Elegantly Solving the Expression Problem: Multiple Dispatch and Open Methods

This article delves into the 'expression problem,' a challenge plaguing both object-oriented and functional programming: adding new data types and operations without modifying existing code. Using C++ and Haskell examples, the author illustrates the problem's core. Traditional OOP struggles to extend types and operations simultaneously, and functional programming faces similar limitations. The article deeply analyzes the visitor pattern and its extensions, ultimately using Clojure's multimethods and protocols to demonstrate how multiple dispatch and open methods elegantly solve the expression problem, enabling flexible and extensible code.