clojure中带有第一个自变量引用的列表的值是最后一个自变量? [英] Value of a list with quoted first argument in clojure is the last argument?

问题描述

为什么要表达

('+ 10 20)

评估为 20 ?这对我来说真的很奇怪！我本来希望给出一个错误，因为列表的第一个元素'+ 我不是可以评估的东西！

evaluate to 20? This is realy bizarre to me! I would have expected to give an error, because the first element of the list, '+ i not a thing that can be evaluated!

推荐答案

无论是好是坏，这都是Clojure的常见问题解答.因此，'+ 是(quote +)的简写，其结果为符号 + .符号(和关键字)可以视为函数，并在其参数中查找自身，因此('+ 10 20)本质上是(get 10'+ 20)，这就是 get 的 not-found Arity，因此，如果"collection" 10 不包含符号'+ ，则它将返回第三个参数 20 .

This is pretty much an FAQ with Clojure, for good or bad. So '+ is shorthand for (quote +) which evaluates to the symbol +. Symbols (and keywords) can be treated as functions and look themselves up in their argument, so ('+ 10 20) is essentially (get 10 '+ 20) and that's the not-found arity of get so if the "collection" 10 does not include the symbol '+ then it will return the third argument, 20.

您可能希望针对 10 的关联查询失败并引发异常，但是Clojure中有很多情况会导致查询返回 nil (或未找到版本)，因为它从 nil -punning的角度来看是有益的，并且是导致惯用Clojure的一部分.

You might expect associative lookup against 10 to fail and throw an exception but there are a lot of situations in Clojure where lookup will return nil (or the not-found version) because it's beneficial from a nil-punning point of view and is part of what leads to idiomatic Clojure.

某些函数 do 执行这种参数检查:例如，(包含?10'+)会引发异常，而两个(得到10'+)和(得到'+ 10)都将返回 nil .

Some functions do perform that sort of argument checking: (contains? 10 '+) will throw an exception, for example, whereas both (get 10 '+) and (get '+ 10) will both return nil.

可能还值得注意的是，以下内容引发了异常:

It's probably also worth noting that the following throws an exception:

user=> ('+ 10 20 30)
Execution error (ArityException) at user/eval39476 (REPL:1).
Wrong number of args (3) passed to: clojure.lang.Symbol

您询问在clojure中带有第一个自变量引用的列表的值是否为最后一个自变量"?并且答案是否定的，由于上述逻辑，这非常具体地是两个参数的函数调用:具有3个参数的符号，只能具有1或2.

You asked if the "value of a list with quoted first argument in clojure is the last argument" and the answer is no, this is very specifically a two-argument function invocation, because of the above logic: you can't "call" a symbol with 3 args, only with 1 or 2.

值得一提的是0 args情况的区别:

Worth noting the difference here for the 0 args case:

user=> ('+)
Execution error (ArityException) at user/eval39478 (REPL:1).
Wrong number of args (0) passed to: clojure.lang.Symbol
user=> (+)
0

第一种情况是符号查找-至少需要一个集合才能在其中查找自身-而第二种情况是函数调用( clojure.core/+ )为零参数，其返回 + 的标识值的方式与(*)返回 1 的方式相同.

The first case is a symbol lookup -- which requires at least a collection to look itself up in -- whereas the second case is a function invocation (of clojure.core/+) with zero arguments, which returns the identity value of + in the same way that (*) returns 1.

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