Mutants!

Part of the rationale of Clojure is to get rid of free-for-all mutation: "for the concurrent programming future, pervasive, unmoderated mutation simply has to go."

Atoms are a way of performing mutation in a safe way, free from any potential race conditions.

An atom is associated with a Clojure value, in this case an integer.

  (def position (atom 0))

If you want to observe the value, use deref or the @ notation.

  user> (prn @position)
  0
  nil
  user> (prn position)
  #
  nil
  user> (prn (deref position))
  0
  nil

So how do you actually change the value inside the atom? swap! and compare-and-set! change the values of atoms. Both have the ! (bang) suffix to indicate that they are destructive operations.

swap! takes two arguments, the atom itself and a function to apply to the value and swap with the current value of the atom. For example:

  user> (swap! position inc)
  1
  user> (swap! position inc)
  2

compare-and-set! (CAS)is a lower level function which sets the value of an atom given the original value and the new. The value is only changed if it is observed first to be equal to the original value. A Boolean return value indicates whether the atom was changed.

  user> (swap! position (fn [x] 0))
  0
  user> (compare-and-set! position 0 1)
  true
  user> (compare-and-set! position 0 2)
  false

I'll use atoms to change state when animating my little sorting visualization.