R is for ...

R#

is a free programming language based on REBOL.

RELFUN

is a logic-programming language with call-by-value (eager) expressions of non-deterministic, non-ground functions [Boley99]. Clauses are `hornish', succeeding with true(s), or `footed', returning any value(s). They define operations (relations and functions) permitting (apply-reducible) higher-order syntax with arbitrary terms (constants, structures, and variables) as operators. The language explicitly distinguishes structures [passive] and expressions (active). All structures and expressions, not only lists or tuples, can have varying arities. RELFUN generalizes PROLOG's is-primitive to .=, unifying a structure with the value(s) of an arbitrary expression; expressions may become flattened via further .=-calls. Finite domains and exclusions [Boley93] as well as sorts [Hall95] are first-class citizens built into the unification. Extensions include single-cut clauses and relational-functional primitives such as a value-returning tupof. Versions of RELFUN have been used in the hybrid COLAB shell [BoleyHanschke+93], as VEGA's Declarative Representation Language, and for developing ontologies, e.g. for Mathematics International (Mathint Ontology).

RapidBatch

is a professional, easy-to-learn and powerful batch- and scripting-language for 32-Bit Windows. With RapidBATCH, you can automate Windows and tune up the productivity of your computer system by writing software tools for your pretensions and usage simply yourself. Due to its high flexibility and easy syntax, RapidBATCH is not only a powerful tool for computer professionals. Even the unskilled newbie creates his first own, useful scripts just after a very short practice.

Rigal

is compiler construction language developed at the University of Latvia, Institute of Mathematics and Computer Science in 1987. The main data structures are atoms, lists and labelled trees. The control structures are based on advanced pattern matching. All phases of compilation, including parsing, optimization and code generation, can be programmed in this language in short and readable form.

Ruri

is a compiler for a programming language (also called ruri). The ruri language mirrors the feature set of IJVM and IJVMA, and the compiler targets those assembly languages.
There's also a demo page, which allows you to type in ruri-language code and have the appropriate IJVM or IJVMA output produced.