TITLE: The Theory of Patches AUTHOR: Eugene Wallingford DATE: December 03, 2004 10:11 AM DESC: A confluence between patterns, refactoring, and version control tools ----- BODY: Recently, I ran across an interesting article by David Roundy called The Theory of Patches, which is actually part of the manual for Darcs, a version-control alternative to CVS and the like. Darcs uses an inductive definition to define the state of a software artifact in the repository:

So how does one define a tree, or the context of a patch? The simplest way to define a tree is as the result of a series of patches applied to the empty tree. Thus, the context of a patch consists of the set of patches that precede it.

Starting with this definition, Roundy goes on to develop a set of light theorems about patches, their composition, and their merging. Apparently these theorems guide the design and implementation of Darcs. (I haven't read that far.) The reason I found this interesting was the parallel between repeated application of patches to an empty tree and

the repeated application of patterns to a design, leading to piecemeal growth of a system, and
the agile methods' cyclic process of extension and refactoring.

This led me to think about how an IDE such as Eclipse might have version control build right into it. Each time the programmer adds new behavior or applies a refactoring, the change can be recorded as a patch, a diff to the previous system. The theory of patches needed for a version control system like Darcs would be directly useful in implementing such a system. I used to program a lot in Smalltalk, and Smalltalk's .changes file served as a lightweight, if incomplete, version control system. I suppose that a Smalltalker could implement something more complete rather easily. Someone probably already has! Thanks to Chad Fowler's bookmarks for the link. -----