Diagrammatic representation of Haskell datatypes

Hi, I'm desining a visual programming language (just for fun...). The language is pretty much based on "Visual Haskell": http://ptolemy.eecs.berkeley.edu/~johnr/papers/visual.html But it uses another approach to model higher-order functions: http://citeseer.nj.nec.com/dami96higherorder.html The "functional side" of the language is pretty much finished so what's left is the "datatype side". With Object-Oriented languages such as C++ and Java you can use diagram languages such as UML to model the data. I was wondering if there exists diagrammatic languages for modelling Haskell datatypes? It would be great if someone can point me to some research papers. Thanks, Bas van Dijk.