Diagrammatic representation of Haskell datatypes

[Bas van Dijk]

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.