Part III

Strategies for Program Transformation

Programmable Rewriting Strategies



Rewrite rules provide a good formalism for expressing basic program transformation steps. However, rewriting has a number of limitations. First, since most sets of rules are not confluent and/or terminating, exhaustive application of rules is not adequate; more careful control over the application of rewrite rules is needed.

Chapter In Control of Rewriting surveys several solutions to controlling rewriting. Most solutions for the encoding of control suffer from a substantial overhead in the definition of traversals and/or intertwine transformation rules and strategy. The paradigm of programmable rewriting strategies provides a solution with minimal overhead for traversal, while maintaining the separation of rules and strategies.

Chapter Composing Strategies to Chapter Generic Traversal Strategies introduce combinators for the composition of programmable strategies. Chapter Composing Strategies introduces combinators for sequential non-deterministic programming and for data type specific traversal using congruence operators. Chapter First Class Pattern Matching reduces rewrite rules to the more primitive strategy actions match, build, and scope. The availability of first class patterns allows many other operations involving pattern matching. Data type specific traversal with congruence operators does solve the separation between rules and strategies, but not the overhead caused by the specification of traversals. Chapter Generic Traversal Strategies shows how combinators for generic traversal can concisely capture traversal schemata independent of any object language.

Another limitation of rewriting is that rewrite rules are context-free. That is, only local information available during the pattern match can be used in a transformation step. In Chapter Scoped Dynamic Rewrite Rules the paradigm of rewriting strategies is extended with scoped dynamic rewrite rules, rules which are generated at run-time to inherit context information.