Revised^6 Report on the Algorithmic Language Scheme

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Introduction

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. 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 number objects, 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.

### Guiding principles

To help guide the standardization effort, the editors have adopted a set of principles, presented below. Like the Scheme language defined in Revised5 Report on the Algorithmic Language Scheme [14], the language described in this report is intended to:

- allow programmers to read each other’s code, and allow development of portable programs that can be executed in any conforming implementation of Scheme;

- derive its power from simplicity, a small number of generally useful core syntactic forms and procedures, and no unnecessary restrictions on how they are composed;

- allow programs to define new procedures and new hygienic syntactic forms;

- support the representation of program source code as data;

- make procedure calls powerful enough to express any form of sequential control, and allow programs to perform non-local control operations without the use of global program transformations;

- allow interesting, purely functional programs to run indefinitely without terminating or running out of memory on finite-memory machines;

- allow educators to use the language to teach programming effectively, at various levels and with a variety of pedagogical approaches; and

- allow researchers to use the language to explore the design, implementation, and semantics of programming languages.

In addition, this report is intended to:

- allow programmers to create and distribute substantial programs and libraries, e.g., implementations of Scheme Requests for Implementation, that run without modification in a variety of Scheme implementations;

- support procedural, syntactic, and data abstraction more fully by allowing programs to define hygiene-bending and hygiene-breaking syntactic abstractions and new unique datatypes along with procedures and hygienic macros in any scope;

- allow programmers to rely on a level of automatic run-time type and bounds checking sufficient to ensure type safety; and

- allow implementations to generate efficient code, without requiring programmers to use implementation-specific operators or declarations.

While it was possible to write portable programs in Scheme as described in Revised5 Report on the Algorithmic Language Scheme, and indeed portable Scheme programs were written prior to this report, many Scheme programs were not, primarily because of the lack of substantial standardized libraries and the proliferation of implementation-specific language additions.

In general, Scheme should include building blocks that allow a wide variety of libraries to be written, include commonly used user-level features to enhance portability and readability of library and application code, and exclude features that are less commonly used and easily implemented in separate libraries.

The language described in this report is intended to also be backward compatible with programs written in Scheme as described in Revised5 Report on the Algorithmic Language Scheme to the extent possible without compromising the above principles and future viability of the language. With respect to future viability, the editors have operated under the assumption that many more Scheme programs will be written in the future than exist in the present, so the future programs are those with which we should be most concerned.

### Acknowledgements

Many people contributed significant help to this revision of the report. Specifically, we thank Aziz Ghuloum and André van Tonder for contributing reference implementations of the library system. We thank Alan Bawden, John Cowan, Sebastian Egner, Aubrey Jaffer, Shiro Kawai, Bradley Lucier, and André van Tonder for contributing insights on language design. Marc Feeley, Martin Gasbichler, Aubrey Jaffer, Lars T Hansen, Richard Kelsey, Olin Shivers, and André van Tonder wrote SRFIs that served as direct input to the report. Marcus Crestani, David Frese, Aziz Ghuloum, Arthur A. Gleckler, Eric Knauel, Jonathan Rees, and André van Tonder thoroughly proofread early versions of the report.

We would also like to thank the following people for their help in creating this report: Lauri Alanko, Eli Barzilay, Alan Bawden, Brian C. Barnes, Per Bothner, Trent Buck, Thomas Bushnell, Taylor Campbell, Ludovic Courtès, Pascal Costanza, John Cowan, Ray Dillinger, Jed Davis, J.A. “Biep” Durieux, Carl Eastlund, Sebastian Egner, Tom Emerson, Marc Feeley, Matthias Felleisen, Andy Freeman, Ken Friedenbach, Martin Gasbichler, Arthur A. Gleckler, Aziz Ghuloum, Dave Gurnell, Lars T Hansen, Ben Harris, Sven Hartrumpf, Dave Herman, Nils M. Holm, Stanislav Ievlev, James Jackson, Aubrey Jaffer, Shiro Kawai, Alexander Kjeldaas, Eric Knauel, Michael Lenaghan, Felix Klock, Donovan Kolbly, Marcin Kowalczyk, Thomas Lord, Bradley Lucier, Paulo J. Matos, Dan Muresan, Ryan Newton, Jason Orendorff, Erich Rast, Jeff Read, Jonathan Rees, Jorgen Schäfer, Paul Schlie, Manuel Serrano, Olin Shivers, Jonathan Shapiro, Jens Axel Søgaard, Jay Sulzberger, Pinku Surana, Mikael Tillenius, Sam Tobin-Hochstadt, David Van Horn, André van Tonder, Reinder Verlinde, Alan Watson, Andrew Wilcox, Jon Wilson, Lynn Winebarger, Keith Wright, and Chongkai Zhu.

We would like to thank the following people for their help in creating the previous revisions of this report: Alan Bawden, Michael Blair, George Carrette, Andy Cromarty, Pavel Curtis, Jeff Dalton, Olivier Danvy, Ken Dickey, Bruce Duba, Marc Feeley, Andy Freeman, Richard Gabriel, Yekta Gürsel, Ken Haase, Robert Hieb, Paul Hudak, Morry Katz, Chris Lindblad, Mark Meyer, Jim Miller, Jim Philbin, John Ramsdell, Mike Shaff, Jonathan Shapiro, Julie Sussman, Perry Wagle, 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 [26]. We gladly acknowledge the influence of manuals for MIT Scheme [20], T [21], Scheme 84 [12], Common Lisp [25], Chez Scheme [8], PLT Scheme [11], and Algol 60 [1].

We also thank Betty Dexter for the extreme effort she put into setting this report in TEX, and Donald Knuth for designing the program that caused her troubles.

The Artificial Intelligence Laboratory of the Massachusetts Institute of Technology, the Computer Science Department of Indiana University, the Computer and Information Sciences Department of the University of Oregon, and the NEC Research Institute supported the preparation of this report. Support for the MIT work was provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-80-C-0505. Support for the Indiana University work was provided by NSF grants NCS 83-04567 and NCS 83-03325.