Clojure符号在用作函数时做什么？ [英] What do Clojure symbols do when used as functions?

问题描述

在尝试解决涉及重新执行评估的通用计算引擎时遇到的4Clojure问题，我不小心结束了调用这样的：

 （apply'/'（16 8））
  
 （apply / '（16 8））
  
这会产生混淆的副作用： 8 
 
 
 
我后来发现我的错误后一些调试 - 我没有评估的 / 符号，然后尝试调用它 - 因此意识到 clojure.lang.Symbol 必须实现 clojure.lang.IFn 。但是，这个实现怎么做？我可以得到它做的是返回 nil 有一个参数，或第二个参数，如果给定。
符号在地图中自己查找，与关键字一样。请参见 Symbol的实现：
  ... 
 122 public Object invoke（Object obj）{
 123 return RT.get（obj，this）; 
 124} 
 125 
 126 public Object invoke（Object obj，Object notFound）{
 127 return RT.get（obj，this，notFound）; 
 128} 
 ... 
  
（ code>是  clojure.lang。 
 
 
 
在给定的示例中，查找失败（因为16不是映射），因此正在返回 notFound 值（8）。
 
While trying to solve a 4Clojure problem Universal Computation Engine involving reimplementing evaluation, I accidentally ended up calling something like this:
(apply '/ '(16 8))
rather than the intended:
(apply / '(16 8))
This had the confusing side effect of returning 8, which made me think I had messed up my maths.

I later realised my error after some debugging—I was failing to evaluate the / symbol before attempting to call it—and so realised that clojure.lang.Symbol must implement clojure.lang.IFn. But what does that implementation do? All I can get it to do is return nil with one argument, or the second argument if given.
解决方案
Symbols look themselves up in a map, much as keywords do.  See Symbol's implementation:
…
122 public Object invoke(Object obj) {
123         return RT.get(obj, this);
124 }
125
126 public Object invoke(Object obj, Object notFound) {
127         return RT.get(obj, this, notFound);
128 }
…
(RT is clojure.lang.RT, which does just about everything. "RunTime"?)

In the example given, the lookup is failing (because 16 is not a map), and therefore the notFound value (8) is being returned.

                        
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