Refactoring tools and their kin

Abstract

Refactoring is the process of changing a program in such a way that its design improves with respect to some specific goal, while its observable behaviour remains the same. Trivially, the latter includes the preservation of the program’s well-formedness, since arguably, a malformed program has no behaviour to be preserved. While the problem of refactoring is easily stated, casting it into fully functional refactoring tools for contemporary programming languages is surprisingly hard. In fact, most refactoring tools in use today cannot even guarantee to preserve well-formedness, let alone behaviour, not even for some of the most basic refactorings (such as Rename or Pull Up Member). In Part I of this briefing, I will report on some of the most promising techniques for implementing correct refactoring tools. Common to these techniques is that they give up the notion of behaviour preservation in favour of the more basic (and less demanding) notion of invariant preservation: to be correct, a refactoring tool must not accidentally change the binding of names, the overriding of methods, the synchronization on a monitor, etc. Preservation of well-formedness is then the preservation of invariants relating to well-formedness. With invariant preservation tackled, it is straightforward to transfer refactoring technology to other programming tools, including tools for automatic repair and completion of programs, mutation testing, and program generation. How these are related to refactoring tools, and how they can be developed in concert, I will propose in Part II of this briefing.