在clojure中，如何根据定义宏本身来定义呢？

Question

我一直在看def宏的源代码，它在其定义中使用了"let“：

(def

 ^{:doc "Like defn, but the resulting function name is declared as a
  macro and will be used as a macro by the compiler when it is
  called."
   :arglists '([name doc-string? attr-map? [params*] body]
                 [name doc-string? attr-map? ([params*] body)+ attr-map?])
   :added "1.0"}
 defmacro (fn [&form &env 
                name & args]
             (let [prefix (loop [p (list name) args args]

然而，"let“被定义为宏本身：

(defmacro let
  "binding => binding-form init-expr

  Evaluates the exprs in a lexical context in which the symbols in
  the binding-forms are bound to their respective init-exprs or parts
  therein."
  {:added "1.0", :special-form true, :forms '[(let [bindings*] exprs*)]}
  [bindings & body]
  (assert-args
     (vector? bindings) "a vector for its binding"
     (even? (count bindings)) "an even number of forms in binding vector")
  `(let* ~(destructure bindings) ~@body))

有人能解释一下这是如何工作的吗，因为我不能理解为什么“def宏”可以被定义为需要“def宏”已经被定义的东西。(如果这是有意义的:)

Answer 1

这是可能的，因为在core.clj中定义defmacro函数之前，this location中已经有了let的定义(稍后会重新定义)。宏只是普通函数，它们绑定到的变量具有值为true的元数据键:macro，以便编译器在编译时可以区分宏(在编译时执行)和函数，没有这个元键就无法区分宏和函数，因为宏本身就是一个恰好处理S表达式的函数。