when-let

(when-let bindings & body)

bindings => binding-form test

如果 test 的值 value 为真，那么结合 binding-form 绑定，对 body 进行求值。

user=> (defn drop-one [coll]
           (when-let [s (seq coll)]
               (rest s)))
#'user/drop-one

user=> (drop-one [1 2 3])
(2 3)