Does this meet your criteria of "first-class" property? It works on any entity that "has" and "age", at least according to my definition of those things!

Does this meet your criteria of "first-class" property? It works on any entity that "has" and "age", at least according to my definition of those things! -- The class definition can often be entirely elided -- with Generics or Template Haskell. Instance -- definitions can generally require minimal or no -- implementation either class HasAge r age | r -> age where getAge :: r -> age setAge :: r -> age -> r isOld :: HasAge r => r -> Bool isOld r = getAge r > 30