在Clojure中，函数，带引号的函数和尖引号的函数之间的差异 [英] In Clojure, difference between function, quoted function and sharp-quote function

问题描述

在Clojure中，我想知道以下三个之间的区别.

In clojure, I'd like to know what are the differences between the three below.

(println (map + '(1 2 3) '(4 5 6))) 

(println (map '+ '(1 2 3) '(4 5 6))) 

(println (map #'+ '(1 2 3) '(4 5 6))) 

结果是

(5 7 9) 

(4 5 6) 

(5 7 9) 

我听不懂第二个人的行为.

I can't understand the second one's behavior.

我觉得Clojure中的第一个和第三个是相同的Lisp-1并且不区分评估变量和同名函数.

I feel the first one and the third one are the same in clojure which is Lisp-1 and doesn't distinguish between evaluating a variable and the identically named function.

这可能是一个基本问题，但似乎信息量不足.请教我.

This may be a basic question, but there seems not to be enough infomation. Please teach me.

谢谢.

推荐答案

关于第三种情况，与Common Lisp相比，#'+ 不能理解为(function +)并在函数名称空间中引用符号 + 的值，因为Clojure没有函数名称空间.而是将其读取为(var +)，并引用称为 + 的 var .应用 var 与应用存储在 var 中的值相同.

Regarding the third case, in contrast to Common Lisp, #'+ does not read as (function +) and refer to the value of the symbol + in the function namespace, since Clojure does not have a function namespace. Instead, it reads as (var +) and refers to the var called +. Applying a var is the same as applying the value stored in the var.

在第二种情况下，您将一个符号重复地应用于一对数字.这是偶然发生的.在地图上应用符号与在该地图中建立索引相同:

In the second case, you are repeatedly applying a symbol to a pair of numbers. This is valid by accident. Applying a symbol to a map is the same as indexing into that map:

user> ('a {'a 1, 'b 2, 'c 3, '+ 4})
1
user> ('+ {'a 1, 'b 2, 'c 3, '+ 4})
4

如果提供第二个参数，则在地图中找不到匹配键的情况下，它将用作默认值:

If you supply a second argument, it is used as the default value in case no matching key is found in the map:

user> ('+ {'a 1, 'b 2, 'c 3} 4)
4

由于在每次迭代中，您都将符号 + 应用于一对数字，并且由于数字不是映射，因此不包含 + 作为键，则返回第二个参数作为匹配失败的默认值.

Since in each iteration, you apply the symbol + to a pair of numbers, and since a number isn't a map and therefore doesn't contain + as a key, the second argument is returned as the default value of a failed match.

user> ('+ 'foo 4)
4
user> ('+ {} 4)
4
user> ('+ 1 4)
4

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