In this thesis headlined ”Functional programming in Haskell” we will elaborate functional programming paradigm through the programming language Haskell. Functional programming is permeated with concepts like pure functions, referential transparency, higher order functions, recursion, etc. In contrast to the imperative programming paradigm in which computer is explicitly given what to do step-by-step, the functional programming paradigm is based on a notion that the computer is told what represents what and that computations are made by combining functions. Haskell is programming language that fully obeys functional programming paradigm. Unlike most of the programming languages that implement some of its principles Haskell doesn’t mix it with imperative or any other programming paradigm. The first version of Haskell was defined in April of 1990. At the time of writing of this thesis the actual version is Haskell 2010. The motive for creating Haskell was to consolidate the development efforts that existed at time in the development of functional programming languages because fragmentation in that field was considerable. At the end of the thesis, there is an example of the program written in Haskell that is using parallelism for processing some data considering that is one of the areas where some of the advantages of functional programming languages can be shown.