Heist is a Scheme interpreter written in Ruby. It provides an executable for running Scheme code directly, and also allows the interpreter to be easily embedded in, and extended using, Ruby applications. It nominally targets R5RS, though is likely to be more liberal than other implementations. Scheme is a dialect of Lisp, one of the oldest programming languages still in use today. Lisp was the first language to implement conditionals, first class functions (lambdas), closures and recursion. Some Lisp features, such as closures and macros, have yet to penetrate mainstream languages and it has been observed that mainstream languages are slowly converging on the feature set of Lisp.