Pattern/idiom for functional state updates in Clojure

I've decided to try and write a simulation program in Clojure (as a proof of concept) where:

• The entire state of the simulation is stored in a single immutable data structure
• State updates are represented as functions that map a state to the next state

This means that I could write an update function as something like:

(defn example-update-function [old-state]
  (let [state (atom old-state)]
    (swap! state some-other-update-function-1)
    (if (some-condition @state)
      (swap! state some-conditional-update-function))
    (swap! state some-other-update-function-2)
    (reset! state (some-function @state some-other-param))
    @state))

The approach seems to work but the following gives me two causes for concern:

• I'm having to use a mutable atom to manage the intermediate states... doesn't seem very functional!
• The code seems a bit ugly with all the swap!s and @state dereferences every开发者_StackOverflow中文版where.

Is there a better / more elegant approach?

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You could write it like so:

  (defn change-when 
    "If (test val) is truethy, returns (fun val), else returns val"
    [val test fun] (if (test val) (fun val) val))

  (defn example-update-function [old-state]
    (-> old-state
        some-other-update-function-1
        (change-when some-condition some-conditional-update-function)
        some-other-update-function-2
        (some-function some-other-param)
        identity))

Probably half-way to the road of monads ...

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This is why the State monad was invented, to encode state modification patterns in a way that looks idiomatic, yet under the hood uses sharing of immutable data structures. You typically get three operations, two core, one derived:

get :: m s   
  -- Return the state from the internals of the monad.

put :: s -> m ()  
  -- Replace the state inside the monad.

modify :: (s -> s) -> m ()   
  -- Maps an old state to a new state inside a state monad. The old state is thrown away.

So perhaps encode those primitives, to make plumbing your state easier.

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Monads? Everyone's always excited about the State monad, which I gather is designed for this sort of thing.