interactions do not follow a very
1944 rigid format anymore. See below for an overview, though.
1946 @subheading Does the parser make Scheme procedure calls or C++ function calls?
1948 Both. And the Scheme calls can call C++ and vice versa. It's nested,
1949 with the SCM datatype as lubrication between the interactions
1951 (I think the word "lubrication" describes the process better than the
1952 traditional word "glue")
1954 @subheading How do the front-end and back-end get started?
1956 Front-end: a file is parsed, the rest follows from that. Specifically,
1958 Parsing leads to a Music + Music_output_def object (see parser.yy,
1959 definition of toplevel_expression )
1961 A Music + Music_output_def object leads to a Global_context object (see
1962 ly_run_translator ())
1964 During interpreting, Global_context + Music leads to a bunch of
1965 Contexts (see Global_translator::run_iterator_on_me ()).
1967 After interpreting, Global_context contains a Score_context (which
1968 contains staves, lyrics etc.) as a child. Score_context::get_output ()
1969 spews a Music_output object (either a Paper_score object for notation
1970 or Performance object for MIDI).
1972 The Music_output object is the entry point for the backend (see
1973 ly_render_output ()).
1975 The main steps of the backend itself are in
1980 @file{paper-score.cc} , Paper_score::process_
1983 @file{system.cc} , System::get_lines()
1986 The step, where things go from grobs to output, is in
1987 System::get_line(): each grob delivers a Stencil (a Device
1988 independent output description), which is interpreted by our
1989 outputting backends (@file{scm/output-tex.scm} and
1990 @file{scm/output-ps.scm}) to produce TeX and PS.
1994 Interactions between grobs and putting things into .tex and .ps files
1995 have gotten a little more complex lately. Jan has implemented
1996 page-breaking, so now the backend also involves Paper_book,
1997 Paper_lines and other things. This area is still heavily in flux, and
1998 perhaps not something you should want to look at.
2000 @subheading How do the front-end and back-end communicate?
2002 There is no communication from backend to front-end. From front-end to
2003 backend is simply the program flow: music + definitions gives
2004 contexts, contexts yield output, after processing, output is written
2007 @subheading Where is the functionality associated with KEYWORDs?
2009 See @file{my-lily-lexer.cc} (keywords, there aren't that many)
2010 and @file{ly/*.ly} (most of the other backslashed @code{/\words} are identifiers)
2012 @subheading What Contexts/Properties/Music/etc. are available when they are processed?
2014 What do you mean exactly with this question?
2016 See @file{ly/engraver-init.ly} for contexts,
2017 see @file{scm/define-*.scm} for other objects.
2019 @subheading How do you decide if something is a Music, Context, or Grob property?
2020 Why is part-combine-status a Music property when it seems (IMO)
2021 to be related to the Staff context?
2023 The Music_iterators and Context communicate through two channels
2025 Music_iterators can set and read context properties, idem for
2026 Engravers and Contexts
2028 Music_iterators can send "synthetic" music events (which aren't in
2029 the input) to a context. These are caught by Engravers. This is
2030 mostly a one way communication channel.
2032 part-combine-status is part of such a synthetic event, used by
2033 Part_combine_iterator to communicate with Part_combine_engraver.
2036 @subheading Deciding between context and music properties
2038 I'm adding a property to affect how \autochange works. It seems to
2039 me that it should be a context property, but the Scheme autochange
2040 procedure has a Music argument. Does this mean I should use
2043 \autochange is one of these extra strange beasts: it requires
2044 look-ahead to decide when to change staves. This is achieved by
2045 running the interpreting step twice (see
2046 @file{scm/part-combiner.scm} , at the bottom), and
2047 storing the result of the first step (where to switch
2048 staves) in a Music property. Since you want to influence that
2049 where-to-switch list, your must affect the code in
2050 make-autochange-music (@file{scm/part-combiner.scm}).
2051 That code is called directly from the parser and there are no
2052 official "parsing properties" yet, so there is no generic way
2053 to tune \autochange. We would have to invent something new
2054 for this, or add a separate argument,
2057 \autochange #around-central-C ..music..
2061 where around-central-C is some function that is called from
2062 make-autochange-music.
2064 @subheading More on context and music properties
2066 From Neil Puttock, in response to a question about transposition:
2068 Context properties (using \set & \unset) are tied to engravers: they
2069 provide information relevant to the generation of graphical objects.
2071 Since transposition occurs at the music interpretation stage, it has
2072 no direct connection with engravers: the pitch of a note is fixed
2073 before a notehead is created. Consider the following minimal snippet:
2079 This generates (simplified) a NoteEvent, with its pitch and duration
2080 as event properties,
2086 (ly:make-duration 2 0 1 1)
2088 (ly:make-pitch 0 0 0)
2091 which the Note_heads_engraver hears. It passes this information on to
2092 the NoteHead grob it creates from the event, so the head's correct
2093 position and duration-log can be determined once it's ready for
2096 If we transpose the snippet,
2099 \transpose c d @{ c' @}
2102 the pitch is changed before it reaches the engraver (in fact, it
2103 happens just after the parsing stage with the creation of a
2104 TransposedMusic music object):
2110 (ly:make-duration 2 0 1 1)
2112 (ly:make-pitch 0 1 0)
2115 You can see an example of a music property relevant to transposition:
2119 \transpose c d @{ c'2 \withMusicProperty #'untransposable ##t c' @}
2122 -> the second c' remains untransposed.
2124 Take a look at @file{lily/music.cc} to see where the transposition takes place.
2127 @subheading How do I tell about the execution environment?
2129 I get lost figuring out what environment the code I'm looking at is in when it
2130 executes. I found both the C++ and Scheme autochange code. Then I was trying
2131 to figure out where the code got called from. I finally figured out that the
2132 Scheme procedure was called before the C++ iterator code, but it took me a
2133 while to figure that out, and I still didn't know who did the calling in the
2134 first place. I only know a little bit about Flex and Bison, so reading those
2135 files helped only a little bit.
2137 @emph{Han-Wen:} GDB can be of help here. Set a breakpoint in C++, and run. When you
2138 hit the breakpoint, do a backtrace. You can inspect Scheme objects
2139 along the way by doing
2142 p ly_display_scm(obj)
2145 this will display OBJ through GUILE.
2147 @node Music functions and GUILE debugging
2148 @subsection Music functions and GUILE debugging
2150 Ian Hulin was trying to do some debugging in music functions, and
2151 came up with the following question
2154 I'm working on the Guile Debugger Stuff, and would like to try
2155 debugging a music function definition such as:
2158 conditionalMark = #(define-music-function (parser location) ()
2159 #@{ \tag #'instrumental-part @{\mark \default@} #@} )
2162 It appears conditionalMark does not get set up as an
2163 equivalent of a Scheme
2166 (define conditionalMark = define-music-function(parser location () ...
2170 although something gets defined because Scheme apparently recognizes
2173 #(set-break! conditionalMark)
2177 later on in the file without signalling any Guile errors.
2179 However the breakpoint trap is never encountered as
2180 define-music-function passed things on to ly:make-music-function,
2181 which is really C++ code ly_make_music_function, so Guile never
2182 finds out about the breakpoint.
2184 Han-Wen answered as follows:
2186 You can see the definition by doing
2189 #(display conditionalMark)
2193 inside the @file{.ly} file.
2195 The breakpoint failing may have to do with the call sequence. See
2196 @file{parser.yy}, run_music_function(). The function is called directly from
2197 C++, without going through the GUILE evaluator, so I think that is why
2198 there is no debugger trap.