Shifting Expression Procedures into Reverse

The best known approach to program transformation is the unfold fold methodology of Burstall and Darlington a sim ple intuitive and expressive approach which serves as the basis of many automatic program transformation algorithms such as partial evaluation and deforestation Unfortu nately unfold fold does not preserve total correctness and requires maintaining a transformation history of the pro gram Scherlis invented a similar approach expression pro cedures which solved these two problems expression pro cedures preserve total correctness and require no transfor mation history Motivated by our desire to make expression procedures more expressive by eliminating their one directional nature they are designed to specialize but not to generalize func tions we have developed an equational speci cation of ex pression procedures in which the essence of expression pro cedures is expressed in a single transformation rule Our approach has the following advantages over expression pro cedures all program derivations are reversible trans formations can be done which expression procedures cannot do fewer and simpler rules are used and the proof of correctness is simpler