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 programming language that is flexible enough to support most of the major programming paradigms in use today.
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 GOTOs that pass arguments, thus allowing a programming style that is both coherent and efficient. Scheme was the first widely used programming language to embrace first-class escape procedures, from which all previously known sequential control structures can be synthesized. A subsequent version of Scheme introduced the concept of exact and inexact numbers, an extension of Common Lisp’s generic arithmetic. More recently, Scheme became the first programming language to support hygienic macros, which permit the syntax of a block-structured language to be extended in a consistent and reliable manner.
The first description of Scheme was written in 1975 [35]. A revised report [31] appeared in 1978, which described the evolution of the language as its MIT implementation was upgraded to support an innovative compiler [32]. Three distinct projects began in 1981 and 1982 to use variants of Scheme for courses at MIT, Yale, and Indiana University [27, 24, 14]. An introductory computer science textbook using Scheme was published in 1984 [1].
As Scheme became more widespread, local dialects began to diverge until students and researchers occasionally found it difficult to understand code written at other sites. Fifteen representatives of the major implementations of Scheme therefore met in October 1984 to work toward a better and more widely accepted standard for Scheme. Their report, the RRRS [8], was published at MIT and Indiana University in the summer of 1985. Further revision took place in the spring of 1986, resulting in the R3RS [29]. Work in the spring of 1988 resulted in R4RS [10], which became the basis for the IEEE Standard for the Scheme Programming Language in 1991 [18]. In 1998, several additions to the IEEE standard, including high-level hygienic macros, multiple return values, and eval, were finalized as the R5RS [20].
In the fall of 2006, work began on a more ambitious standard, including many new improvements and stricter requirements made in the interest of improved portability. The resulting standard, the R6RS, was completed in August 2007 [33], and was organized as a core language and set of mandatory standard libraries. Several new implementations of Scheme conforming to it were created. However, most existing R5RS implementations (even excluding those which are essentially unmaintained) did not adopt R6RS, or adopted only selected parts of it.
In consequence, the Scheme Steering Committee decided in August 2009 to divide the standard into two separate but compatible languages — a “small” language, suitable for educators, researchers, and users of embedded languages, focused on R5RS compatibility, and a “large” language focused on the practical needs of mainstream software development, intended to become a replacement for R6RS. The present report describes the “small” language of that effort: therefore it cannot be considered in isolation as the successor to R6RS.
We intend this report to belong to the entire Scheme community, and so we grant permission to copy it in whole or in part without fee. In particular, we encourage implementers of Scheme to use this report as a starting point for manuals and other documentation, modifying it as necessary.
We would like to thank the members of the Steering Committee, William Clinger, Marc Feeley, Chris Hanson, Jonathan Rees, and Olin Shivers, for their support and guidance.
This report is very much a community effort, and we’d like to thank everyone who provided comments and feedback, including the following people: David Adler, Eli Barzilay, Taylan Ulrich Bayırlı/Kammer, Marco Benelli, Pierpaolo Bernardi, Peter Bex, Per Bothner, John Boyle, Taylor Campbell, Raffael Cavallaro, Ray Dillinger, Biep Durieux, Sztefan Edwards, Helmut Eller, Justin Ethier, Jay Reynolds Freeman, Tony Garnock-Jones, Alan Manuel Gloria, Steve Hafner, Sven Hartrumpf, Brian Harvey, Moritz Heidkamp, Jean-Michel Hufflen, Aubrey Jaffer, Takashi Kato, Shiro Kawai, Richard Kelsey, Oleg Kiselyov, Pjotr Kourzanov, Jonathan Kraut, Daniel Krueger, Christian Stigen Larsen, Noah Lavine, Stephen Leach, Larry D. Lee, Kun Liang, Thomas Lord, Vincent Stewart Manis, Perry Metzger, Michael Montague, Mikael More, Vitaly Magerya, Vincent Manis, Vassil Nikolov, Joseph Wayne Norton, Yuki Okumura, Daichi Oohashi, Jeronimo Pellegrini, Jussi Piitulainen, Alex Queiroz, Jim Rees, Grant Rettke, Andrew Robbins, Devon Schudy, Bakul Shah, Robert Smith, Arthur Smyles, Michael Sperber, John David Stone, Jay Sulzberger, Malcolm Tredinnick, Sam Tobin-Hochstadt, Andre van Tonder, Daniel Villeneuve, Denis Washington, Alan Watson, Mark H. Weaver, Göran Weinholt, David A. Wheeler, Andy Wingo, James Wise, Jörg F. Wittenberger, Kevin A. Wortman, Sascha Ziemann.
In addition we would like to thank all the past editors, and the people who helped them in turn: Hal Abelson, Norman Adams, David Bartley, Alan Bawden, Michael Blair, Gary Brooks, George Carrette, Andy Cromarty, Pavel Curtis, Jeff Dalton, Olivier Danvy, Ken Dickey, Bruce Duba, Robert Findler, Andy Freeman, Richard Gabriel, Yekta Gürsel, Ken Haase, Robert Halstead, Robert Hieb, Paul Hudak, Morry Katz, Eugene Kohlbecker, Chris Lindblad, Jacob Matthews, Mark Meyer, Jim Miller, Don Oxley, Jim Philbin, Kent Pitman, John Ramsdell, Guillermo Rozas, Mike Shaff, Jonathan Shapiro, Guy Steele, Julie Sussman, Perry Wagle, Mitchel Wand, Daniel Weise, Henry Wu, and Ozan Yigit. We thank Carol Fessenden, Daniel Friedman, and Christopher Haynes for permission to use text from the Scheme 311 version 4 reference manual. We thank Texas Instruments, Inc. for permission to use text from the TI Scheme Language Reference Manual [37]. We gladly acknowledge the influence of manuals for MIT Scheme [24], T [28], Scheme 84 [15], Common Lisp [34], and Algol 60 [25], as well as the following SRFIs: 0, 1, 4, 6, 9, 11, 13, 16, 30, 34, 39, 43, 46, 62, and 87, all of which are available at http://srfi.schemers.org.