Purely functional programming languages like Haskell do not allow mutable state or other side effects in plain functions. A popular misconception says that this could make real-life tasks like programming graphical user interfaces (GUI) more difficult. This is not the case. The aim of this tutorial is to show that Haskell is, in fact, well-suited for GUI programming.

In this tutorial, I will give a hands-on introduction to writing GUI applications with Haskell using my GUI library “Threepenny”. This library is easy to install, because it uses the web browser as a display. First, I will give a brief overview of the library architecture. Then, I will present the basic concepts of GUI programming in Haskell by walking through a few example programs. At the end, we will look at how the paradigm of functional reactive programming (FRP), a radical departure from the traditional imperative paradigm, can simplify GUI programming.