let.clj

(defmacro let?
  [bindings & body]
  (let [[bind [kwd pred & more]] (split-with (complement #{:ensure}) bindings)]
    `(let [~@bind]
       ~@(cond (and kwd more) [`(when ~pred (check-let [~@more] ~@body))]
               kwd            [`(when ~pred ~@body)]
               :else body))))

(let? [x 100
       y 300
       :ensure (pos? y)
       z (- y 250)]
      z)

;; expands to

(let [x 100
      y 300]
  (when (pos? y)
    (let [z (- y 250)]
      z))) ;; => 50

;; and returns 50. The following returns nil:

(let? [x 100
       y 300
       :ensure (neg? y)
       z (- y 250)]
      z) ;; => nil