store/[org.clojure/clojure "1.8.0"] clj::clojure.core/if-not

Official Documentation

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Arities
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   [test then]
   [test then else]

Docstring
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  Evaluates test. If logical false, evaluates and returns then expr, 
  otherwise else expr, if supplied, else nil.

Source

(defmacro if-not
  "Evaluates test. If logical false, evaluates and returns then expr, 
  otherwise else expr, if supplied, else nil."
  {:added "1.0"}
  ([test then] `(if-not ~test ~then nil))
  ([test then else]
   `(if (not ~test) ~then ~else)))

Example 1

Edit
(if-not (zero? 0) :then :else)
;; => :else

Example 2

Edit
;; See examples for "if" explaining Clojure's idea of logical true
;; and logical false.

Example 3

Edit
(defn has-neg [coll] 
  (if-not (empty? coll)   ;;  = (if (not (empty? coll)) ...
    (or (neg? (first coll)) (recur (rest coll)))))
;; => #'user/has-neg

(has-neg [])
;; => nil 

(has-neg [1 2 -3 4])
;; => true

Uses on crossclj