I've personally never thought the cost of tracking mutation and IO in a type system is particularly worth the complexity and hassle. But for everything else, an

I've personally never thought the cost of tracking mutation and IO in a type system is particularly worth the complexity and hassle. But for everything else, an advanced Hindley Milner derived type system is absolutely, 100% worth it. I do think there might be some middle ground where you can annotate non pure functions in order to help the compiler with aggressive optimization.