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

问题描述

  scala>这是一个小小的Scala会话，它定义并尝试了一些函数。 def test1（str：String）= str + str; 
 test1：（str：String）java.lang.String 
 
 scala> test1（ab）
 res0：java.lang.String = abab 
  

。

  scala> val test2 = test1 
< console>：6：错误：对象$ iw中缺少方法test1的参数; 
如果您想将其视为部分应用函数，则使用`_'跟随此方法
 val test2 = test1 
 ^ 
  

oops。

  scala> val test2 = test1 _ 
 test2：（String）=> java.lang.String =< function1> 
 
 scala> test2（ab）
 res1：java.lang.String = abab 
  

效果很好！

现在，我在折叠时看到了 _ 语法（ _ + _ 等）。据我所知， _ 基本上是指一个参数。所以 test1 _ 基本上意味着一个带有参数的函数，它被赋予 test1 。

所以我一直在探索......

  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> 
  

这里没有 _ ！ def ed函数与 val ed函数之间的区别？

解决方案

def'ed函数和val'ed函数之间没有区别：

  scala> def test1 =（str：String）=> str + str 
 test1：（String）=> j ava.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> 
  

请参阅？所有这些都是函数，它由 X => Y 类型。

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

您是否看到 X => Y 键入？如果你这样做，去看眼科医生，因为没有。这里的类型是（X）Y ，通常用来表示一个方法。

。

实际上， test1 ， test2 ， test3 和 test4 都是返回函数的方法。 test5 是一个返回 java.lang.String 的方法。另外， test1 至 test4 不接受参数（只有 test1 可以，无论如何），而 test5 不会。

所以，区别非常简单。在第一种情况下，您尝试将方法分配给val，但未填写方法所需的参数。所以它失败了，直到你添加了一个尾部下划线，这意味着把我的方法变成一个函数。

在第二个例子中，你有一个函数，所以你不需要做任何事情。

方法不是函数，反之亦然。函数是 FunctionN 类之一的对象。一个方法是与某个对象相关的一段代码的句柄。

查看堆栈溢出中有关方法与函数的各种问题。

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

works nicely.

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
                   ^

oops.

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

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

works well!

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 _?

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>

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

解决方案

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>

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

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.

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.

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.

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

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.

See various questions about methods vs functions on Stack Overflow.

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