[mbrock]

You could also say it's just an implementation of mutable state. The "State" type and its operations are just a few lines of simple Haskell code. The implementation uses parameters and return values to thread state between functions. It's just a simple abstraction of a simple pattern of programming. It uses a monadic interface, which is convenient and gives you some syntactic sugar, but there's nothing magical about it.

From the Haskell point of view, so called "actually-mutable" state is on the other hand deeply magical, because it lets you do exotic things like having one thread modify a value in another thread. And that's possible through things like IORefs. But it should be avoided as much as possible.

[mp8]

Really I just wanted to point out that the State Monad is not actually hiding any "real" mutation anywhere, nor is it related to the IO Monad- I was confused on this point for some time when learning Haskell.