Duplication and Partial Evaluation | for a Better Understanding of Re Blockinective Languages |

This paper presents a general implementation framework for re ective languages. It allows us to systematically build re ective languages which have the following favorable properties: (1) user programs are allowed to access and change (parts of) metalevel interpreters, (2) re ective facilities are available at every level, (hence there exists conceptually an in nite tower of interpreters), and (3) the interpreter runs as e ciently as the conventional (directly implemented) metacircular interpreter when re ection is not used. Our scheme is divided into three stages. First, we de ne interpreters which give the operational semantics of each level, and conceptually construct the in nite tower of these interpreters. They are then duplicated to obtain directly executed interpreters, while introducing double interpretation to maintain rede nability of interpreters. Finally, partial evaluation is employed to collapse the double interpretation into single interpretation. We illustrate our scheme by implementing a particular re ective language called Black in Scheme, but it is general enough to be applied to other re ective languages. The paper gives the complete Scheme implementation of Black and demonstrates some examples. We also show how a system with the delta abstraction introduced by Blond can be constructed in our framework.