解构时需要的密钥

Question

如果您尝试解构的键不在传递给您的函数的映射中,是否有一种方法可以引发异常？这对宏来说是一个很好的用例吗？

例如:

(defn x [{:keys [a b]}] (println a b))

我想要这个工作:

(x {:a 1 :b 2})

但这会引发异常(:b缺失)

(x {:a 1})

Answer 1

怎么样一个前提条件？

(defn x [{:keys [a b]}]
  {:pre [(some? a) (some? b)]}
  (println a b))

user=> (x {:a 1})
AssertionError Assert failed: (some? b)  user/x (NO_SOURCE_FILE:1)

编辑:是的,您可以使用宏来为您处理此问题.也许是这样的:

(defmacro defnkeys [name bindings & body]
  `(defn ~name [{:keys ~bindings}]
     {:pre ~(vec (map #(list 'some? %) bindings))}
     ~@body))

(defnkeys foo [a b]
  (println a b))

(foo {:a 1 :b 2})
(foo {:a 1}) ;; AssertionError Assert failed: (some? b)
(foo {:a 1 :b nil}) ;; AssertionError Assert failed: (some? b)