Structure and Interpretation of Computer Programs

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Preface to the Second Edition

Is it possible that software is not like anything else, that it

is meant to be discarded: that the whole point is to always

see it as a soap bubble?

Alan J. Perlis

The material in this book has been the basis of MIT’s entry-level computer science subject since 1980.

We had been teaching this material for four years when the first edition was published, and twelve

more years have elapsed until the appearance of this second edition. We are pleased that our work has

been widely adopted and incorporated into other texts. We have seen our students take the ideas and

programs in this book and build them in as the core of new computer systems and languages. In literal

realization of an ancient Talmudic pun, our students have become our builders. We are lucky to have

such capable students and such accomplished builders.

In preparing this edition, we have incorporated hundreds of clarifications suggested by our own

teaching experience and the comments of colleagues at MIT and elsewhere. We have redesigned most

of the major programming systems in the book, including the generic-arithmetic system, the

interpreters, the register-machine simulator, and the compiler; and we have rewritten all the program

examples to ensure that any Scheme implementation conforming to the IEEE Scheme standard (IEEE

1990) will be able to run the code.

This edition emphasizes several new themes. The most important of these is the central role played by

different approaches to dealing with time in computational models: objects with state, concurrent

programming, functional programming, lazy evaluation, and nondeterministic programming. We have

included new sections on concurrency and nondeterminism, and we have tried to integrate this theme

throughout the book.

The first edition of the book closely followed the syllabus of our MIT one-semester subject. With all

the new material in the second edition, it will not be possible to cover everything in a single semester,

so the instructor will have to pick and choose. In our own teaching, we sometimes skip the section on

logic programming (section 4.4), we have students use the register-machine simulator but we do not

cover its implementation (section 5.2), and we give only a cursory overview of the compiler 

(section 5.5). Even so, this is still an intense course. Some instructors may wish to cover only the first

three or four chapters, leaving the other material for subsequent courses.

The World-Wide-Web site 

www-mitpress.mit.edu/sicp

 provides support for users of this

book. This includes programs from the book, sample programming assignments, supplementary

materials, and downloadable implementations of the Scheme dialect of Lisp.

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Preface to the First Edition

A computer is like a violin. You can imagine a novice

trying first a phonograph and then a violin. The latter, he

says, sounds terrible. That is the argument we have heard

from our humanists and most of our computer scientists.

Computer programs are good, they say, for particular

purposes, but they aren’t flexible. Neither is a violin, or a

typewriter, until you learn how to use it.

Marvin Minsky, ‘‘Why Programming Is a Good

Medium for Expressing Poorly-Understood and

Sloppily-Formulated Ideas’’

‘‘The Structure and Interpretation of Computer Programs’’ is the entry-level subject in computer

science at the Massachusetts Institute of Technology. It is required of all students at MIT who major in

electrical engineering or in computer science, as one-fourth of the ‘‘common core curriculum,’’ which

also includes two subjects on circuits and linear systems and a subject on the design of digital systems.

We have been involved in the development of this subject since 1978, and we have taught this material

in its present form since the fall of 1980 to between 600 and 700 students each year. Most of these

students have had little or no prior formal training in computation, although many have played with

computers a bit and a few have had extensive programming or hardware-design experience.

Our design of this introductory computer-science subject reflects two major concerns. First, we want

to establish the idea that a computer language is not just a way of getting a computer to perform

operations but rather that it is a novel formal medium for expressing ideas about methodology. Thus,

programs must be written for people to read, and only incidentally for machines to execute. Second,

we believe that the essential material to be addressed by a subject at this level is not the syntax of

particular programming-language constructs, nor clever algorithms for computing particular functions

efficiently, nor even the mathematical analysis of algorithms and the foundations of computing, but

rather the techniques used to control the intellectual complexity of large software systems.

Our goal is that students who complete this subject should have a good feel for the elements of style

and the aesthetics of programming. They should have command of the major techniques for

controlling complexity in a large system. They should be capable of reading a 50-page-long program,

if it is written in an exemplary style. They should know what not to read, and what they need not

understand at any moment. They should feel secure about modifying a program, retaining the spirit

and style of the original author.

These skills are by no means unique to computer programming. The techniques we teach and draw

upon are common to all of engineering design. We control complexity by building abstractions that

hide details when appropriate. We control complexity by establishing conventional interfaces that

enable us to construct systems by combining standard, well-understood pieces in a ‘‘mix and match’’

way. We control complexity by establishing new languages for describing a design, each of which

emphasizes particular aspects of the design and deemphasizes others.

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