Understanding Scheme In LilyPond

GNU LilyPond

GNU LilyPond is an extremely powerful and versatile text based music notation system with a strong focus on traditional craftsmanship and the aesthetics of manual plate engraving. LilyPond reads input files in which the user has textually specified the content of a score, and it compiles these files to graphical scores in PDF, PNG, or SVG format (or additionally to non-graphical MIDI files). LilyPond's output can be tweaked to the least detail, but in fact even LilyPond's behaviour can be modified and extended right through to its inner gears! This is possible through LilyPond's built in extension language, Scheme

Extending LilyPond

In a way, extending Lilypond with Scheme is like open heart surgery, and it is definitely not necessary for regular use and engraving scores. However, getting familiar with Scheme at least basically is always a good idea as one encounters its language constructs even in entry level LilyPond documents. Understanding these elements and their integration in LilyPond will make you feel more comfortable writing LilyPond files, and it opens up the horizon, even without ambitions to become a serious LilyPond “programmer”.

The Thorny Path

For anybody except seasoned computer scientists this seems to be a thorny path, to which the LilyPond user mailing lists blatantly bear witness. The basic concepts of the language are different to what most users may know from more familiar script languages such as Python, JavaScript or even VisualBasic. The integration of two different languages (LilyPond and Scheme) adds to the complexity, and the advanced interaction with LilyPond's internals gives yet another layer on top.

User-friendly Learning Materials

The introductory chapters outline the different aspects to the complexity of understanding Scheme in LilyPond. But in addition I have always felt that part of the problem is a lack of suitable learning resources. Unfortunately the official documentation is not exactly helpful in a smooth introduction to the world of extending LilyPond with Scheme, presumably because it's written too much from a developer's perspective. This was my motivation to start a category of Scheme tutorials on Scores of Beauty, the semi-official LilyPond blog. The idea behind these tutorials is to focus on a single task and explain it at a slow pace and sufficiently verbosely to give non-programmers the chance to get an idea why something is done a certain way, rather than just having them copy some ready-made code fragments.

About This Book

When preparing a university course about Scheme in LilyPond I found myself starting to write the book that I would have needed several years ago. More and more it started heading towards a certain level of comprehensiveness, although it still doesn't claim to be a proper computer science textbook. In this online book I'll cover a lot of Scheme topics, from the perspective of its use in LilyPond. What I'm trying to achieve is giving thorough explanations to the fundamental concepts and language structures. This tries to overcome or at least alleviate the obstacles that LilyPond users typically face when trying to approach Scheme. The book won't be “easy reading”, but it is slow-paced enough to enable any reader to get a firm understanding of how and why things have to be written in certain ways when using Scheme in LilyPond.

Who Should Read This?

This book is for you if you

  • want to be more comfortable with the scripting extension in LilyPond
  • want to learn something new and powerful
  • want to stretch your limits with LilyPond
  • want to look under LilyPond's hood and experience the power of extending a program to its inner gears
  • are simply a curious nature

The Book's Structure

This book has to cover a range of different topics in several areas of interest. While slicing content in web-friendly, digestible chunks of information seems rather straightforward, the number of topics might quickly get out of hand for a website navigation. Therefore the book is split into a parent and a number of “child” books, each covering a specific area. The book parts are essentially equal with regard to design and navigation, but they are differentiated by color variations. The main book's left-hand navigation includes “downward” links to the sub books, while each of them points back to the main book.

The main book (which you are currently reading) introduces the topic and gives an overview about the context of LilyPond as a compiling batch program and its extensibility with Scheme.

The first part will introduce Scheme as a programming language. It covers very fundamental details but doesn't claim to be a proper computer science book. Introducing the programming language is done strictly from the perspective of a LilyPond user, both regarding possible preconditions and use cases and by covering exactly LilyPond's version of Scheme.

The second part deals with the integration of Scheme in LilyPond. This aims at providing familiarity with switching languages in input files, and at explaining the fundamentals of extending LilyPond's functionality through Scheme.

The third part will then dive into the internals of LilyPond, giving assistance in the tasks of tweaking LilyPond's behaviour to its inner core. This is where a number of wizards perform their black magic, and at the point of this writing (Jan 2020) it is totally unclear how far the journey will go with this book. But I really hope to be able to give you some foundations to build on so in future you will not have to fully and helplessly rely on the generosity of some friendly community member to donate you some copy&paste-able code.

Contributing

Like everything in the LilyPond and Free Software world I would be more than happy for this “book” to become a community-driven resource. If you feel you have something to say about a topic feel free to contact me directly; if you have something to comment on or complain about please click on the Github link at the top right and open an issue in the tracker. In addition, the first heading of each page includes an “edit” link leading to the actual page source; if you have an account on Github you will be able to edit the page and open a pull request for me to review.


Last update: January 31, 2020