Yeah, I came across Barr and Wells 5 years ago in grad school and found it almost impossible to buy. I ended up getting a copy from the library and photocopying and rebinding it. Like you said, it was ok, but hardly comprehensive. Maybe it's easier to find now.

Anyway, nice paper..and thanks for making me think of category theory again. Maybe I'll pull out my old copy of "CT for the working mathematician" and give it (and haskell) another go.

Edit: I came across this: http://www.amazon.com/gp/product/0521422264/ref=pd_lpo_k2_dp...

Anyone gone through this book?