Gah. The very interesting question that the author asks is whether we should use the type system to help us enforce validation constraints, and in particular,

Gah. The very interesting question that the author asks is whether we should use the type system to help us enforce validation constraints, and in particular, security constraints -- a great idea that I've seen discussed elsewhere. But unfortunately he takes a wrong turn and assumes that this implies a subclassing relationship, where these validated string class derive from an abstract string class. This makes no sense from an OO perspective; nothing about a string class is "abstract," and it's better to favor composition over inheritance.