在 Scala 中定义函数的两种方式.有什么不同? [英] Two ways of defining functions in Scala. What is the difference?

问题描述

这是一个小的 Scala 会话，它定义并尝试了一些函数:

Here is a little Scala session that defines and tries out some functions:

scala> def test1(str: String) = str + str;    
test1: (str: String)java.lang.String

scala> test1("ab")
res0: java.lang.String = abab

效果很好.

scala> val test2 = test1
<console>:6: error: missing arguments for method test1 in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
       val test2 = test1
                   ^

哎呀.

scala> val test2 = test1 _
test2: (String) => java.lang.String = <function1>

scala> test2("ab")
res1: java.lang.String = abab

效果很好！

现在，我已经看到了折叠时的 _ 语法(_ + _ 等).所以据我了解 _ 基本上意味着一个论点".所以 test1 _ 基本上意味着一个带有参数的函数，它被赋予 test1".但为什么这与 完全 不一样test1?为什么我附加一个_会有不同?

Now, I've seen the _ syntax when folding (_ + _, etc). So as I understand it _ basically means "an argument". So test1 _ basically means a function with an argument, which is given to test1". But why isn't that exactly the same as just test1? Why is there a difference if I append a _?

所以我一直在探索......

So I kept exploring...

scala> val test3 = (str: String) => str + str
test3: (String) => java.lang.String = <function1>

scala> test3("ab")
res2: java.lang.String = abab

scala> val test4 = test3
test4: (String) => java.lang.String = <function1>

这里无需_即可工作！defed 函数和 valed 函数有什么区别?

Here it works without _! What's the difference between a defed function, and a valed function?

推荐答案

def'ed 函数和 val'ed 函数没有区别:

There's no difference between a def'ed function and a val'ed function:

scala> def test1 = (str: String) => str + str
test1: (String) => java.lang.String

scala> val test2 = test1
test2: (String) => java.lang.String = <function1>

scala> val test3 = (str: String) => str + str
test3: (String) => java.lang.String = <function1>

scala> val test4 = test2
test4: (String) => java.lang.String = <function1>

看到了吗?所有这些都是函数，用 X => 表示.Y 他们拥有的类型.

See? All of these are functions, which is indicated by the X => Y type they have.

scala> def test5(str: String) = str + str
test5: (str: String)java.lang.String

你看到一个 X =>Y 类型?如果你这样做，去看眼科医生，因为没有.这里的类型是(X)Y，常用来表示一个方法.

Do you see an X => Y type? If you do, go see an ophthalmologist, because there's none. The type here is (X)Y, commonly used to denote a method.

其实test1、test2、test3和test4都是方法，返回函数.test5 是一个返回 java.lang.String 的方法.此外，test1 到 test4 不带参数(无论如何只有 test1 可以)，而 test5 可以.

Actually, test1, test2, test3 and test4 are all methods, which return functions. test5 is a method which returns a java.lang.String. Also, test1 through test4 do not take parameters (only test1 could, anyway), while test5 does.

所以，区别很简单.在第一种情况下，您尝试将一个方法分配给一个 val，但没有填写该方法采用的参数.所以它失败了，直到你添加了一个尾随下划线，这意味着把我的方法变成一个函数.

So, the difference is pretty simple. In the first case, you tried to assign a method to a val, but did not fill in the parameters the method take. So it failed, until you added a trailing underscore, which meant turn my method into a function.

在第二个示例中，您有一个函数，因此您无需执行任何其他操作.

In the second example you had a function, so you didn't need to do anything else.

方法不是函数，反之亦然.函数是 FunctionN 类之一的对象.方法是与对象关联的某些代码段的句柄.

A method is not a function, and vice versa. A function is an object of one of the FunctionN classes. A method is a handle to some piece of code associated with an object.

在 Stack Overflow 上查看有关方法与函数的各种问题.

See various questions about methods vs functions on Stack Overflow.

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