From the Gtk2Hs web site: Gtk2Hs is a GUI library for Haskell based on Gtk+. Gtk+ is an extensive and mature multi-platform toolkit for creating graphical user interfaces.
This tutorial is aimed at the intermediate level Haskell programmer, and covers what is needed to write the most common GUI interfaces. This is summed up by the table of contents of this tutorial. For advanced users, however, the Gtk2Hs API documentation contains much more.
Gtk+ is largely a library of components called 'widgets', which are organized in an object oriented hierarchy. In Gtk2Hs this is implemented by giving each widget both a Haskell type and a Haskell type class. Thus the Haskell type system is rigorously preserved, and the programmer has all the advantages of Haskell type checking by the Glasgow Haskell compiler and interpreter. Gtk2Hs also has many functions for type casting.
Widgets have attributes which control their behavior and appearance. There are two general functions to get and to set attributes.
To set one or more attributes of a widget use:
set widget [widgetAttr1 := foo, widgetAttr2 := fie, widgetAttr3 := bar]
To get an attribute of a widget use:
x <- get widget widgetAttr1
Almost always there are specific functions to set and get widget attributes too, but in future these will be deprecated. For now, however, use of the functions is more common.
What happens in a Gtk2Hs application is determined by user actions, like mouse clicks, button
presses, selection of radio buttons and so on. These are called events and may result in signals
being emitted. Information about events, for example whether the user clicked a right or left
mouse button, may be captured in data fields that can be queried by the Gtk2Hs programmer.
Signals are usually handled by specific functions for that type of widget. They are listed
in the Signals section of the API documentation for that widget.
Most take a function of type IO () as parameter, but many have some result.
The Gtk2Hs programmer, of course, must write these parameter functions, which determine the response
to the user action.
Finally, setting up a widget tree for your application and determining the visual layout (packing) can be done visually with the Glade interface designer. Gtk2Hs fully supports Glade and there is an introductory tutorial on the Gtk2Hs web site. Using Glade is actually recommended over the programmatic approach which is described in this tutorial.