Does this solve the problem though? I'll be honest, I'm not very familiar with Haskell (and frankly only casually use FP in general), but I don't see how the pa

Does this solve the problem though? I'll be honest, I'm not very familiar with Haskell (and frankly only casually use FP in general), but I don't see how the pattern in your link alleviates my concern. The problem (truly the problem) is about _where_ one defines behavior. It's the expression problem (EP) at its core. A FP approach separates data and behavior in a way that makes adding new behaviors to the same data types easy and OOP combines them in a way to make adding new data types to the same behaviors easy. It's literally the difference between: let model' = update model msg and: let model' = model.update msg Like I said, I don't know Haskell very well (and had a hard time grokking the link), but any purely functional language will suffer from the former over the latter. The side of the EP a language falls on is simply as a matter of the data types/type system available.