Explain Type Classes in Haskell

Question

I am a C++ / Java programmer and the main paradigm I happen to use in everyday programming is OOP. In some thread I read a comment that Type classes are more intuitive in na

Answer 1

In addition to what xtofl and camccann have already written in their excellent answers, a useful thing to notice when comparing Java's interfaces to Haskell's type classes is the following:

1. Java interfaces are closed, meaning that the set of interfaces any given class implements is decided once and for all when and where it is defined;

2. Haskell's type classes are open, meaning that any type (or group of types for multi-parameter type classes) can be made a member of any type class at any time, as long as suitable definitions can be provided for the functions defined by the type class.

This openness of type classes (and Clojure's protocols, which are very similar) is a very useful property; it is quite usual for a Haskell programmer to come up with a new abstraction and immediately apply it to a range of problems involving pre-existing types through clever use of type classes.