为什么Clojure有“关键字”除了“符号”吗？ [英] Why does Clojure have "keywords" in addition to "symbols"?
问题描述
我有一个传递知识的其他Lisps（特别是Scheme）从回来。最近我一直在阅读关于 Clojure 的信息。我看到它有符号和关键字。
I have a passing knowledge of other Lisps (particularly Scheme) from way back. Recently I've been reading about Clojure. I see that it has both "symbols" and "keywords". Symbols I'm familiar with, but not with keywords.
别的Lisps有关键字吗？
Do other Lisps have keywords? How are keywords different from symbols other than having different notation (ie: colons)?
推荐答案
这里是 Clojure documentation for Keywords and Symbols。
Here's the Clojure documentation for Keywords and Symbols.
关键字是对自身进行评估的符号标识符。它们提供非常快速的等式测试...
Keywords are symbolic identifiers that evaluate to themselves. They provide very fast equality tests...
符号是通常用于指代别的东西的标识符。它们可以在程序形式中用于引用函数参数，允许绑定，类名和全局变量...
Symbols are identifiers that are normally used to refer to something else. They can be used in program forms to refer to function parameters, let bindings, class names and global vars...
用作轻量级的常量字符串，例如用于哈希映射的键或多方法的分派值。符号通常用于命名变量和函数，除了宏等之外，直接操作它们作为对象不太常见。但是没有什么能阻止你在使用关键字的任何地方使用符号（如果你不介意一直引用它们）。
Keywords are generally used as lightweight "constant strings", e.g. for the keys of a hash-map or the dispatch values of a multimethod. Symbols are generally used to name variable and functions and it's less common to manipulate them as objects directly except in macros and such. But there's nothing stopping you from using a symbol everywhere you use a keyword (if you don't mind quoting them all the time).
最简单的方法来看看差异是在Clojure源中读取 Keyword.java 和 Symbol.java 。有一些明显的实现差异。例如，Clojure中的Symbol可以有元数据，而Keyword不能。
The easiest way to see the difference is to read Keyword.java and Symbol.java in the Clojure source. There are a few obvious implementation differences. For example a Symbol in Clojure can have metadata and a Keyword can't.
除了单冒号语法之外，还可以使用双冒号来创建命名空间-qualified keyword。
In addition to single-colon syntax, you can use a double-colon to make a namespace-qualified keyword.
user> :foo
:foo
user> ::foo
:user/foo
Common Lisp有关键字，语言。他们在那些语言当然略有不同。 Common Lisp关键字和Clojure关键字之间的一些区别：
Common Lisp has keywords, as do Ruby and other languages. They are slightly different in those languages of course. Some differences between Common Lisp keywords and Clojure keywords:
-
Clojure中的关键字不是符号。
Keywords in Clojure are not Symbols.
user> (symbol? :foo)
false
关键字不属于任何命名空间，除非您具体限定他们：
Keywords don't belong to any namespace unless you specifically qualify them:
user> (namespace :foo)
nil
user> (namespace ::foo)
"user"
$ b b
（感谢 Rainer Joswig 为我提供有关事物的想法。）
(Thanks Rainer Joswig for giving me ideas of things to look at.)
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