Introduction to Haggis

Haggis is a framework for creating graphical user interfaces using Haskell, a lazy functional programming language (See section Related resources.) It provides the basic functionality required to express and execute a user interface in a functional language.

Haggis has two overriding goals, one is to provide a good, useable framework for writing programs with graphical user interfaces, and secondly to act as a research system for me (sof@dcs.gla.ac.uk) to investigate issues of how graphics and interaction can be expressed and structured using a functional language.

But why do we need Yet Another Windowing Toolkit/Framework, I hear you ask. Good question :-), the reasons are many:

• Through the introduction of monadic I/O, Haskell programmers can interact comfortably with the outside world. However, no proper support exist for writing graphical user interfaces using this I/O model (the impressive Fudgets system uses the (older) I/O Dialogues (See section Related resources for more info.)
• Current mainstream user interface technology impose severe restrictions on programming style and structuring, where whole applications are structured around the delivery of events from the user. Having a binding to such toolkits in Haskell would result in functional programs with the same style and structuring problems. So, instead of importing this user interface technology wholesale into a functional language, would a more fundamental approach where a functional programming language is used from the bottom up lead to a `better' approach to user interface construction? Haggis is partly an attempt to explore this issue of expressiveness of functional programming applied to graphical user interfaces.
• The egalitarian view of values in functional programming does not fit well with the rather special status that standard toolkits provide of user interface objects. For example, combining two such objects to make up a new one with equal status is not well supported, this contrasts with the inherent compositional mechanisms that FP provides. Will a structured, first-class approach lead to a more extensible user interface framework where the building of an application is not separated from the construction of a new user interface abstraction.
• A user interface reacts and responds to interaction from the outside. It is often stated that simultaneously dealing with the interaction from multiple sources in a UI is best done using concurrency. One aim of examine the expressiveness that result. Haggis is to try to combine concurrency and a functional language, and