Events in Haskell, and how to implement them

We describe a new and simpler implementation in Haskell of CML's events, wh ich encode reactions by a thread to combinations of messages from other thr eads. We add a new type of Guarded Events, by which recipients can filter m essages with conditions on their value known as Guards. We implement guarde d channels. The guard type and the indexing algorithm are not part of the c hannel definition, so that the user can trade off what guards are required against the cost of indexing. As an example we sketch the encapsulation of a graphical user interface too lkit. This can be done concisely not only because of guarded events, but al so because we construct events monadically. Monadic events are especially h elpful for representing concurrent processes which transform themselves in reaction to external communications.