10.1 Overview of LilyPond architecture (LilyPond Contributor’s Guide)

### 10.1 Overview of LilyPond architecture

LilyPond processes the input file into graphical and musical output in a number of stages. This process, along with the types of routines that accomplish the various stages of the process, is described in this section. A more complete description of the LilyPond architecture and internal program execution is found in Erik Sandberg’s master’s thesis.

The first stage of LilyPond processing is parsing. In the parsing process, music expressions in LilyPond input format are converted to music expressions in Scheme format. In Scheme format, a music expression is a list in tree form, with nodes that indicate the relationships between various music events. The LilyPond parser is written in Bison.

The second stage of LilyPond processing is iterating. Iterating assigns each music event to a context, which is the environment in which the music will be finally engraved. The context is responsible for all further processing of the music. It is during the iteration stage that contexts are created as necessary to ensure that every note has a Voice type context (e.g. Voice, TabVoice, DrumVoice, CueVoice, MensuralVoice, VaticanaVoice, GregorianTranscriptionVoice), that the Voice type contexts exist in appropriate Staff type contexts, and that parallel Staff type contexts exist in StaffGroup type contexts. In addition, during the iteration stage each music event is assigned a moment, or a time in the music when the event begins.

Each type of music event has an associated iterator. Iterators are defined in `*-iterator.cc`. During iteration, an event’s iterator is called to deliver that music event to the appropriate context(s).

The final stage of LilyPond processing is translation. During translation, music events are prepared for graphical or midi output. The translation step is accomplished by the polymorphic base class Translator through its two derived classes: Engraver (for graphical output) and Performer (for midi output).

Translators are defined in C++ files named `*-engraver.cc` and `*-performer.cc`. Much of the work of translating is handled by Scheme functions, which is one of the keys to LilyPond’s exceptional flexibility.

#### LilyPond Contributor’s Guide v2.26.0 (stable-branch).

- 1 Introduction to contributing
  - 1.1 Help us
  - 1.2 Overview of work flow
  - 1.3 Summary for experienced developers
  - 1.4 Mentors
- 2 Quick start
  - 2.1 LilyDev
    - Installing LilyDev in VirtualBox
    - Configuring LilyDev in VirtualBox
  - 2.2 Compiling with LilyDev
  - 2.3 Now start work!
- 3 Working with source code
  - 3.1 Setting up
    - 3.1.1 Installing Git
    - 3.1.2 Creating a GitLab account and setting up SSH
    - 3.1.3 Cloning and forking the repository
    - 3.1.4 Configuring Git
  - 3.2 Git cheat sheet
  - 3.3 Lifecycle of a merge request
    - 3.3.1 Uploading a patch for review
    - 3.3.2 Automated testing
    - 3.3.3 Patch countdown
    - 3.3.4 Merging to master
    - 3.3.5 Abandoned patches
  - 3.4 Writing good commit messages
  - 3.5 Commit access
  - 3.6 Further Git documentation resources
  - 3.7 Repository directory structure
- 4 Compiling
  - 4.1 Overview of compiling
  - 4.2 Requirements
    - 4.2.1 Requirements for running LilyPond
    - 4.2.2 Requirements for compiling LilyPond
      - Fedora
      - Linux Mint
      - OpenSUSE
      - Ubuntu
      - Other
    - 4.2.3 Requirements for building documentation
  - 4.3 Getting the source code
  - 4.4 Configuring `make`
    - 4.4.1 Build modes
    - 4.4.2 Running `autogen.sh`
    - 4.4.3 Running `configure`
      - Configuration options
      - Checking build dependencies
      - Configuring target directories
  - 4.5 Compiling LilyPond
    - 4.5.1 Using `make`
    - 4.5.2 Saving time with the `-j` option
    - 4.5.3 Useful `make` variables
  - 4.6 Post-compilation options
    - 4.6.1 Installing LilyPond from a local build
    - 4.6.2 Generating documentation
      - Documentation editor’s edit/compile cycle
      - Building documentation
      - Building a single document
      - Saving time with `CPU_COUNT`
      - Installing documentation
      - Building documentation without compiling
    - 4.6.3 Testing LilyPond binary
  - 4.7 Problems
  - 4.8 Concurrent stable and development versions
    - 4.8.1 Replacing the notation fonts in development versions
  - 4.9 Build system
- 5 Documentation work
  - 5.1 Introduction to documentation work
  - 5.2 `\version` in documentation files
  - 5.3 Documentation suggestions
  - 5.4 Texinfo introduction and usage policy
    - 5.4.1 Texinfo introduction
    - 5.4.2 Documentation files
    - 5.4.3 Sectioning commands
    - 5.4.4 Menus
    - 5.4.5 LilyPond formatting
    - 5.4.6 Text formatting
    - 5.4.7 Syntax survey
      - Comments
      - Cross-references
      - External links
      - Fixed-width font
      - Indexing
      - Lists
      - Special characters
      - Miscellany
    - 5.4.8 Other text concerns
  - 5.5 Documentation policy
    - 5.5.1 Books
    - 5.5.2 Section organization
    - 5.5.3 Checking cross-references
    - 5.5.4 General writing
    - 5.5.5 Technical writing style
  - 5.6 Tips for writing documentation
    - 5.6.1 Working on subsections
    - 5.6.2 Searching
  - 5.7 Scripts to ease documentation work
    - 5.7.1 Building only one section of the documentation
    - 5.7.2 Updating documentation with `convert-ly`
  - 5.8 Documentation strings in the Internals Reference
  - 5.9 Translating the documentation
    - 5.9.1 Getting started with documentation translation
      - Translation requirements
      - Which documentation can be translated
      - Starting translation in a new language
    - 5.9.2 Documentation translation details
      - Files to be translated
      - Translating the website and other Texinfo documentation
      - Adding a Texinfo manual
    - 5.9.3 Documentation translation maintenance
      - Check state of translation
      - Updating documentation translation
      - Updating translation committishes
      - Maintaining without updating translations
    - 5.9.4 Technical background
- 6 Website work
  - 6.1 Introduction to website work
  - 6.2 Uploading website
  - 6.3 Debugging website and docs locally
  - 6.4 Translating the website
- 7 LilyPond Wiki work
  - 7.1 Introduction to the LilyPond Wiki
  - 7.2 Adding and editing snippets
  - 7.3 Snippet administration
  - 7.4 The `makelsr.pl` script
  - 7.5 LilyPond Wiki to Git
- 8 Issues
  - 8.1 Introduction to issues
  - 8.2 Triaging bugs
  - 8.3 Issue classification
  - 8.4 Adding issues to the tracker
- 9 Regression tests
  - 9.1 Introduction to regression tests
  - 9.2 Precompiled regression tests
  - 9.3 Compiling regression tests
  - 9.4 Regtest comparison
  - 9.5 Pixel-based regtest comparison
  - 9.6 Finding the cause of a regression
  - 9.7 MusicXML tests
- 10 Programming work
  - 10.1 Overview of LilyPond architecture
  - 10.2 LilyPond programming languages
  - 10.3 Programming without compiling
  - 10.4 Finding functions
  - 10.5 Code style
    - 10.5.1 Languages
    - 10.5.2 Filenames
    - 10.5.3 Code formatting
    - 10.5.4 Naming Conventions
    - 10.5.5 Broken code
    - 10.5.6 Code comments
    - 10.5.7 Handling errors
    - 10.5.8 Localization
  - 10.6 Warnings, Errors, Progress and Debug Output
  - 10.7 Debugging LilyPond
    - 10.7.1 Debugging overview
    - 10.7.2 Debugging C++ code
    - 10.7.3 Debugging Scheme code
    - 10.7.4 Debugging scoring algorithms
    - 10.7.5 Debugging skylines
  - 10.8 Tracing object relationships
  - 10.9 Tracing processing time
  - 10.10 Adding or modifying features
    - 10.10.1 Write the code
    - 10.10.2 Write regression tests
    - 10.10.3 Write convert-ly rule
    - 10.10.4 Automatically update documentation
    - 10.10.5 Manually update documentation
    - 10.10.6 Edit changes.tely
    - 10.10.7 Verify successful build
    - 10.10.8 Verify regression tests
    - 10.10.9 Post patch for comments
    - 10.10.10 Push patch
    - 10.10.11 Closing the issues
  - 10.11 Iterator tutorial
  - 10.12 Engraver tutorial
    - 10.12.1 Useful methods for information processing
    - 10.12.2 Translation process
    - 10.12.3 Listening to music events
    - 10.12.4 Acknowledging grobs
    - 10.12.5 Engraver declaration/documentation
  - 10.13 Callback tutorial
  - 10.14 Understanding pure properties
    - 10.14.1 Purity in LilyPond
    - 10.14.2 Writing a pure function
    - 10.14.3 How purity is defined and stored
    - 10.14.4 Where purity is used
    - 10.14.5 Case studies
    - 10.14.6 Debugging tips
  - 10.15 LilyPond scoping
  - 10.16 Regular expressions
  - 10.17 Scheme->C interface
    - 10.17.1 Comparison
    - 10.17.2 Conversion
  - 10.18 Garbage collection for dummies
  - 10.19 LilyPond miscellany
    - 10.19.1 Spacing algorithms
    - 10.19.2 Info from Han-Wen email
    - 10.19.3 Music functions and Guile debugging
    - 10.19.4 Articulations on EventChord
- 11 Release work
  - 11.1 Development phases
  - 11.2 Release checklist
  - 11.3 Major release checklist
- 12 Modifying the Emmentaler font
  - 12.1 Overview of the Emmentaler font
  - 12.2 Font creation tools
  - 12.3 Adding a new font section
  - 12.4 Adding a new glyph
  - 12.5 Building the changed font
  - 12.6 METAFONT formatting rules
- 13 Administrative policies
  - 13.1 LilyPond is GNU Software
  - 13.2 Environment variables
  - 13.3 Performing yearly copyright update (“grand-replace”)
- Appendix A GNU Free Documentation License