Closure: ClassCastException java.lang.Boolean cannot be cast to clojure.lang.IFn

Question

I am defining a function "true-or-false" that will take an argument and print "1" if it is true and "0" if it is false but when I run my function with the argument:

(= 5 4)

it returns the error: "ClassCastException java.lang.Boolean cannot be cast to clojure.lang.IFn"

Code:

(defn true-or-false [x] (if (x)
  (println "1")
  (println "0")))

(def a (= 5 4))

(true-or-false a)

Answer 1

The clojure.lang.IFn interface provides access to invoking functions, but what you are passing to true-or-false appears to be a number. You shouldn't be wrapping x in parentheses inside if – that would mean you are invoking the x function call (see clojure.org reference on the if special form).