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

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/ http://wiki.portal.chalmers.se/agda/) or Idris (http://www.cs.st-andrews.ac.uk/~eb/Idris/ http://www.cs.st-andrews.ac.uk/~eb/Idris/).