W. G. Griswold, ``Program Restructuring as an Aid to Software Maintenance'', Ph.D. Thesis, Technical Report 91-08-04, Department of Computer Science and Engineering, University of Washington, July 1991.


Maintenance tends to degrade the structure of software, ultimately making maintenance more costly. At times, then, it is worthwhile to manipulate the structure of a system to make changes easier. However, it is shown that manual restructuring is an error-prone and expensive activity. By separating structural manipulations from other maintenance activities, the semantics of a system can be held constant by a tool, assuring that no errors are introduced by restructuring. To allow the maintenance team to focus on the aspects of restructuring and maintenance requiring human judgment, a transformation-based tool can be provided--based on a model that exploits preserving data flow-dependence and control flow-dependence--to automate the repetitive, error-prone, and computationally demanding aspects of restructuring. A set of automatable transformations is introduced; their impact on structure is described, and their usefulness is demonstrated in examples. A model to aid building meaning-preserving restructuring transformations is described, and its realization in a functioning prototype tool for restructuring Scheme programs is discussed.