This is used to create a reference value. When creating a reference value, there is an option to provide a validator function, which will validate the value created.
Following is the syntax.
(ref x options)
Parameters − ‘x’ is the value which needs to be provided to the reference. ‘Options’ is a set of options that can be provided, such as the validate command.
Return Value − The reference and its corresponding value.
An example on how this is used is shown in the following program.
(ns clojure.examples.example (:gen-class)) (defn Example  (def my-ref (ref 1 :validator pos?)) (println @my-ref)) (Example)
To access the value of the reference value, you can use the @ symbol.
The above program produces the following output.