4.3. Refactoring

In discussing isotopes in Section 4.1.1, I stated that a pattern isotope instance might look the same as another instance of that pattern from the outside but have a buggy implementation that causes problems later. Worse than a single poorly implemented design pattern, however, is having multiple interacting design patterns that all bring well-meaning and rational implementation choices to the system but form a problem when combined. Recall that the initial driving problem for SPQR, described in Section 2.1, was that a design pattern arose organically and unintentionally. Malignant designs can form just as easily, and because the number of ways to produce poor code is greater than the number of good designs, they tend to arise much more readily.