Formal Program Development with Approximations

We describe a method for combining formal program development with a disciplined and documented way of introducing realistic compromises, for example necessitated by resource bounds. Idealistic specifications are identified with the limits of sequences of more “realistic” specifications, and such sequences can then be refined in their entirety. Compromises amount to focusing the attention on a particular element of the sequence instead of the sequence as a whole. This method addresses the problem that initial formal specifications can be abstract or complete but rarely both. Various potential application areas are sketched, some illustrated with examples. Key research issues are found in identifying metric spaces and properties that make them usable for refinement using approximations.