Coalgebras and Automata

fold is a really useful operator in a number of programming languages, including Haskell. It also hs a dual partner, unfold. But more importantly, both can be generalised, using a little category theory, to F-algebras and F-coalgebras. Amazingly, unfold, in the right category (as I discovered after a weekend of frenzied category theory) gives a really nice way to convert descriptions of automata into runnable automata. Anyway, I wrote the whole thing up as a piece of literate Haskell.

UPDATE: That came out formatted ugly so I've moved it to my web site where I'll eventually format it better. Here's the link.