clojureの便利なif-let
if-letとwhen-letという便利なマクロの解説
http://blog.jayfields.com/2011/03/clojure-if-let-and-when-let.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+jayfields/mjKQ+(Jay+Fields+Thoughts)
(if-let [a ] )
例
user=> (if-let [a 4] (+ a 4) (+ 10 10))
8