Maintaining the consistency of multiple program representations in a program manipulation tool is difficult. I describe a hybrid software architecture for a meaning-preserving program restructuring tool. Layering is the primary architectural paradigm, which successively provides increasingly integrated and unified abstract machines to implement the tool. However, layering does not provide adequate control over extensibility or the independence of components, so I also adopt the paradigm of keeping the key program abstractions separate throughout the layering, providing independent columns of abstract data types. A pair of columns is integrated by a mapping column that translates elements in one column's data type into related elements in the other column's data type. Thus, integration of function and separation of representation can be achieved simultaneously. This hybrid architecture was crucial in overcoming severe performance problems that became apparent once the basic tool was completed. By taking advantage of the independence of the columns and the special characteristics of meaning-preserving restructuring, it was possible to extend one representation column of the architecture to the uppermost layer to provide the required access for efficient updating without compromising independence. The cost of the extended architecture is that the upper layers are no longer as simple because they expose operations that only guarantee consistency under careful usage. However, the structural constraints of the hybrid architecture and the models for building the more complicated layers minimizes the negative impact of this tradeoff. >
['William G. Griswold', 'David Notkin']
Architectural tradeoffs for a meaning-preserving program restructuring tool