为什么以及何时需要在方法名称后加上_? [英] Why and when do I need to follow a method name with _?

问题描述

对于何时需要将_用作函数的方法，我对规则有些不确定.例如，为什么下面的Foo和Nil的::之间有区别?

I'm a bit shaky on the rules as to when you need a _ after a method to use it as a function. For example, why is there a difference between Foo's and Nil's :: in the following?

def square(n: Int) = n * n  
object Foo { def ::(f: Int => Int) = f(42) }

// ...

scala> Foo.::(square)
res2: Int = 1764

scala> Nil.::(square) 
<console>:6: error: missing arguments for method square in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
   Nil.::(square)
          ^
scala> Nil.::(square _) 
res3: List[(Int) => Int] = List(<function1>)

推荐答案

在部分应用的函数表达式中省略所有参数时，除非编译器 ，否则您需要在其后跟随_ em>需要在您使用该功能的地方.

When you omit all parameters in a partially applied function expression, then you need to follow it with _ unless the compiler requires a function type in the place where you use it.

当您在Foo上调用方法::时，编译器期望该参数的类型为Int => Int.因此，您可以安全地在该位置的square之后省略下划线.

When you call the method :: on Foo, the compiler expects a type Int => Int for the parameter. So you can safely omit the underscore after square in that position.

但是，Nil上的::方法可以采用 any 类型的参数.因此，除非您通过添加_使其绝对明确，否则它会抱怨，而不是假设您打算部分应用该功能.

However, the :: method on Nil can take a parameter of any type. So rather than assume that you meant to partially apply the function, it complains unless you make it absolutely explicit by adding _.

这些就是规则...我真的不能启发您为什么这些规则；也许其他对编译器，类型系统和语言设计有更好了解的人可以告诉您原因.但是我认为，如果没有这些规则，许多地方就会有造成意外歧义的危险.

So those are the rules... I can't really enlighten you about why those are the rules; maybe somebody else who has better knowledge of the compiler, the type system, and the language design will be able to tell you why. But I assume that without these rules, there would be a danger of accidental ambiguity in many places.

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