|Elemental Design Patterns|
|author||Jason McC. Smith|
What's most fascinating about Elemental Design Patterns is the analysis performed in working out a solution to the question of how a pattern may be defined in a way that's language-agnostic and flexible to differing implementations. This was a success: his resulting pattern recognition tool even found unintentional usages of well-known design patterns in a large legacy code base, which could then be refactored from the ugly 'accidental usage' to transform apparent chaos into maintainable order.
The basic idea is that every pattern is composed of elemental patterns. For example, the 'Factory Method' pattern may be decomposed into four EDPs (elemental design patterns): 'Create Object', 'Fulfill Method', 'Conglomeration' and 'Retrieve'. The 'Pattern Instance Notation', introduced in this book, and which serves as an extension to UML, helps visualize the relationships between the four sub-patterns and the larger pattern. No doubt readers will find the notation useful in their own work.
This premise's success or failure hinges on two questions: are the set of patterns really elemental? and can the set of patterns be complete? Oddly, the patterns listed in the book are NOT complete: "this book touches on only one-quarter, at best, of the possible EDPs that exist" (p. 107). The fact that this book (which defines 16 patterns in depth) is only the beginning of a project is not well-communicated. Those who might benefit from a complete listing of EDPs (i.e. analysis tool makers) might be puzzled at how to immediately put this book to use if it's not complete. After all, Smith insists in the Preface that "this book is meant to be used." To me, this implies it should serve as more than a basis for research or design-skills edification.
As for them being elemental, in the sense that all possible 'macro-patterns' may be built from them, Smith backs up this claim with the help of a mathematical formal system called rho-calculus, which is introduced in some depth in the appendix, but avoided in the body of the text for readability's sake. Readers wanting a full mathematical treatment are referred to Smith's Ph.D thesis.
What makes the book worth reading and re-reading is in the methods employed to analytically derive EDPs. As dull as I probably make it sound, Smith gives the entertaining first half of the book an almost 'novelistic', first-person quality in which the reader is engaged to experience Smith's insights first-hand. In a sense, the EDPs are 'unfolded' from simple concepts like the degrees of method or object similarity in a method-call relationship.
Understanding this point is important to understanding EDPs: a method call is not just a method call from a 'micro-patterns' perspective. Calling a very different method on the same object is semantically distinct from calling a very similar method on a very different object. The first is described by the EDP 'Conglomeration' (breaking larger tasks into subtasks); the second, by the EDP 'Redirection' (redirecting a process to another object). Of course, the terms 'similar' and 'different' are fuzzy qualifiers to programmers and there's bound to be some debate on how these terms are applied. Smith, in order to make this distinction, puts faith in the developer's ability to name classes and methods. But anyway, such 'semantic' relationships are the real building-blocks of the higher-level patterns. Once other object-oriented principles are considered, like inheritance, the EDP list grows.
You can purchase Elemental Design Patterns from amazon.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.