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Type-classes are nothing like interfaces

Posted on April 2, 2010, in Programming

A beginner is confused about Haskell’s type-classes and so asks the question, “What is a type-class?” The response is often devastating, “You know, like Java or C# interfaces.” This wildly misleading statement can leave the beginner in a state of disrepair. I’ll tell you why but first I must emphasise.

Type-classes are nothing like interfaces (emphasis on nothing).

Haskell has something like interfaces. They are called data types and are expressed using the data or newtype keywords. Languages like Java/C# have nothing like type-classes; there is not even a close analogy. Consider for example, Java’s [Comparator](http://java.sun.com/javase/6/docs/api/java/util/Comparator.html) interface. We would express this in Haskell like so:

newtype Comparator a = C { compare :: a -> a -> Int }

Then if we wanted to sort a list, the type would be sort :: Comparator a -> [a] -> [a]. Notice the explicit passing of the Comparator that would be required at the call site. This is just like Java.

Now suppose we did something that Java cannot do. We used a type-class.

class Comparator a where
  compare :: a -> a -> Int

The type for sorting a list now becomes sort :: Comparator a => [a] -> [a]. This is just like the previous signature except for the way the left-most arrow is written. This is an important distinction. When the caller uses this function, it implicitly passes the Comparator. Also, the type-class instance is decoupled from the data type. These are essential properties of type-classes. Indeed, it is its single-most defining property and since Java/C# have nothing like this, then it has nothing like type-classes.

Scala has implicit parameters which give you the ability to implement the essential property of type-classes (and more). Therefore, Scala does have something very much like type-classes. This is evident in a library such as Scalaz.

But let’s try to save the idea.

Java has implicit type-conversion by virtue of inheritance. For example, a method that accepts a T can be passed a U and its implicit conversion to a T is denoted by the way of U extends T. Notice that no side-effect can be performed during this conversion. This is unlike Scala’s implicit where it’s simply a bad idea, not enforced (Scala also has inheritance like Java).

So, if you are to say type-classes are like anything in these languages, it’s sort-of-like-yeah-ok-not-really inheritance. However, I’m sure you’ll agree, this will only cause confusion for the poor beginner, so it’s best not to draw the analogy at all.

Type-classes are a new concept to people coming from Java/C#. It is most appropriate to explain it as a new concept. I often use the contention between using java.util.Comparable, where the caller has the convenience of implicitly pass the implementation by way of inheritance but inconvenience of carrying the implementation with the data type versus using java.util.Comparator where the caller has the convenience of decoupling the implementation from the data type but requiring explicit passing at the call site. Type-classes (and Scala implicits) resolve this contention with a new concept.