Definitely. Using Flow, type refinement has been one of my biggest annoyances, because the list of things the type system recognizes as such is hardcoded and ca

Definitely. Using Flow, type refinement has been one of my biggest annoyances, because the list of things the type system recognizes as such is hardcoded and can't be extended: instanceof typeof Array.isArray() == null And if you want to refine object types it gets even worse; you have to just check for properties that may or may not be there, which means those properties have to be rigidly distinguishable between object types in a union, etc. etc. Being able to do custom, rich object validation and give it first-class hooks into the type system sounds incredible.