Dylan Programming


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Dylan (DYnamic LANguage) is a new programming language invented by Apple Computer and several partners. Dylan is dynamic, is object-oriented, and delivers efficient applications.

The Dylan language is defined by The Dylan Reference Manual, written by Andrew Shalit, and published by Addison-Wesley (1996). That manual is the definitive reference on Dylan. The Dylan Reference Manual is available on the World Wide Web; see Resources on Dylan, for details.

Dylan is up and running. You can get it from Harlequin, Carnegie Mellon University, Apple Computer, Digitool, and other organizations. Dylan implementations run on most of the popular computer platforms. Full-fledged implementations provide both a compiler and a development environment. You can obtain public-domain implementations. See Resources on Dylan.


This book is written for application programmers who have experience working in a conventional language, such as C, Pascal, COBOL, FORTRAN, or BASIC, or in an object-oriented language, such as C++, Java, Smalltalk, or Common LISP with CLOS. Familiarity with object-oriented programming and dynamic languages is not required. We do compare Dylan to C, C++, and Java in this book, but you can read and understand the book without any knowledge of C, C++, or Java.

Goals of this book

The primary goals of this book are to teach you how to program in Dylan, and how to write programs in an object-oriented style. Along the way, we hope to convince you to use Dylan. It is intended to be a practical, elegant, and fun language to use. This book is a tutorial on programming in Dylan, and it does the following:

  • Begins with the most basic use of Dylan, and gradually expands to show the more powerful and advanced techniques.
  • Gives the flavor of working with the Dylan language in a typical Dylan environment.
  • Shows how to define classes and methods that work together to solve a problem.
  • Shows how to use many of Dylan’s classes, functions, and features to good effect within the context of an example application.
  • Introduces the more advanced features of Dylan, including multiple inheritance, performance, exceptions, and macros.

This book does not attempt to be as complete as The Dylan Reference Manual, and does not provide the following kinds of material:

  • Complete descriptions of all classes and functions provided by Dylan
  • Complete descriptions of the detailed mechanisms in Dylan
  • To make full use of Dylan, programmers need The Dylan Reference Manual, as well as this book.

Organization of this book

We have divided the chapters of this book into four parts:

Program examples

This book includes many program examples. Our approach is to show how evolutionary programming might work by presenting an example simply at first, and then expanding it gradually.

In Part 1. Basic Concepts, we develop an example of a simple library that represents time and position. That library is needed for the sample airport application that we develop in Part 3. Sample Application. The airport application simulates airplanes, runways, gates, flights, and airports. Its goal is to schedule gates for arriving and departing aircraft. To do scheduling, we need the library that represents and manipulates time and position.

Harlequin and Addison-Wesley provide World Wide Web pages containing the source code of the program examples. See Source Code of Program Examples.

Dylan’s core language is lean. It does not include input–output facilities, support for a user interface, or interfaces for communicating with programs written in other languages. These features are available in libraries supplied by vendors or in the public domain. We want this book to be applicable to the widest possible range of Dylan implementations, so we focus on the core Dylan language, and use only those library interfaces that are widely available.

Conventions used in this book

  • We use boldface when we introduce new terms, such as library.

  • We use bold typewriter font for code examples and names of Dylan functions and objects, such as define method. Code comments appear in oblique typewriter font — for example,

    // Method that says a greeting
    define method say-greeting (greeting :: <object>);
      format-out("%s\n", greeting);
  • Many Dylan environments provide a listener, which enables you to type in expressions and to see their return values and output. We use a hypothetical Dylan listener to show the result of evaluating Dylan expressions:

    ? say-greeting("hi, there");
    => hi, there

    In our hypothetical listener, the Dylan prompt is the question mark, ?. The bold typewriter font shows what the user types. The bold-oblique typewriter font shows what the listener displays.

  • We use boxes to give information about Dylan’s naming conventions, cautions, performance implications, comparisons to other languages such as C or C++, environment notes, and automatic-storage-management notes. Here is an example:

Environment note:

Our hypothetical development environment does not represent any particular Dylan development environment. Also note that the Dylan language does not require a development environment, so any given implementation may not provide one.

An image of Dylan

Jonathan Bachrach designed the image on the cover of this book. He played with the meaning that Dylan has for him by creating colorful tiles that appear to take off and fly. Each tile has its own vibrant color, unique personality, and individual strength. The tiles fly independently, but tend to flock with other tiles to achieve harmony within a community. Each tile could represent a Dylan component, or a Dylan programmer. Once Bachrach was satisfied with the still image, he took the next step, and built an animation of the tiles flying gracefully through space, flocking together, and creating a dynamic new world.

Bachrach wrote the animation and physical-modeling portions of the program in Dylan, using Open GL as the three-dimensional rendering substrate. Steve Rowley provided the physics equations. Bachrach demonstrated his animation at the Apple Worldwide Developers Conference in 1995.


We are fortunate to have at Harlequin a great pool of Dylan talent and expertise, including original inventors of the language, compiler gurus, and environment designers. A core group of Dylan experts and two expert C programmers gave us valuable technical advice and encouragement from the first to the final days of our project: Freeland Abbott, Jonathan Bachrach, Kim Barrett, Paul Butcher, Paul Haahr, Tony Mann, and Keith Playford. Other people reviewed our drafts along the way: Roman Budzianowski, Bob Cassels, Edward Cessna, Bill Chiles, Christopher Fry, David Gray, Eliot Miranda, Scott McKay, Nosa Omorogbe, Mike Plusch, and Andy Sizer. We are grateful to Harlequin people whose expertise lies in programming languages other than Dylan, for giving us their perspectives on our book: Judy Anderson, Wesley Dunnington, David Jones, Andy Latto, Peter Norvig, Kent Pitman, Steve Rowley, Craig Swanson, Jason Trenouth, Helen Vickers, and Evan Williams.

Andrew Shires carefully tested all our program examples. Brent Tennefoss gave us a great deal of help with graphics. Gary Palter shared his Macintosh expertise, and Leah Bateman shared her Windows expertise. Richard Brooksby let us steal time from other projects to write this book. Anne Altherr, Sharon Van Gundy, Clive Harris, and Sang Lee helped us to navigate the legal and business issues. Ken Jackson helped us to get the ball rolling, and gave it an extra push when needed. Jo Marks is one of Dylan’s biggest fans — he urged us to write this book as a way to explain the power of Dylan to a wider audience.

We are grateful to Dylan experts outside of Harlequin who gave us thoughtful and thorough reviews of the book: Scott Fahlman, Robert Futrelle, David Moon, and Andrew Shalit.

Our editors at Addison-Wesley cheerfully and capably steered us through the process and helped to shape our book. We are grateful to Sarah Hallet Corey, Lyn Dupré, Nancy Fenton, and Helen Goldstein. Eileen Hoff designed the cover using Bachrach’s image. It was, once again, a great pleasure to work with Peter Gordon.

We thank the people at Apple Computer who combined their vision of the future with hard work to make Dylan a reality. We thank the people at Carnegie Mellon University and Harlequin who continue to move Dylan forward with insight and creativity.

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