Why would ad-hoc scripting require more flexibility with a type system? What kind of flexibility? Is it that you need more types? Or more union types to handle all the different variations that are the same, but all slightly different?
Anonymous records and variants are convenient in a scripting language because you don't have to declare them. This in turn leads to anonymous record types and anonymous sum types, which require something beyond HM for type inference, and I wanted to avoid introducing row variables.
TopShell uses type constraints for this, and I've never implemented a type constraint solver before TopShell. There are also some extra things to do around checking explicit type annotations, when the user provides them.
