Background ¶

The first description of Scheme was written in 1975². A revised report³ appeared in 1978, which described the evolution of the language as its MIT (Massachusetts Institute of Technology) implementation was upgraded to support an innovative compiler⁴. Three distinct projects began in 1981 and 1982 to use variants of Scheme for courses at MIT, Yale, and Indiana Uni­ver­sity^{5 6 7}. An introductory computer science textbook using Scheme was published in 1984⁸.

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 (Revised Revised Report on Scheme)⁹, 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 (Revised³ Report on the Algorithmic Language Scheme)¹⁰. Work in the spring of 1988 resulted in R⁴RS (Revised⁴ Report on the Algorithmic Language Scheme)¹¹, which became the basis for the IEEE (Institute of Electrical and Electronics Engineers) Standard for the Scheme Program­ming Language in 1991¹². In 1998, several additions to the IEEE standard, including high-level hygienic macros, multiple return values, and eval, were finalized as the R⁵RS¹³).

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 2007¹⁴, 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.