Over the last year or so, I have found myself making more and more use of Scala's Type Class system to add flexibility to my code. This is especially evident in the MongoDB Scala Driver, http://github.com/mongodb/casbah, where the most recent work has been to simplify many features by migrating them to type classes.
During this work however, I've found during that many otherwise adroit Scala engineers seem befuddled or daunted by the Type Class. It does me no good to take advantage of clever features that my users don't understand, and many will benefit from introducing these concepts to their own code. So let's take a look at what type classes are, as well as how & why we can utilize them.
Wikipedia defines a Type Class as "... a type system construct that supports ad-hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types". Admittedly, a bit of a mouthful -- and not very helpful to those of us who are self taught and lack the benefit of a comprehensive academic Computer Science education (myself included). Surely, there must be a way to simplify this concept?
In evaluating these ideas, I've found it easiest to think of a Type Class (in Scala, at least) as a special kind of adapter, which can impart additional capabilities upon a given type or set of types. In Scala the Type Class is communicated through implicits, and imparts one, or both, of two behaviors. First, a Type Class can be to utilized to filter what types are valid for a given method call (which I detailed in this earlier post). Second, a Type Class can impart additional features and behaviors upon a type at method invocation time. This latter is much along the lines of an enhanced kind of composition, rather than the weaker inheritance which often plagues similar behaviors in a language like Java.
To better understand what I am describing, let's compare a few concepts around the creation and interaction of custom domain objects. I have several sets of tasks I have had to accomplish in Scala in the past -- and Scala solutions show some elegant Type Class oriented approaches which are rooted in the Standard Library. While this may seem a bit contrived, it is exactly the kind of problem through which I initially came to understand Type Classes –– and is thus an ideal lesson.