Scrap Your Reprinter

Refactoring tools are extremely useful in software development: they provide automatic source code transformation tools for a variety of laborious tasks like renaming, formatting, standardisation, modernisation, and modularisation. A refactoring tool transforms part of a code base and leaves everything else untouched, including secondary notation like whitespace and comments. We describe a novel datatype generic algorithm for the last step of a refactoring tool – the reprinter – which takes a syntax tree, the original source file, and produces an updated source file which preserves secondary notation in the untransformed parts of the code. As a result of being datatype generic, the algorithm does not need changing even if the underlying syntax tree definition changes. We impose only modest preconditions on this underlying data type. The algorithm builds on existing work on datatype generic programming and zippers. This is useful technology for constructing refactoring tools as well as IDEs, interactive theorem provers, and verification and specification tools.