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Grapefruit is a library for creating graphical user interfaces in a declarative way. It is currently based on Gtk2Hs but implementations on top of other GUI libraries are planned for the future.

Grapefruit makes it possible to implement graphical user interfaces by describing them as systems of interconnected components. Components can be visible components like widgets and windows but also invisible components which provide certain control functionality. Component systems can be built from components by using methods from the Arrow and ArrowLoop classes.

Components communicate via signals and event streams. A signal denotes a time-varying value, and an event stream denotes a sequence of events occuring at discrete points in time. Several functions allow the construction of signals and event streams in a purely functional manner.

Grapefruit has also support for list signals, special signals denoting time-varying lists where each element has a unique identity. An algebra for list signals provides operations like concatenation, filtering and sorting. The key point is that when the value of a list signal changes, the values of the dependent list signals do not have to be recalculated completely but can be just updated to reflect the change.

User interfaces with a changing collection of widgets are also possible with Grapefruit. There is a special widget which takes a list signal of widgets as its input and always contains the elements of it as its child widgets.

With Grapefruit, user interface descriptions always cover the complete lifetime of the respective interface. No explicit event handler registrations and no explicit recalculations of values are necessary. This is in line with the declarative nature of Haskell because it stresses how the user interface operates instead of how this operation is achieved. Internally though, signals and event streams are implemented efficiently using the event dispatching and handling mechanism of the underlying GUI toolkit.

The roots of Grapefruit lie in systems like FranTk and wxFruit. Grapefruit tries to combine concepts of these systems with new ideas to become a system which maintains a reasonable balance between ease of use and efficiency.


As of July 2007, Grapefruit is in an early stage. The basic concepts are implemented but the implementation is still subject to notable change.

Things planned for the next months cover the introduction of the model-view principle into Grapefruit as well as the development of a framework for animations. Furthermore, work on formal specification and verification of GUI properties based on Grapefruit’s GUI programming interface is about to start. This will probably be done using Ivor but it is intended to (also) use Epigram for this in the future.

Source code

The source code of Grapefruit resides inside a darcs repository under To get a copy, you can use the command darcs get


There is also documentation generated by Haddock. This documentation comes in two flavors. The API documentation covers only the public interface while the complete documentation covers also the internals. Note that the documentation might be a bit out of date since it is not automatically regenerated on every commit yet.

Grapefruit-related Talks

  • Wolfgang Jeltsch: Funktionale GUI-Programmierung in Haskell mit Grapefruit. Haskell in Leipzig 2. July 10, 2007.