什么是 Scala 中的 apply 方法，特别是在类型定义中使用 [英] what is apply method in Scala, especially used in type definition

问题描述

我知道 apply 方法在伴生对象中使用时是语法糖.但是，当 apply 方法用于下面的类型定义时，它是什么?

I know that apply method is syntactic sugar when used in companion object. However, what is apply method for when it is used in type definition just like below?

type Applyn = {
    def apply[A](f: A=>A, n: Int, x: A): A
}

这句话之间有区别吗?我猜这句话是用来给Applyn分配泛型函数值的.

Is there a deference between this sentence? As I guess, this sentence is used for assigning generic function value to Applyn.

例如，通过使用上面的句子，我们可以使

For example, by using above sentence, we can make

(Int=>Int, Int, Int)=>Int

(String=>String, Int, String)=>String

等，仅转换为单一类型 Applyn.

etc., into only a single type Applyn.

这是正确的吗?

推荐答案

您正在寻找的是结构类型.它不指任何特定类型，而是指任何具有方法 apply[A](f: A=>A, n: Int, x: A): A 的类型.函数具有 apply 方法，因此具有正确签名的函数适用，但也可以创建具有不是函数的 apply 方法的类型.想象一下.

What you are looking is a structural type. It doesn't refer to any specific type but any type that has a method apply[A](f: A=>A, n: Int, x: A): A. Functions have an apply method, so a function with the correct signature applies, but it's also possible to create types that have that apply method that are not functions. Imagine the following.

object NotAFunction {
  def apply[A](f: A=>A, n: Int, x: A): A = {...}
}

这个对象没有扩展Function，所以它不是一个函数但它仍然满足结构类型要求.

This object doesn't extend Function, so it is not a function but it still satisfies the structural type requirement.

结构类型提供了一种很好的语法，但它们非常慢，因此应尽可能避免使用，尤其是在性能关键代码中.

Structural types provide a nice syntax but they are very slow, so they should be avoided where possible, especially in performance critical code.

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