为什么要在下面的代码中得到NPE?

看看正在发生的扩张:

(macroexpand '(exec-all (cons 2 [4 5 6])))
=>
((clojure.core/println "Code: " (quote (cons 2 [4 5 6])) "\t=>\tResult: " (cons 2 [4 5 6])))

为了解决这个问题,我建议修改宏,在前面加一个“do”,例如。

(defmacro exec-all [& commands]
  (cons 'do (map (fn [c] `(println "Code: " '~c "\t=>\tResult: " ~c)) commands)))

(defmacro exec-all [& commands]
  `(doseq [c# ~(vec (map (fn [c]
                           `(fn [] (println "Code: " '~c "=> Result: " ~c)))
                         commands))]
     (c#)))

这扩展到

(doseq [c [(fn []
             (println "Code: "      '(conj [2 3 4] 5)
                      "=> Result: " (conj [2 3 4] 5)))
           (fn []
             (println "Code: "      '(+ 1 2)
                      "=> Result: " (+ 1 2)))]]
  (c))

fn c 在宏展开时收集在向量中。

(do ...)

互动示例:

user=> (exec-all (conj [2 3 4] 5) (+ 1 2))                                                                                                    
Code:  (conj [2 3 4] 5) => Result:  [2 3 4 5]
Code:  (+ 1 2) => Result:  3
nil