R4RS

R4RS, or the Revised^4 Report on the Algorithmic Language Scheme, is a historic standard specification for Scheme, a dialect of Lisp. It represents the fourth revision of the Scheme report and functioned as a portable baseline for implementing Scheme across diverse platforms.

The document defines a core language with a minimal, regular syntax and a standard library intended to support practical programming while remaining implementation-dependent in some areas. It formalizes the evaluation rules for Scheme expressions, the treatment of identifiers and environments, and the object model of procedures as first-class values. A defining feature of Scheme that R4RS codified is the presence of proper tail calls, enabling efficient tail recursion.

R4RS covers data types such as numbers, booleans, symbols, pairs, and lists, as well as core control structures and abstraction mechanisms. It provides standard forms and procedures for functional programming (lambda, define, apply, call/cc) and basic I/O, as well as predicates and list processing facilities. The standard also discusses portability considerations, error handling, and library conventions intended to ease cross-implementation compatibility.

Impact and legacy: R4RS was later superseded by R5RS and then by subsequent standards, such as R6RS and R7RS. However, many Scheme implementations offered R4RS compatibility or modes, and the document helped establish the language’s core semantics and portability expectations that influenced later revisions.