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The Expression Problem and its solution
craft of programming is almost universally concerned with different types of data and operations/algorithms that act on this data [1]. Therefore, it's hardly surprising that designing abstractions for data types and operations has been on the mind of software engineers and programming-language designers since...
In that email, Wadler points to the paper"Synthesizing Object-Oriented and Functional Design to Promote Re-Use" by Krishnamurthi, Felleisen and Friedman as an earlier work describing the problem and proposed solutions. We kinda added a new type FunctionCall and can now visit it without changing existing code (assuming the virtual inheritance was built into the design from the start to anticipate this approach). Protocols leverage the host platform's (which in Clojure's case is mostly Java) ability to provide quick virtual dispatch, so using them is a very efficient way to implement runtime polymorphism.
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