. navigate
LISP Conference    Page 26 left arrow
right arrow Page 28    The Scheme Environment
The Scheme Environment

LISP Pointers
Volume I, Number 1
April-May 1987

The Scheme Environment
page 27

Prev Article               Start of This Article                Next Article

The Scheme Environment

William Clinger
Tektronix, Inc.

I can think of no better way to introduce this department than to quote the first two paragraphs of the introduction to the new Scheme standard, the Revised3 Report on the Algorithmic Language Scheme [4], which I will refer to as R3RS. The first paragraph conveys a sense of the Scheme philosophy, while the second paragraph lists technical features that distinguish Scheme from other programming languages. One of my goals for future articles will be to explain and illustrate those features, as well as some other features that were considered too experimental to be included in R3RS.
Programming languages should be designed not by piling feature on top of feature, but by removing the weaknesses and restrictions that make additional features appear necessary. Scheme demonstrates that a very small number of rules for forming expressions, with no restrictions on how they are composed, suffice to form a practical and efficient prograrmning language that is flexible enough to support most of the major programming paradigms in use today.

Scheme has influenced the evolution of Lisp. Scheme was one of the first programming languages to incorporate first class procedures as in the lambda calculus, thereby proving the usefulness of static scope rules and block structure in a dynamically typed language. Scheme was the first major dialect of Lisp to distinguish procedures from lambda expressions and symbols, to use a single lexical environment for all variables, and to evaluate the operator position of a procedure call in the same way as an operand position. By relying entirely on procedure calls to express iteration, Scheme emphasized the fact that tail-recursive procedure calls are essentially goto's that pass arguments. Scheme was the first widely used programming language to embrace first class escape procedures, from which all known sequential control structures can be synthesized. A few of these innovations have recently been incorporated into Common Lisp, while others remain to be adopted.
The two features of Scheme that have been adopted by Common Lisp are first class procedures and static scope rules. These are the key features that make Common Lisp superior to most older dialects of Lisp. It can also be argued that Common Lisp has adopted a third feature: the distinction between procedures, lambda expressions and symbols. Given the technical meaning of "function" on page 32 of [6] and the acceptability of symbols and lambda expressions as arguments to procedures like funcall, apply, and mapcar, it's probably more accurate to say that Common Lisp has gone part of the way toward adopting this important feature of Scheme but is constrained by a desire to maintain compatibility with the past.

Several books that use Scheme have been published recently, and more are on the way. Much academic interest in Scheme was sparked by Structure and Interpretation of Computer Programs by Hal Abelson and Gerry Sussman with Julie Sussman [1]. This introductory computer science textbook is unusual in its emphasis upon serious intellectual concepts rather than details of a particular programming language. Scheme helped to make this approach work by being simple enough for anyone to learn and powerful enough for everyone to use. Dan Friedman and Matthias Felleisen also used Scheme when they wrote the second edition of Friedman's The Little Lisper [3]. Oddly enough, neither the introduction nor the preface to this book mentions Scheme, so their readers have to study the footnotes to find out which dialect of Lisp they're learning. Other books are keyed to particular implementations. The SCHEME Programming Language by Kent Dybvig [2] places particular emphasis on Chez Scheme, while The T Programming Language by Stephen Slade [5] describes Yale's implementation.

The Scheme development team at the MIT AI Lab maintains a list of available Scheme implementations, primarily as an aid to people using the Abelson and Sussman textbook. The table below was adapted from their release note #1 and from information solicited from implementors by Jonathan Rees. It is my hope that implementors of Scheme will take part in the more detailed survey of Lisp implementations being conducted by Walter van Roggen, so I

Page Left left arrow         
right arrow Page Right
Vol. I, No. 1
Table of Contents

Home   Favorites   Computers   Map

IME logo Copyright © 2009, Mary S. Van Deusen