Improved Clojure calculator

calculator.clj

(ns calculator.core
  (:gen-class))

(def operators { "-" - "+" + "/" / "*" *})

(defn convert-to-number [value]
  (let [n (read-string value)]
       (if (number? n) n nil)))

(defn read-operator []
  (println "Enter an operator")
	(read-line))

(defn read-value []
	(println "Enter value")
	(convert-to-number (read-line)))

(defn execute-calculation [running-total selected-operator value]
  ((operators selected-operator) running-total value))

(defn calculator
	([] (calculator 0))
	([running-total]
	 (println (str "Current total " running-total))
	 (def new-total (execute-calculation running-total (read-operator) (read-value)))
	 (calculator new-total)))	

(defn -main
  "Run the calculator"
  [& args]
  (calculator))