Scala:了解参数多态性 [英] Scala: understanding parametric polymorphism

问题描述

两者之间有什么区别

def drop1[A](l: List[A]) = l.tail

和

def drop1(l: List[Int]) = l.tail

提供的用法类似于

drop1(List(1,2,3))

?

何时应使用一个或另一个?为什么?尽管我可以理解第二个示例，但我并不真正理解第一个示例的目的.

When should one or the other be used and why? Whereas I can understand the second example, I don't really understand the purpose of the first one.

推荐答案

真的很简单.您的第一个示例涉及泛型的概念.

It's very simple really. Your first example refers to the concept of generics.

泛型有一个简单的目标，就是使某些方法成为泛型，例如非类型依赖.

Generics have a simple goal, to make certain methods generic, e.g non-type dependant.

让我们看一下这个简单的例子.假设我要为List编写drop1方法.

Lets look at this trivial example. Say I want to write a drop1 method for List.

我可以为每种类型写一个:(慢，重复):

I can either write one for every single type:(slow, repetitive):

def drop1(l: List[Int]): List[Int] = l.tail // this only works for Int

def drop1(l: List[String]): List[String] = l.tail // this only works for String 

您将看到如何为每种类型编写以上代码.为了克服这个问题，您可以使用泛型:

You can see how you'd have to write the above for every single type. To overcome this, you have generics:

def drop1[A](l: List[A]): List[A] = l.tail // this works for any given type.

本质上是这样说的:无论List中包含什么类型，请给我尾巴. 无需为几乎无限数量的类型编写成千上万的drop1变体，我只需要编写一个.

Which essentially says: Whatever the type contained in the List is, give me the tail. Instead of writing thousands of variants of drop1 for the virtually infinite number of types, I only need to write one.

现在在Scala中，最好使用以下方法来实现:

Now in Scala, your implementation is best done with:

implicit class ListOps[A](val l: List[A]) extends AnyVal {
   def drop1: List[A] = l match {
     case head :: tail => tail
     case Nil => Nil
   }
}
// you can now have
List(1, 2, 3).drop1

重命名众所周知的库方法通常也是一个坏主意. tail操作不安全，而drop是安全的.您所引起的只是混乱，因为有一个默认的drop方法.

It is also generally a bad idea to rename well known library methods. A tail operation is unsafe and a drop is safe. All you are causing is confusion, since there is a default drop method.

List(1, 2, 3) drop 1

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