(1998). Java existed, but neither Scala nor Java-with-generics did.

From the conclusion:

"We have presented a programming protocol, Extensible Visitor, that can be used to construct systems with extensible recursive data domains and toolkits. It is a novel combination of the functional and object-oriented programming styles that draws on the strengths of each. The object-oriented style is essential to achieve extensibility along the data dimension, yet tools are organized in a functional fashion, enabling extensibility in the functional dimension. Systems based on the Extensible Visitor can be extended without modification to existing code or recompilation (which is an increasingly important concern)."