This isn't about encoding the validation rules in the type system, only the requirement that validation with respect to some properties has been done. So in you

This isn't about encoding the validation rules in the type system, only the requirement that validation with respect to some properties has been done. So in your example you would just define a function createPurchaseRequest :: UserRequest -> Purchase -> Maybe PurchaseRequest now everywhere in the code taking a PurchaseRequest can assume it is valid.