any type theory book recommendation?
chriseidhof: I've only read Pierce, and it's good. As an alternative, you could look at dependent types if you want to know more about types. Dependent types are like GADTs on steroids. A good place to start is Agda (http://wiki.portal.chalmers.se/agda/) or Idris (http://www.cs.st-andrews.ac.uk/~eb/Idris/).