A Simple Re ective Interpreter Stanley Je erson

Procedurally re ective programming languages enable user programs to semantically extend the language itself, by permitting them to run at the level of the language implementation with access to their context. The re ective tower, rst introduced by Brian Smith [9, 10], is the principal architecture for such languages. It is informally described as an in nitely ascending tower of meta-circular interpreters, connected by a mechanism that allows programs at one level to run at the next higher level. Various accounts of the re ective tower have been published, including a meta-circular de nition, operational de nitions, and denotational de nitions. We present an operational account of the main aspects of the re ective tower, which we claim is simpler than previous accounts. Our approach is to implement a nite tower where each level literally runs the level directly below it. A complete Scheme implementation is included.