[LukeHoersten]

The author states the "extreme" case of OOP is "everything is an object" and the extreme case of FP is "purity."

The problem with this analogy is that everything being an object doesn't by you much. But in Haskell, the type system helps you keep track of what's pure and what's not. So having a type system that keeps track of purity buys you something: lots of pure and extremely easy to reason about code. Non-deterministic (unpure) code is the same difficulty to reason about in any language. Isolating it from pure code just reduces the amount of code which is difficult to reason about.