Final remarks

We have in this tutorial presented an overview of the features and capabilities of Haggis, a graphical user interface in and for the lazy functional language Haskell. By way of a series of examples, the features and ideas underlying Haggis were presented:

A user interface component is viewed as a virtual I/O device

The model for creating and accessing user interface components is similar to the way devices such as files are interacted with in Haskell 1.3.

Graphics is described declaratively

All graphical output in Haggis is described using Picture values. Pictures can be transformed, combined, printed, saved etc.

User interface components are represented uniformly

User interface components are represented uniformly using the DisplayHandle type. A DisplayHandle is a reference or handle to a component, and is used to issue window system commands and requests along.

Interactive behaviour is added by encapsulating components

To catch input or make a user interface component come alive, Haggis uses encapsulation to catch and interpret user input events performed on the component. Haggis provides a basic combinator catchDeviceEv, which encapsulates an existing component by rerouteing all the events representing user actions to separate handle.

Layout described using functional layout combinators

The layout of a user interface is described by combinators of the form

hbox :: [DisplayHandle] -> DisplayHandle

taking a list of DisplayHandle and returning a new value representing the set of components laid out.

Communication between user interface and the underlying

application is done via abstract interactive device handles Although there is no rigid separation between application and user interface in Haggis, the application tends to be added piecemeal to the user interface, building more and more application semantics to the constructed components.

Style environments used to customise component instances

To deal with the problem of configuring a component's look (font to use, colours to use etc.), functions that create user interface components are passed an environment containing a Style environment. The Style contains a mapping between customisation options and values to use.

If the tutorial has left you with an unfilled stomach, help yourself to some more Haggis by looking at the Examples section, See section Some examples of Haggis programs/A haggis primer.