defcontract-proposed.clj

(defcontractedfn1 sqr
  "Calculates the square of a number."
  ([n]
   :requires
   (number? n)
   (not (zero? n))

   :body
   (* n n)

   :ensures
   (pos? %)))

;;;;;;  OR

(defcontractedfn2 sqr
  "Calculates the square of a number."
  ([n]
   :requires
   (number? n)
   (not (zero? n))

   :ensures
   (pos? %)

   :body
   (* n n)))

;;;;;;  OR

(defcontractedfn3 sqr
  "Calculates the square of a number."
  ([n]
   :requires
   number?
   (not (zero? n))

   :ensures
   pos?

   :body
   (* n n)))


(sqr 0)
; java.lang.AssertionError: Assert failed: (not (zero? n))

(sqr 7)
;=> 49

(sqr -7)
;=> 49

I'm not sure I follow.

ensure and require conditions are always checked and enforced? Can I opt out of that behavior, for instance for performance reasons in production environments?

I see what you mean. Nothing will happen if *assert* is set to false. In fact that pre and post conditions will not even be generated.