Scheme was introduced in 1975 by Gerald J. Sussman and Guy L. Steele Jr. [28,29], and was the first dialect of Lisp to fully support lexical scoping, first-class procedures, and continuations. In its earliest form it was a small language intended primarily for research and teaching, supporting only a handful of predefined syntactic forms and procedures. Scheme is now a complete general-purpose programming language, though it still derives its power from a small set of key concepts. Early implementations of the language were interpreter-based and slow, but some current Scheme implementations boast sophisticated compilers that generate code on par with code generated by the best optimizing compilers for lower-level languages such as C and Fortran.

This book is intended to provide an introduction to the Scheme programming language but not an introduction to programming in general. The reader is expected to have had some experience programming and to be familiar with terms commonly associated with computers and programming languages. Readers unfamiliar with Scheme or Lisp should also consider reading The Little Schemer [13] to become familiar with the concepts of list processing and recursion. Readers new to programming should begin with an introductory text on programming.

Scheme has been standardized both formally and informally. The IEEE Standard for the Scheme Programming Language [18], describes a formal ANSI/IEEE Standard for Scheme but dates back to 1991. A related series of reports, the "Revised Reports on the Algorithmic Language Scheme," document an evolving informal standard that most implementations support. The current report in this series is the "Revised6 Report on the Algorithmic Language Scheme" [24], which was completed in 2007.

This book covers the language of the Revised6 Report. It is not intended to supplant the Revised6 Report but rather to provide a more comprehensive introduction and reference manual for the language, with more explanatory text and examples, suitable more for users than for implementors. Features specific to particular implementations of Scheme are not included. In particular, features specific to the author's Chez Scheme and Petite Chez Scheme implementations are described separately in the Chez Scheme User's Guide [9]. On the other hand, no book on Scheme would be complete without some coverage of the interactive top level, since nearly every Scheme system supports interactive use in one form or another, even though the behavior is not standardized by the Revised6 Report. Chapters 2 and 3 are thus written assuming that the reader has available a Scheme implementation that supports an interactive top level, with behavior consistent with the description of the top-level environment in earlier reports and the IEEE/ANSI standard.

A large number of small- to medium-sized examples are spread throughout the text, and one entire chapter is dedicated to the presentation of a set of longer examples. Many of the examples show how a standard Scheme syntactic form or procedure might be implemented; others implement useful extensions. All of the examples can be entered directly from the keyboard into an interactive Scheme session.

This book is organized into twelve chapters, plus back matter. Chapter 1 describes the properties and features of Scheme that make it a useful and enjoyable language to use. Chapter 1 also describes Scheme's notational conventions and the typographical conventions employed in this book.

Chapter 2 is an introduction to Scheme programming for the novice Scheme programmer that leads the reader through a series of examples, beginning with simple Scheme expressions and working toward progressively more difficult ones. Each section of Chapter 2 introduces a small set of related features, and the end of each section contains a set of exercises for further practice. The reader will learn the most from Chapter 2 by sitting at the keyboard and typing in the examples and trying the exercises.

Chapter 3 continues the introduction but covers more advanced features and concepts. Even readers with prior Scheme experience may wish to work through the examples and exercises found there.

Chapters 4 through 11 make up the reference portion of the text. They present each of Scheme's primitive procedures and syntactic forms in turn, grouping them into short sections of related procedures and forms. Chapter 4 describes operations for creating procedures and variable bindings; Chapter 5, program control operations; Chapter 6, operations on the various object types (including lists, numbers, and strings); Chapter 7, input and output operations; Chapter 8, syntactic extension; Chapter 9, record-type definitions; Chapter 10, libraries and top-level programs; and Chapter 11, exceptions and conditions.

Chapter 12 contains a collection of example procedures, libraries, and programs, each with a short overview, some examples of its use, the implementation with brief explanation, and a set of exercises for further work. Each of these programs demonstrates a particular set of features, and together they illustrate an appropriate style for programming in Scheme.

Following Chapter 12 are bibliographical references, answers to selected exercises, a detailed description of the formal syntax of Scheme programs and data, a concise summary of Scheme syntactic forms and procedures, and the index. The summary of forms and procedures is a useful first stop for programmers unsure of the structure of a syntactic form or the arguments expected by a primitive procedure. The page numbers appearing in the summary of forms and procedures and the italicized page numbers appearing in the index indicate the locations in the text where forms and procedures are defined.

Because the reference portion describes a number of aspects of the language not covered by the introductory chapters along with a number of interesting short examples, most readers will find it profitable to read through most of the material to become familiar with each feature and how it relates to other features. Chapter 6 is lengthy, however, and may be skimmed and later referenced as needed.

An online version of this book is available at The summary of forms and index in the online edition include page numbers for the printed version and are thus useful as searchable indexes.

About the illustrations: The cover illustration and the illustration at the front of each chapter are algorithmic line fields created by artist Jean-Pierre Hébert, based on an idea inspired by the writings of John Cage. Each line field is created by the composition of any number of grids of parallel lines. The grids are regular, but they are not. For instance, the lines are of irregular length, which creates ragged edges. Their tone and thickness vary slightly. They are not exactly equidistant. They intersect with each other at a certain angle. When this angle is small, patterns of interference develop. The lines are first steeped into various scalar fields that perturb their original straight shape, then projected on the plane of the paper. Masks introduce holes in some layers. For the cover illustration, the grids are colored in different hues.

All the images are created by a single Scheme program that makes most of the decisions, based heavily on chance. The artist controls only canvas size, aspect ratio, the overall palette of colors, and levels of chance and fuzziness. The task of the artist is to introduce just enough chance at the right place so that the results are at the same time surprising, interesting, and in line with the artist's sense of aesthetics. This is a game of uncertainty, chaos, and harmony.

Acknowledgments: Many individuals contributed in one way or another to the preparation of one or more editions of this book, including Bruce Smith, Eugene Kohlbecker, Matthias Felleisen, Dan Friedman, Bruce Duba, Phil Dybvig, Guy Steele, Bob Hieb, Chris Haynes, Dave Plaisted, Joan Curry, Frank Silbermann, Pavel Curtis, John Wait, Carl Bruggeman, Sam Daniel, Oscar Waddell, Mike Ashley, John LaLonde, John Zuckerman, John Simmons, Bob Prior, Bob Burger, and Aziz Ghuloum. Many others have offered minor corrections and suggestions. Oscar Waddell helped create the typesetting system used to format the printed and online versions of this book. A small amount of text and a few examples have been adapted from the Revised6 Report for this book, for which credit goes to the editors of that report and many others who contributed to it. Finally and most importantly, my wife, Susan Dybvig, suggested that I write this book in the first place and lent her expertise and assistance to the production and publication of this and the previous editions.

R. Kent Dybvig / The Scheme Programming Language, Fourth Edition
Copyright © 2009 The MIT Press. Electronically reproduced by permission.
Illustrations © 2009 Jean-Pierre Hébert
ISBN 978-0-262-51298-5 / LOC QA76.73.S34D93
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