Lists, Option, Either, Reader, Logging (non-effectful), STM etc... There is even a probability monad that I have found quite useful. ST & State are far more u

Lists, Option, Either, Reader, Logging (non-effectful), STM etc... There is even a probability monad that I have found quite useful. ST & State are far more useful than updating a local variable. If you are going to choose the simplest use case for them ever and then say that is the only thing they can do it is not a productive discussion. The state monad is a lot like a context object that allows state to flow through function composition (for functions that have no knowledge of such state). For example you could compose a bunch of functions and have the results of each function call stored without changing any of the functions to know about the operation. It's a method of abstraction and encapsulation. It's not just monads that are useful either, it is the full hierarchy of Monoids, functors, applicatives, monads, etc...