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Comment by mp8 | Hacker News Reader

> So as I understand it,languages like Haskell(which I dont know) wrap states into a type that can be passed to pure functions right?

I would restate this as: "In Haskell you write a description of a stateful computation." The "State Monad" is really just the illusion of mutable state, provided by passing the underlying state to each described computation. Actually-mutable state is possible with things like IORef, but these are impure.

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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.

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.

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