So this type system was designed such that I could: Get a value of a type -> Convert it to lua types -> call lua code with it -> Convert into my type system -> Type check the result

I think you're describing a functor here, at least a commutative square. You're looking at a subset of Haskell, described by the types of your scripting language, whose functions can be mapped to another language (lua in your case). Personally, I would try to avoid defining, parsing and type-checking an entirely new language if there is a chance of embedding it into Haskell. Especially when it must be quite feature-rich as for writing games. Perhaps a quasi-quoter is sufficient? Also have a look at 'zoom' and 'focus' from the lens package. They let you write state transformers that care only about a subset of your huge game state, where the subset is described by a lens. Olaf