S-expression - Standardization

Standardization

Standards for some Lisp-derived programming languages include a specification for their S-expression syntax. These include Common Lisp (ANSI standard document ANSI INCITS 226-1994 (R2004)), Scheme (R5RS and R6RS), and ISLISP.

In May 1997, Ron Rivest submitted an Internet-Draft to be considered for publication as an RFC. The draft defined a syntax based on Lisp S-expressions but intended for general-purpose data storage and exchange (similar to XML) rather than specifically for programming. It was never approved as an RFC, but it has since been cited and used by other RFCs (e.g. RFC 2693) and several other publications. It was originally intended for use in SPKI.

Rivest's format defines an S-expression as being either an octet-string (a series of bytes) or a finite list of other S-expressions. It describes three interchange formats for expressing this structure. One is the "advanced transport", which is very flexible in terms of formatting, and is syntactically similar to Lisp-style expressions, but they are not identical. The advanced transport, for example, allows octet-strings to be represented verbatim (the string's length followed by a colon and the entire raw string), a quoted form allowing escape characters, hexadecimal, Base64, or placed directly as a "token" if it meets certain conditions. (Rivest's tokens differ from Lisp tokens in that the former are just for convenience and aesthetics, and treated exactly like other strings, while the latter have specific syntactical meaning.) Another interchange format, intended to be more compact, easier to parse, and unique for any abstract S-expression, is the "canonical representation" which only allows verbatim strings, and prohibits whitespace as formatting outside strings. Finally there is the "basic transport representation", which is either the canonical form or the same encoded as Base64 and surrounded by braces, the latter intended to safely transport a canonically encoded S-expression in a system which might change spacing (e.g. an email system which has 80-character-wide lines and wraps anything longer than that).

This format has not been widely adapted for use outside of SPKI. Rivest's S-expressions web page provides C source code for a parser and generator (available under the MIT license, which could be adapted and embedded into other programs. In addition, there are no restrictions on independently implementing the format.