You may find Computational Category Theory by Rydeheard and Burstall interesting. It's available online at Rydeheard's page: http://www.cs.manchester.ac.uk/~david/categories/

The Manual there is the textbook and The Programs provide a great deal of the ML code in the text in its development of category theory.

For someone new to CT it's very different than most introductions but the code helps make some of the ideas much more concrete.