-@c -*- coding: us-ascii; mode: texinfo; -*-
+@c -*- coding: utf-8; mode: texinfo; -*-
@node Programming work
@chapter Programming work
* Finding functions::
* Code style::
* Debugging LilyPond::
+* Tracing object relationships::
* Adding or modifying features::
* Iterator tutorial::
* Engraver tutorial::
programming in Scheme is available in its entirety online.
An introduction to Guile/Scheme as used in LilyPond can be found in the
-Learning Manual, see @rlearning{Scheme tutorial}.
+@rextend{Scheme tutorial}.
@subsection MetaFont
PostScript is used to generate graphical output. A brief PostScript tutorial
is @uref{http://local.wasp.uwa.edu.au/~pbourke/dataformats/postscript/,
available online}. The
-@uref{http://www.adobe.com/devnet/postscript/pdfs/PLRM.pdf, PostScript Lanugage
+@uref{http://www.adobe.com/devnet/postscript/pdfs/PLRM.pdf, PostScript Language
Reference} is available online in PDF format.
@subsection Python
-Python is used for XML2ly and is used for buillding the documentation and the
+Python is used for XML2ly and is used for building the documentation and the
website.
Python documentation is available at @uref{http://www.python.org/doc/,
@subsection Using the ROADMAP
The file ROADMAP is located in the main directory of the lilypond source.
-ROADMAP lists all of the directories in the LilPond source tree, along
+ROADMAP lists all of the directories in the LilyPond source tree, along
with a brief description of the kind of files found in each directory.
This can be a very helpful tool for deciding which directories to search
when looking for a function.
@node Code style
@section Code style
+This section describes style guidelines for LilyPond
+source code.
+
@menu
+* Languages::
+* Filenames::
+* Indentation::
+* Naming conventions::
+* Broken code::
+* Code comments::
+* Handling errors::
+* Localization::
@end menu
-@subsection Handling errors
-
-As a general rule, you should always try to continue computations,
-even if there is some kind of error. When the program stops, it
-is often very hard for a user to pinpoint what part of the input
-causes an error. Finding the culprit is much easier if there is
-some viewable output.
-
-So functions and methods do not return errorcodes, they never
-crash, but report a programming_error and try to carry on.
+@node Languages
@subsection Languages
C++ and Python are preferred. Python code should use PEP 8.
+
+@node Filenames
@subsection Filenames
Definitions of classes that are only accessed via pointers (*) or
The class Class_name is coded in @q{class-name.*}
+
+@node Indentation
@subsection Indentation
Standard GNU coding style is used. In emacs:
is the case, it is desirable to fix the improper indenting when the
file is modified, with the hope of continually improving the code.
-@subsection Indenting files with emacs in script mode
+
+@subheading Indenting files with fixcc.py
+
+LilyPond provides a python script that will correct the indentation
+on a c++ file:
+
+@example
+scripts/auxiliar/fixcc.py lily/my-test-file.cc
+@end example
+
+Be sure you replace @code{my-test-file.cc} with the name of the file
+that you edited.
+
+If you are editing a file that contains an ADD_TRANSLATOR or ADD_INTERFACE
+macro, the fixcc.py script will move the final parenthesis up one line
+from where it should be. Please check the end of the file before
+you run fixcc.py, and then put the final parenthesis and semicolon
+back on a line by themselves.
+
+
+@subheading Indenting files with emacs in script mode
@c email to wl@gnu.org when I get here.
Save it as a shell script, then run on the file(s) you modified.
-@subsection Indenting with vim
+
+@subheading Indenting with vim
Although emacs indentation is the LilyPond standard, acceptable
indentation can usually be accomplished with vim. Some hints for
autocmd BufWritePre * :%s/\s\+$//e
@end verbatim
-With this .vimrc, files can be reindented automatically by highlihting
+With this .vimrc, files can be reindented automatically by highlighting
the lines to be indented in visual mode (use V to enter visual mode)
and pressing =.
@verbatim
" Additional Guile-specific 'forms'
-syn keyword schemeSyntax define-public define* define-safe-public
+syn keyword schemeSyntax define-public define*-public
+syn keyword schemeSyntax define* lambda* let-keywords*
+syn keyword schemeSyntax defmacro defmacro* define-macro
+syn keyword schemeSyntax defmacro-public defmacro*-public
syn keyword schemeSyntax use-modules define-module
-syn keyword schemeSyntax defmacro-public define-macro
-syn keyword schemeSyntax define-markup-command
-syn keyword schemeSyntax define-markup-list-command
-syn keyword schemeSyntax let-keywords* lambda* define*-public
-syn keyword schemeSyntax defmacro* defmacro*-public
+syn keyword schemeSyntax define-method define-class
+
+" Additional LilyPond-specific 'forms'
+syn keyword schemeSyntax define-markup-command define-markup-list-command
+syn keyword schemeSyntax define-safe-public define-music-function
+syn keyword schemeSyntax def-grace-function
" All of the above should influence indenting too
-set lw+=define-public,define*,define-safe-public,use-modules,define-module
-set lw+=defmacro-public,define-macro
+set lw+=define-public,define*-public
+set lw+=define*,lambda*,let-keywords*
+set lw+=defmacro,defmacro*,define-macro
+set lw+=defmacro-public,defmacro*-public
+set lw+=use-modules,define-module
+set lw+=define-method,define-class
set lw+=define-markup-command,define-markup-list-command
-set lw+=let-keywords*,lambda*,define*-public,defmacro*,defmacro*-public
+set lw+=define-safe-public,define-music-function
+set lw+=def-grace-function
" These forms should not influence indenting
set lw-=if
syn match schemeFunc "ly:[^) ]\+"
@end verbatim
-@subsection Classes and Types
+
+@node Naming conventions
+@subsection Naming Conventions
+
+Naming conventions have been established for LilyPond
+source code.
+
+@subheading Classes and Types
+
+Classes begin with an uppercase letter, and words
+in class names are separated with @code{_}:
@verbatim
This_is_a_class
@end verbatim
-
-@subsection Members
+@subheading Members
Member variable names end with an underscore:
Type Class::member_
@end verbatim
+@subheading Macros
-@subsection Macros
+Macro names should be written in uppercase completely,
+with words separated by @code{_}:
-Macro names should be written in uppercase completely.
+@verbatim
+THIS_IS_A_MACRO
+@end verbatim
+@subheading Variables
+Variable names should be complete words, rather than abbreviations.
+For example, it is preferred to use @code{thickness} rather than
+@code{th} or @code{t}.
+
+Multi-word variable names in C++ should have the words separated
+by the underscore character (@q{_}):
+
+@verbatim
+cxx_multiword_variable
+@end verbatim
+
+Multi-word variable names in Scheme should have the words separated
+by a hyphen (@q{-}):
+
+@verbatim
+scheme-multiword-variable
+@end verbatim
+
+@node Broken code
@subsection Broken code
Do not write broken code. This includes hardwired dependencies,
you can not avoid it, mark the place clearly, and add a comment
explaining shortcomings of the code.
-We reject broken-in-advance on principle.
-
-@subsection Naming
-
-Variable names should be complete words, rather than abbreviations.
-For example, it is preferred to use @code{thickness} rather than
-@code{th} or @code{t}.
+Ideally, the comment marking the shortcoming would include
+TODO, so that it is marked for future fixing.
-Multi-word variable names in C++ should have the words separated
-by the underscore character (@q{_}).
+We reject broken-in-advance on principle.
-Multi-word variable names in Scheme should have the words separated
-by a hyphen (@q{-}).
-@subsection Comments
+@node Code comments
+@subsection Code comments
Comments may not be needed if descriptive variable names are used
in the code and the logic is straightforward. However, if the
preparing a patch, it would be wise to add comments reflecting your
understanding to make future work easier.
-@subsection Messages
-Messages need to follow Localization.
+@node Handling errors
+@subsection Handling errors
+
+As a general rule, you should always try to continue computations,
+even if there is some kind of error. When the program stops, it
+is often very hard for a user to pinpoint what part of the input
+causes an error. Finding the culprit is much easier if there is
+some viewable output.
+
+So functions and methods do not return errorcodes, they never
+crash, but report a programming_error and try to carry on.
+
+Error and warning messages need to be localized.
+@node Localization
@subsection Localization
-This document provides some guidelines for programmers write user
+This document provides some guidelines to help programmers write
+proper user
messages. To help translations, user messages must follow
uniform conventions. Follow these rules when coding for LilyPond.
Hopefully, this can be replaced by general GNU guidelines in the
@item
Think about translation issues. In a lot of cases, it is better to
-translate a whole message. The english grammar must not be imposed
+translate a whole message. The English grammar must not be imposed
on the translator. So, instead of
@example
@item
Do not modularize too much; words frequently cannot be translated
-without context. It is probably safe to treat most occurences of
+without context. It is probably safe to treat most occurrences of
words like stem, beam, crescendo as separately translatable words.
@item
The GNU debugger, gdb, is the principal tool for debugging C++ code.
-@unnumberedsubsubsec Compiling LilyPond for use with gdb
+@subheading Compiling LilyPond for use with gdb
In order to use gdb with LilyPond, it is necessary to compile
LilyPond with debugging information. This is accomplished by running
with LilyPond. The @var{make install} command will strip debugging
information from the LilyPond binary.
-@unnumberedsubsubsec Typical gdb usage
+@subheading Typical gdb usage
Once you have compiled the Lilypond image with the necessary
debugging information it will have been written to a location in a
This is important as you will need to let gdb know where to find the
image containing the symbol tables. You can invoke gdb from the
-command line using
+command line using the following:
@example
gdb out/bin/lilypond
@end example
@noindent
This loads the LilyPond symbol tables into gdb. Then, to run
-LilyPond on @code{test.ly} under the debugger, enter
+LilyPond on @code{test.ly} under the debugger, enter the following:
@example
run test.ly
@end example
+
@noindent
at the gdb prompt.
As an alternative to running gdb at the command line you may try
-a graphical interface to gdb such as ddd
+a graphical interface to gdb such as ddd:
@example
ddd out/bin/lilypond
You can also use sets of standard gdb commands stored in a .gdbinit
file (see next section).
-@unnumberedsubsubsec Typical .gdbinit files
+@subheading Typical .gdbinit files
The behavior of gdb can be readily customized through the use of a
@var{.gdbinit} file. A @var{.gdbinit} file is a file named
interactively using just Guile or you can use the debugging
tools available within Guile.
-@unnumberedsubsubsec Using Guile interactively with LilyPond
+@subheading Using Guile interactively with LilyPond
In order to experiment with Scheme programming in the LilyPond
environment, it is necessary to have a Guile interpreter that
The compilation of the .ly file will then continue.
-@unnumberedsubsubsec Using the Guile debugger
+@subheading Using the Guile debugger
To set breakpoints and/or enable tracing in Scheme functions, put
is processed, a guile prompt will be displayed. You may now enter
commands to set up breakpoints and enable tracing by the Guile debugger.
-@unnumberedsubsubsec Using breakpoints
+@subheading Using breakpoints
At the guile prompt, you can set breakpoints with
the @code{set-break!} procedure:
(set-break! print-book-with)
@end example
-@unnumberedsubsubsec Tracing procedure calls and evaluator steps
+@subheading Tracing procedure calls and evaluator steps
Two forms of trace are available:
@code{set-trace-subtree!} traces every step the Scheme evaluator
performs in evaluating the procedure.
+@node Tracing object relationships
+@section Tracing object relationships
+
+Understanding the LilyPond source often boils down to figuring out what
+is happening to the Grobs. Where (and why) are they being created,
+modified and destroyed? Tracing Lily through a debugger in order to
+identify these relationships can be time-consuming and tedious.
+
+In order to simplify this process, a facility has been added to
+display the grobs that are created and the properties that are set
+and modified. Although it can be complex to get set up, once set up
+it easily provides detailed information about the life of grobs
+in the form of a network graph.
+
+Each of the steps necessary to use the graphviz utility
+is described below.
+
+@enumerate
+
+@item Installing graphviz
+
+In order to create the graph of the object relationships, it is
+first necessary to install Graphviz. graphviz is available for a
+number of different platforms:
+
+@example
+@uref{http://www.graphviz.org/Download..php}
+@end example
+
+@item Modifying config.make
+
+In order for the Graphviz tool to work, config.make must be modified.
+It is probably a good idea to first save a copy of config.make under
+a different name. Then, edit config.make by removing every occurrence
+of @code{-DNDEBUG}.
+
+@item Rebuilding LilyPond
+
+The executable code of LilyPond must be rebuilt from scratch:
+
+@example
+make -C lily clean && make -C lily
+@end example
+
+@item Create a graphviz-compatible @file{.ly} file
+
+In order to use the graphviz utility, the @file{.ly} file must include
+@file{ly/@/graphviz@/-init@/.ly}, and should then specify the
+grobs and symbols that should be tracked. An example of this
+is found in @file{input/regression/graphviz.ly}.
+
+@item Run lilypond with output sent to a log file
+
+The Graphviz data is sent to stderr by lilypond, so it is
+necessary to redirect stderr to a logfile:
+
+@example
+lilypond graphviz.ly 2> graphviz.log
+@end example
+
+@item Edit the logfile
+
+The logfile has standard lilypond output, as well as the Graphviz
+output data. Delete everything from the beginning of the file
+up to but not including the first occurrence of @code{digraph}.
+
+@item Process the logfile with @code{dot}
+
+The directed graph is created from the log file with the program
+@code{dot}:
+
+@example
+dot -Tpdf graphviz.log > graphviz.pdf
+@end example
+
+@end enumerate
+
+The pdf file can then be viewed with any pdf viewer.
+
+When compiled without @code{-DNDEBUG}, lilypond may run slower
+than normal. The original configuration can be restored by either
+renaming the saved copy of @code{config.make} or rerunning
+@code{configure}. Then rebuild lilypond with
+
+@example
+make -C lily clean && make -C lily
+@end example
+
+
@node Adding or modifying features
@section Adding or modifying features
its long-term support. This section describes the steps necessary
for feature addition and modification.
+
+@menu
+* Write the code::
+* Write regression tests::
+* Write convert-ly rule::
+* Automatically update auxiliary information::
+* Manually update auxiliary information::
+* Edit changes.tely::
+* Verify successful build::
+* Verify regression tests::
+* Post patch for comments::
+* Push patch::
+* Closing the issues::
+@end menu
+
+@node Write the code
@subsection Write the code
You should probably create a new git branch for writing the code, as that
Please be sure to follow the rules for programming style discussed
earlier in this chapter.
+
+@node Write regression tests
@subsection Write regression tests
In order to demonstrate that the code works properly, you will
Use existing regression tests as templates to demonstrate the type of
header information that should be included in a regression test.
+
+@node Write convert-ly rule
@subsection Write convert-ly rule
If the modification changes the input syntax, a convert-ly rule
rule will simply point out to the user that the feature needs
manual correction.
-@subsection Automatically update documentation, snippets, and regtests
+
+@node Automatically update auxiliary information
+@subsection Automatically update auxiliary information
convert-ly should be used to update the documentation, the snippets,
and the regression tests. This not only makes the necessary syntax
@end example
-@subsection Manually update documentation, snippets, and regtests
+
+@node Manually update auxiliary information
+@subsection Manually update auxiliary information
Where the convert-ly rule is not able to automatically update the inline
lilypond code in the documentation (i.e. if a NOT_SMART rule is used), the
Where the convert-ly rule is not able to automatically update snippets
in Documentation/snippets/, those snippets must be manually updated.
Those snippets should be copied to Documentation/snippets/new. The
-comments at the top of the snippet describing its automatice generation
+comments at the top of the snippet describing its automatic generation
should be removed. All translated texidoc strings should be removed.
The comment @qq{% begin verbatim} should be removed. The syntax of
the snippet should then be manually edited.
or removed from the documentation should be changed only in
the English version.
+
+@node Edit changes.tely
@subsection Edit changes.tely
An entry should be added to Documentation/changes.tely to describe
The changes.tely entry should be written to show how the new change
improves LilyPond, if possible.
+
+@node Verify successful build
@subsection Verify successful build
When the changes have been made, successful completion must be
Developers on Windows who are unable to build LilyPond should
get help from a Linux or OSX developer to do the make tests.
-@subsection Verify regression test
+
+@node Verify regression tests
+@subsection Verify regression tests
In order to avoid breaking LilyPond, it is important to verify that
-the regression tests all succeed. This process is described in
-@ref{Regression tests}.
+the regression tests succeed, and that no unwanted changes are
+introduced into the output. This process is described in
+@ref{Identifying code regressions}.
+@subheading Typical developer's edit/compile/test cycle
+
+TODO: is @code{[-j@var{X} CPU_COUNT=@var{X}]} useful for
+@code{test-baseline}, @code{check}, @code{clean},
+@code{test-redo}? Neil Puttock says it is useful for
+everything but @code{clean}, which is disk-limited.
+Need to check formally.
+
+@itemize
+@item
+Initial test:
+
+@example
+make [-j@var{X}]
+make test-baseline
+make [-j@var{X} CPU_COUNT=@var{X}] check
+@end example
+
+@item
+Edit/compile/test cycle:
+
+@example
+@emph{## edit source files, then...}
+
+make clean @emph{## only if needed (see below)}
+make [-j@var{X}] @emph{## only if needed (see below)}
+make test-redo @emph{## redo files differing from baseline}
+make [-j@var{X} CPU_COUNT=@var{X}] check @emph{## CPU_COUNT here?}
+@end example
+
+@item
+Reset:
+
+@example
+make test-clean
+@end example
+@end itemize
+
+If you modify any source files that have to be compiled (such as
+@file{.cc} or @file{.hh} files in @file{flower/} or @file{lily/}),
+then you must run @command{make} before @command{make test-redo},
+so @command{make} can compile the modified files and relink all
+the object files. If you only modify files which are interpreted,
+like those in the @file{scm/} and @file{ly/} directories, then
+@command{make} is not needed before @command{make test-redo}.
+
+TODO: Fix the following paragraph. You can do @command{rm mf/out/*}
+instead of make clean, and you can probably do
+@command{make -C mf/ clean} as well, but I haven't checked it -- cds
+
+Also, if you modify any font definitions in the @file{mf/}
+directory then you must run @command{make clean} and
+@command{make} before running @command{make test-redo}. This will
+recompile everything, whether modified or not, and takes a lot
+longer.
+
+Running @command{make@tie{}check} will leave an HTML page
+@file{out/@/test@/-results/@/index@/.html}. This page shows all the
+important differences that your change introduced, whether in the
+layout, MIDI, performance or error reporting.
+
+
+
+
+@node Post patch for comments
@subsection Post patch for comments
For any change other than a minor change, a patch set should be
for the same issue can be uploaded by reissuing the git-cl command
with the modified branch checked out.
+Sometimes in response to comments on revisions, the best way to
+work may require creation of a new branch in git. In order to
+associate the new branch with an existing Rietveld issue,
+the following command can be used:
+
+@example
+git cl issue issue-number
+@end example
+
+@noindent
+where @code{issue-number} is the number of the existing Rietveld
+issue.
+
+
+
+@node Push patch
@subsection Push patch
Once all the comments have been addressed, the patch can be pushed.
If the author has push privileges, the author will push the patch.
Otherwise, a developer with push privileges will push the patch.
+
+@node Closing the issues
@subsection Closing the issues
Once the patch has been pushed, all the relevant issues should be
should be sent to bug-lilypond requesting the BugMeister to change
the status.
+
@node Iterator tutorial
@section Iterator tutorial
-FIXME -- this is a placeholder for a tutorial on iterators
+TODO -- this is a placeholder for a tutorial on iterators
Iterators are routines written in C++ that process music expressions
and sent the music events to the appropriate engravers and/or
performers.
+
@node Engraver tutorial
@section Engraver tutorial
-FIXME -- This is a placeholder for a tutorial on how engravers work.
-
Engravers are C++ classes that catch music events and
create the appropriate grobs for display on the page. Though the
majority of engravers are responsible for the creation of a single grob,
in some cases (e.g. @code{New_fingering_engraver}), several different grobs
may be created.
+Engravers listen for events and acknowledge grobs. Events are passed to
+the engraver in time-step order during the iteration phase. Grobs are
+made available to the engraver when they are created by other engravers
+during the iteration phase.
+
+
+@menu
+* Useful methods for information processing::
+* Translation process::
+* Preventing garbage collection for SCM member variables::
+* Listening to music events::
+* Acknowledging grobs::
+* Engraver declaration/documentation::
+@end menu
+
+@node Useful methods for information processing
@subsection Useful methods for information processing
An engraver inherits the following public methods from the Translator
translation: for example, an unterminated spanner might be completed
automatically or reported with a warning message.
+
+@node Translation process
@subsection Translation process
At each timestep in the music, translation proceeds by calling the
following methods in turn:
-@code{start_translation_timestep ()} is called before any user information enters
-the translators, i.e., no property operations (\set, \override, etc.) or events
-have been processed yet.
+@code{start_translation_timestep ()} is called before any user
+information enters the translators, i.e., no property operations
+(\set, \override, etc.) or events have been processed yet.
-@code{process_music ()} and @code{process_acknowledged ()} are called after events
-have been heard, or grobs have been acknowledged. The latter tends to be used
-exclusively with engravers which only acknowledge grobs, whereas the former is
-the default method for main processing within engravers.
+@code{process_music ()} and @code{process_acknowledged ()} are called
+after all events in the current time step have been heard, or all
+grobs in the current time step have been acknowledged. The latter
+tends to be used exclusively with engravers which only acknowledge
+grobs, whereas the former is the default method for main processing
+within engravers.
-@code{stop_translation_timestep ()} is called after all user information has been
-processed prior to beginning the translation for the next timestep.
+@code{stop_translation_timestep ()} is called after all user
+information has been processed prior to beginning the translation for
+the next timestep.
+
+@node Preventing garbage collection for SCM member variables
@subsection Preventing garbage collection for SCM member variables
-In certain cases, an engraver might need to ensure private Scheme variables
-(with type SCM) do not get swept away by Guile's garbage collector: for example,
-a cache of the previous key signature which must persist persist between timesteps.
-The method @code{virtual derived_mark () const} can be used in such cases to mark
-such objects as follows:
+In certain cases, an engraver might need to ensure private Scheme
+variables (with type SCM) do not get swept away by Guile's garbage
+collector: for example, a cache of the previous key signature which
+must persist between timesteps. The method
+@code{virtual derived_mark () const} can be used in such cases:
@example
Engraver_name::derived_mark ()
@end example
+@node Listening to music events
@subsection Listening to music events
-External interfaces to to the engraver are implemented by protected
+External interfaces to the engraver are implemented by protected
macros including one or more of the following:
@itemize
@}
@end example
+
+@node Acknowledging grobs
@subsection Acknowledging grobs
Some engravers also need information from grobs as they are created
@}
@end example
+
+@node Engraver declaration/documentation
@subsection Engraver declaration/documentation
An engraver must have a public macro
and @code{Engraver_writes} is the set of properties written by
the engraver.
+The @code{ADD_ACKNOWLEDGER} and @code{ADD_TRANSLATOR} macros use a
+non-standard indentation system. Each interface, grob, read property,
+and write property is on its own line, and the closing parenthesis
+and semicolon for the macro all occupy a separate line beneath the final
+interface or write property. See existing engraver files for more
+information.
+
+
@node Callback tutorial
@section Callback tutorial
-FIXME -- This is a placeholder for a tutorial on callback functions.
+TODO -- This is a placeholder for a tutorial on callback functions.
@node LilyPond scoping
@section LilyPond scoping
but doesn't yet have a proper home. Ideally, the length of this section
would become zero as items are moved to other homes.
-@subsection Info from Han-Wen Email
+
+@menu
+* Spacing algorithms::
+* Info from Han-Wen email::
+* Music functions and GUILE debugging::
+@end menu
+
+@node Spacing algorithms
+@subsection Spacing algorithms
+
+Here is information from an email exchange about spacing algorithms.
+
+On Thu, 2010-02-04 at 15:33 -0500, Boris Shingarov wrote:
+I am experimenting with some modifications to the line breaking code,
+and I am stuck trying to understand how some of it works. So far my
+understanding is that Simple_spacer operates on a vector of Grobs, and
+it is a well-known Constrained-QP problem (rods = constraints, springs
+= quadratic function to minimize). What I don't understand is, if the
+spacer operates at the level of Grobs, which are built at an earlier
+stage in the pipeline, how are the changes necessitated by differences
+in line breaking, taken into account? in other words, if I take the
+last measure of a line and place it on the next line, it is not just a
+matter of literally moving that graphic to where the start of the next
+line is, but I also need to draw a clef, key signature, and possibly
+other fundamental things -- but at that stage in the rendering
+pipeline, is it not too late??
+
+Joe Neeman answered:
+
+We create lots of extra grobs (eg. a BarNumber at every bar line) but
+most of them are not drawn. See the break-visibility property in
+item-interface.
+
+
+@node Info from Han-Wen email
+@subsection Info from Han-Wen email
In 2004, Douglas Linhardt decided to try starting a document that would
explain LilyPond architecture and design principles. The material below
The headings reflect questions from Doug or comments from Han-Wen;
the body text are Han-Wen's answers.
-@unnumberedsubsubsec Figuring out how things work.
+@subheading Figuring out how things work.
I must admit that when I want to know how a program works, I use grep
and emacs and dive into the source code. The comments and the code
itself are usually more revealing than technical documents.
-@unnumberedsubsubsec What's a grob, and how is one used?
+@subheading What's a grob, and how is one used?
Graphical object - they are created from within engravers, either as
Spanners (derived class) -slurs, beams- or Items (also a derived
class) -notes, clefs, etc.
There are two other derived classes System (derived from Spanner,
-contaning a "line of music") and Paper_column (derived from Item, it
+containing a "line of music") and Paper_column (derived from Item, it
contains all items that happen at the same moment). They are separate
classes because they play a special role in the linebreaking process.
-@unnumberedsubsubsec What's a smob, and how is one used?
+@subheading What's a smob, and how is one used?
A C(++) object that is encapsulated so it can be used as a Scheme
object. See GUILE info, "19.3 Defining New Types (Smobs)"
-@unnumberedsubsubsec When is each C++ class constructed and used
+@@subheading When is each C++ class constructed and used
@itemize
@end itemize
-@unnumberedsubsubsec Can you get to Context properties from a Music object?
+@subheading Can you get to Context properties from a Music object?
You can create music object with a Scheme function that reads context
properties (the \applycontext syntax). However, that function is
properties from Music objects, since music objects are not directly
connected to Contexts. That connection is made by the Music_iterators
-@unnumberedsubsubsec Can you get to Music properties from a Context object?
+@subheading Can you get to Music properties from a Context object?
Yes, if you are given the music object within a Context
object. Normally, the music objects enter Contexts in synchronized
fashion, and the synchronization is done by Music_iterators.
-@unnumberedsubsubsec What is the relationship between C++ classes and Scheme objects?
+@subheading What is the relationship between C++ classes and Scheme objects?
Smobs are C++ objects in Scheme. Scheme objects (lists, functions) are
manipulated from C++ as well using the GUILE C function interface
(prefix: scm_)
-@unnumberedsubsubsec How do Scheme procedures get called from C++ functions?
+@subheading How do Scheme procedures get called from C++ functions?
scm_call_*, where * is an integer from 0 to 4.
Also scm_c_eval_string (), scm_eval ()
-@unnumberedsubsubsec How do C++ functions get called from Scheme procedures?
+@subheading How do C++ functions get called from Scheme procedures?
Export a C++ function to Scheme with LY_DEFINE.
-@unnumberedsubsubsec What is the flow of control in the program?
+@subheading What is the flow of control in the program?
Good question. Things used to be clear-cut, but we have Scheme
and SMOBs now, which means that interactions do not follow a very
rigid format anymore. See below for an overview, though.
-@unnumberedsubsubsec Does the parser make Scheme procedure calls or C++ function calls?
+@subheading Does the parser make Scheme procedure calls or C++ function calls?
Both. And the Scheme calls can call C++ and vice versa. It's nested,
with the SCM datatype as lubrication between the interactions
(I think the word "lubrication" describes the process better than the
traditional word "glue")
-@unnumberedsubsubsec How do the front-end and back-end get started?
+@subheading How do the front-end and back-end get started?
Front-end: a file is parsed, the rest follows from that. Specifically,
Paper_lines and other things. This area is still heavily in flux, and
perhaps not something you should want to look at.
-@unnumberedsubsubsec How do the front-end and back-end communicate?
+@subheading How do the front-end and back-end communicate?
There is no communication from backend to front-end. From front-end to
backend is simply the program flow: music + definitions gives
contexts, contexts yield output, after processing, output is written
to disk.
-@unnumberedsubsubsec Where is the functionality associated with KEYWORDs?
+@subheading Where is the functionality associated with KEYWORDs?
See my-lily-lexer.cc (keywords, there aren't that many) and ly/*.ly
(most of the other backslashed \words are identifiers)
-@unnumberedsubsubsec What Contexts/Properties/Music/etc. are available when they are processed?
+@subheading What Contexts/Properties/Music/etc. are available when they are processed?
What do you mean exactly with this question?
See ly/engraver-init.ly for contexts, see scm/define-*.scm for other
objects.
-@unnumberedsubsubsec How do you decide if something is a Music, Context, or Grob property?
+@subheading How do you decide if something is a Music, Context, or Grob property?
Why is part-combine-status a Music property when it seems (IMO)
to be related to the Staff context?
Part_combine_iterator to communicate with Part_combine_engraver.
-@unnumberedsubsubsec I'm adding a property to affect how \autochange works. It seems to
+@subheading Deciding between context and music properties
+
+I'm adding a property to affect how \autochange works. It seems to
me that it should be a context property, but the Scheme autochange
-procecure has a Music argument. Does this mean I should use
+procedure has a Music argument. Does this mean I should use
a Music property?
\autochange is one of these extra strange beasts: it requires
where around-central-C is some function that is called from
make-autochange-music.
-@unnumberedsubsubsec I get lost figuring out what environment the code I'm looking at is in when it executes.
-I found both the C++ and Scheme autochange code. Then I was
-trying to figure out where the code got called from. I finally figured out that
-the Scheme procedure was called before the C++ iterator code, but it took me a
+@subheading More on context and music properties
+
+From Neil Puttock, in response to a question about transposition:
+
+Context properties (using \set & \unset) are tied to engravers: they
+provide information relevant to the generation of graphical objects.
+
+Since transposition occurs at the music interpretation stage, it has
+no direct connection with engravers: the pitch of a note is fixed
+before a notehead is created. Consider the following minimal snippet:
+
+@example
+@{ c' @}
+@end example
+
+This generates (simplified) a NoteEvent, with its pitch and duration
+as event properties,
+
+@example
+(make-music
+ 'NoteEvent
+ 'duration
+ (ly:make-duration 2 0 1 1)
+ 'pitch
+ (ly:make-pitch 0 0 0)
+@end example
+
+which the Note_heads_engraver hears. It passes this information on to
+the NoteHead grob it creates from the event, so the head's correct
+position and duration-log can be determined once it's ready for
+printing.
+
+If we transpose the snippet,
+
+@example
+\transpose c d @{ c' @}
+@end example
+
+the pitch is changed before it reaches the engraver (in fact, it
+happens just after the parsing stage with the creation of a
+TransposedMusic music object):
+
+@example
+(make-music
+ 'NoteEvent
+ 'duration
+ (ly:make-duration 2 0 1 1)
+ 'pitch
+ (ly:make-pitch 0 1 0)
+@end example
+
+You can see an example of a music property relevant to transposition:
+untransposable.
+
+@example
+\transpose c d @{ c'2 \withMusicProperty #'untransposable ##t c' @}
+@end example
+
+-> the second c' remains untransposed.
+
+Take a look at lily/music.cc to see where the transposition takes place.
+
+
+@subheading How do I tell about the execution environment?
+
+I get lost figuring out what environment the code I'm looking at is in when it
+executes. I found both the C++ and Scheme autochange code. Then I was trying
+to figure out where the code got called from. I finally figured out that the
+Scheme procedure was called before the C++ iterator code, but it took me a
while to figure that out, and I still didn't know who did the calling in the
first place. I only know a little bit about Flex and Bison, so reading those
files helped only a little bit.
this will display OBJ through GUILE.
+@node Music functions and GUILE debugging
@subsection Music functions and GUILE debugging
Ian Hulin was trying to do some debugging in music functions, and
Han-Wen answered as follows:
-You can see the defintion by doing
+You can see the definition by doing
@example
#(display conditionalMark)
projects / lilypond.git / blobdiff