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 a

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.