puffnfresh

How do we add extra information to a tree? This has been called [The
AST Typing
Problem](http://blog.ezyang.com/2013/05/the-ast-typing-problem/).

After being hit with this problem in Roy's new type-inference engine,
I tried figuring out how to represent the algorithm. I eventually
realised that it looked like a comonadic operation. Turns out it's
been done before but I couldn't find any complete example.

Below is some literate Haskell to show how to use the Cofree Comonad