79 lines
3.5 KiB
Markdown
79 lines
3.5 KiB
Markdown
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---
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title: Clojure More on Functions
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localeTitle: Clojure更多关于功能
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---
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功能！它们非常重要。没有功能就很难做任何事情。它们是任何语言的组成部分，尤其是Clojure，因为它是一种拒绝面向对象设计的函数式编程语言。让我们更多地了解它们！
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## 元数
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**Arity**指的是函数期望的参数数量。
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```
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;; add expects 2 arguments. Its arity is 2.
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(defn add [xy] (+ xy))
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(add 2 2)
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; => 4
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;; + itself is a function, and it can have any number of arguments.
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(+ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16) ;; and so on...
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; => 136
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```
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Clojure有一些特殊的语法，允许你让你的函数根据它收到的参数数量做不同的事情。这称为变量arity。
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```
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(defn foo
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([] ; if this function gets no arguments...
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(println "Lisa needs braces!")) ; do this.
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([arg1] ; if this function gets 1 argument...
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(println "Dental plan!"))) ; do this instead!
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(foo)
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; => Lisa needs braces!
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; nil
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(foo "this is a placeholder argument.")
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; => Dental plan!
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; nil
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```
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 [IDEOne吧！](https://ideone.com/sXGplb)
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## 匿名函数
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让我们看一个非常简单的函数：一个向数字加1的函数。
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```
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;; I've called this function "my-inc" so you don't confuse it with inc.
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;; inc is a built-in function that already does this for us.
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(defn my-inc [n] (+ 1 n))
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(inc' 5)
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; => 6
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```
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这看起来很简单。它需要一个参数 - `n` - 并返回`n + 1` 。让我们分开吧。
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```
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(def my-inc-2 (fn [n] (+ 1 n)))
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(inc' 5)
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; => 6
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```
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你可以从中看到，使用`defn`只是使用简写`(def ... (fn ...))` 。但这揭示了一些有趣的东西我们实际上做的不是“定义一个函数”，它只是将一个匿名函数绑定到一个特殊名称 - `inc'` 。如果我们不给它起个名字怎么办？
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```
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((fn [n] (+ 1 n)) 5)
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; => 6
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```
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巴姆！繁荣！ KAPOW！匿名函数。现在这看起来似乎没用了，但是稍后使用`map` ， `reduce`和`filter`将函数应用于列表会非常方便。给你写一个名字的每个功能都会变得枯燥乏味，快速。
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编写匿名函数的方法较短，适用于非常简短的函数。它不允许解构或变量arity。但是，它非常简洁。
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```
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(#(+ 1 %) 5)
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; => 6
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```
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`#(...)`是定义匿名函数的简便方法。 `%`指的是函数的第一个参数。如果您的函数需要多个参数，则可以使用`%1, %2, ... %n` 。
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```
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(#(str %1 %2 %3) "foo" "bar" "baz")
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; => "foobarbaz"
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```
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 [IDEOne吧！](https://ideone.com/roYRgS)
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| [上一页](//forum.freecodecamp.com/t/clojure-loop-recur/18418) | [家](//forum.freecodecamp.com/t/clojure-resources/18422) | [下一个](//forum.freecodecamp.com/t/clojure-collections/18411) |
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| [循环和重复](//forum.freecodecamp.com/t/clojure-loop-recur/18418) | [目录](//forum.freecodecamp.com/t/clojure-resources/18422) | [收藏](/http://forum.freecodecamp.com/t/clojure-collections/18411) |
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