This function is used to alter the value of a reference type but in a safe manner. This is run in a thread, which cannot be accessed by another process. This is why the command needs to be associated with a ‘dosync’ method always. Secondly, to change the value of a reference type, a function needs to be called to make the necessary change to the value.
Following is the syntax.
(alter refname fun)
Parameters − ‘refname’ is the name of the variable holding the reference value. ‘fun’ is the function which is used to change the value of the reference type.
Return Value − The reference and its corresponding new value.
An example on how this is used is shown in the following program.
(ns clojure.examples.example (:gen-class)) (defn Example  (def names (ref )) (defn change [newname] (dosync (alter names conj newname))) (change "John") (change "Mark") (println @names)) (Example)
The above program produces the following output.