A former colleague of mine used to say that thick books about design patterns were evidence of an inadequate programming language. What he meant was that, since design patterns are the common idioms of code, a good programming language should make them very easy to implement. An ideal language would so thoroughly integrate the patterns that they would almost disappear from sight.
To take an extreme example, in the late 80's I worked on a project that produced object oriented code in C. Yes, C, not C++. We pulled this off by having each "object" (actually a C structure) point off to a table of function pointers. We operated on our "objects" by chasing the pointer to the table and calling functions out of the table, thereby simulating a method call on an object. It was awkward and messy, but it worked. Had we thought of it, we might have called this technique the "object oriented" pattern. Of course with the advent of C++ and then Java, our object oriented pattern disappeared, absorbed so thoroughly into the language that it vanished from sight. Today, we don't usually think of object orientation as a pattern—it is too easy.
But many things are still not easy enough. The justly famous Gang of Four book, (Design Patterns: Elements of Reusable Object-Oriented Software by Gamma, Helm, Johnson, and Vlissides) is required reading for every software engineer today. But actually implementing many of the patterns described in Design Patterns with the languages in widespread use today, Java and C++ and perhaps C#, looks and feels a lot like my 1980s vintage handcrafted object system. Too painful. Too verbose. Too prone to bugs.
The Ruby programming language takes us a step closer to my old friend's ideal, a language that makes implementing patterns easy, so easy that sometimes they fade into the background. Building patterns in Ruby is easier for a number of reasons:
- Ruby is dynamically typed. By dispensing with static typing, Ruby dramatically reduces the code overhead of building most programs, including those that implement patterns.
- Ruby has code closures. Ruby allows us to pass around chunks of code and associated scope without having to laboriously construct entire classes and objects that do nothing else.
- Ruby classes are real objects. Since a class in Ruby is just another object, we can do any of the usual runtime things to a Ruby class that we can do to any object. At runtime we can create totally new classes. We can also modify existing classes by adding or deleting methods. We can even clone a class and change the copy, leaving the original alone.
- Ruby has an elegant system of code reuse. In addition to garden variety inheritance, Ruby provides also allows us to define mixins, which are a simple but flexible way to write code that can be shared among several classes.
All of this makes code in Ruby compressible: In Ruby, like Java and C++, you can implement very sophisticated ideas, but with Ruby it is possible to hide the details of your implementations much more effectively. As you will see on the pages that follow, many of the design patterns that require many lines of endlessly repeated boiler plate code in traditional static languages require only one or two lines in Ruby. You can make a class into a singleton with a simple include Singleton. You can delegate as easily as you can inherit. Since Ruby enables you to say more interesting things in each line of code, you end up with less code.
This is not just a question of keyboard laziness, it is an application of the DRY (Don't Repeat Yourself) principal. I don't think anyone today would mourn the passing of my old object oriented pattern in C—it worked for me, but it made me work for it, too. In the same way, the traditional implementations of many design patterns work, but they make you work too. Ruby is a real step forward to be able to do that work only once and compress it out of the bulk of our code. In short, Ruby allows us to concentrate on the real problems that we are trying to solve instead of the plumbing. I hope that this book will help you see how.
Who Is This Book For?
Simply put, this book is for developers who want to know how to build significant software in Ruby. I assume that you are familiar with object oriented programming, but you don't really need any knowledge of design patterns —you can pick that up as you go through the book.
You also don't need a lot of Ruby knowledge to read this book profitably: you can find a quick introduction to the language in Chapter 2 and I do try to explain any Ruby specific language issues as we go.
How Is This Book Organized?
This book is divided into three parts. First there are a couple of introductory chapters, starting with the briefest outline of the history and background of the whole Design Patterns movement and ending with a quick tour of the Ruby language, at the "just enough to be dangerous" level.
The second section, which takes up the bulk of these pages, looks at a number of the original Gang of Four patterns from a Ruby point of view. What problem is this pattern trying to solve? What does the traditional implementation of the pattern, the implementation given by the Gang of Four look like in Ruby? Does the traditional implementation make sense in Ruby? Does Ruby provide us with any alternatives that will make solving the problem easier?
The final section of this book looks at a three patterns that have emerged from the Ruby.
A Word of Warning
I cannot sign my name to a book about design patterns without repeating the mantra that I have been muttering for many years now: design patterns are little spring loaded solutions to common programming problems. Ideally, when the appropriate kind of problem comes along, you should trigger the design pattern and the problem is solved. It is that first part, the bit about waiting for the appropriate problem to come along, that some engineers have trouble with. You cannot say that you are correctly applying a design pattern unless you are confronting the problem that the pattern is supposed to solve.
The reckless use of every design pattern on the menu to solve nonexistent problems has given design patterns a bad name in some circles. I would contend that Ruby makes it easier to write an adapter that uses a factory method to get a proxy to the builder which creates the command which coordinates the operation of adding two plus two. Ruby will make that easier, but even in Ruby it will not make any sense.
Nor can you look at program construction as a simple process of piecing together some existing design patterns in new combinations. Any interesting program is going to have unique sections, bits of code that fit that specific problem perfectly and no other. Design patterns are meant to help you recognize and solve the common problems that arise over and over when you are building software. The advantage of design patterns is that they let you rapidly get past the problems that someone has already solved, so that you can get on to the hard stuff, the code that is unique to your situation. Design patterns are not the universal elixir, the magic potion that will fix all of your design problems. They are simply one technique, albeit a very useful technique, that you can use to build programs.