At the end of the latest issue we managed to write a basic application allowing information retrieval from 3 tables containing data about individuals (data being presented as a list of pairs). Starting from that code (which will be presented here as well) we’ll build the application of this article.

In the last article we offered a short review of Haskell types, both the existing ones and the ones which can be defined by the programmer. We continue to uncover features of this language in this article by giving a short insight into another type system characteristic: the special way in which polymorphism is implemented in Haskell: typeclasses. Though the notions of class and polymorphism bring into mind the object oriented programming, remember that Haskell is different.

The last article was a short introduction to the language, being more concerned with presenting the history of the language and the benefits of using it and giving a lesser importance to syntax. This time, however, we will discuss the types of the Haskell language. The most defining attribute of Haskell is its static typing system: each expression has a type which is known from compile time. Moreover, there are no implicit conversions between similar types: the programmer is the one responsible for doing them in places where they are needed.

Alan Perlis once said A language that doesn’t affect the way you think about programming, is not worth knowing. Judging by the number of Stack Overflow questions and the impressive number of articles and posts on reddit, Slashdot or blogs, it is clear that the Haskell language deserves to be known. In fact, functional programming is indeed a domain which every hacker should be accustomed with.