I just posted a new version of FG, my Arrows GUI Library Based on GTK+. You can find it at http://kevin.atkinson.dhs.org/fg/. I reworked how Events are handled which decreases the number of times a loop is traversed form "2^d" to "d + 1" where d is the depth of the inner most loop. Furthermore only the loop which the event was fired in will need to be traversed multiple times. You can find it at http://kevin.atkinson.dhs.org/fg/. Feedback appreciated. Here are some interesting parts from the documentation: Description: This module is a first attempt of using Arrows to create a GUI Library based on GTK+. A good understanding of how Arrows work is required in order to understand the interface. For more information on Arrows see http://www.haskell.org/arrows/. It uses many ideas from Fruit (http://haskell.org/fruit/). However it is based on discrete events rather than a continuous signal. The interface is only updated during an Event. It also ideas from Fudgets (http://www.md.chalmers.se/Cs/Research/Functional/Fudgets/), some of which were also used by Fruit. ... Implementation Notes: Arrows essentially build up a huge tree like data structure representing the control flow between arrows. In the current implementation most of top-level structure has to be traversed when ever an event is fired -- even if absolutely no actions need to be taken. When a loop is used parts of this structure may be traversed multiple times. In particular the inner most loop where an event was fired from will be traversed "d + 1" times where d is the depth of the loop. If an event was not fired inside a loop (or any of the sub loops) than the loop will only be traversed once. Avoiding this problem of having to traverse most of tree for every event requires information that I'm not sure the compiler can give me. For example I need to know the difference between arr (x -> x) and arr (_ -> 10). The first passes the input to the output the second throws the value away. All I am able to know is that arr was used. What exactly the function does is a black box. -- http://kevin.atkinson.dhs.org