问题描述
在 Programming In Scala 的第 9 章中，有一个这样的示例方法:
In Chapter 9 of Programming In Scala, there is an example method like this:
def twice(op: Double => Double, x: Double) = op(op(x))
作者在书中说:
这个例子中op的类型是双 =>双，这意味着它是一个将一个 Double 作为一个函数参数并返回另一个 Double.
我不明白什么是Double =>"双"在这里，在前面的章节中，="出现只表示函数字面量，从来没有像这样写过Type =>"类型"，因为按照 Scala 函数字面量语法定义，函数字面量的右边部分是函数体，函数体怎么可能是双"呢??
I don't understand what is "Double => Double" here, in previous chapters, where "=>" appears only means function literal, and never wrote like this "Type => Type", because according to Scala function literal syntax definition, the right part of function literal is the function body, how can a function body be "Double" ?
推荐答案
因为它有两种用法.
首先，您可以使用 => 来定义函数字面量.
First, you could use => to define function literal.
scala> val fun = (x: Double) => x * 2
fun: (Double) => Double = <function1>
scala> fun (2.5)
res0: Double = 5.0
这很容易.但这里的问题是，fun 是什么类型?它是一个以 Double 作为参数并返回 double 的函数"，对吗?
It's pretty easy. But the question here is, what type fun is? It is a "function that takes a Double as an argument and return a double", right?
那么我如何用它的类型来注释 fun 呢?那就是 (Double) =>(双).好吧，前面的例子可以改写为:
So how could I annotate fun with its type? That is (Double) => (Double). Well, the previous example could be rewritten to:
scala> val fun: Double => Double = (x: Double) => x * 2
fun: (Double) => Double = <function1>
scala> fun (2.5)
res1: Double = 5.0
好的，那么下面的代码是做什么的?
OK, then what does the following code do?
def twice(op: Double => Double, x: Double) = op(op(x))
好吧，它告诉你 op 是一个 (Double => Double)，这意味着它需要一个函数，它接受一个 Double 并返回一个 Double.
Well, it tells you that op is a (Double => Double), which means it needs a function which takes a Double and return a Double.
所以你可以将前面的 fun 函数传递给它的第一个参数.
So you could pass the previous fun function to its first argument.
scala> def twice(op: Double => Double, x: Double) = op(op(x))
twice: (op: (Double) => Double,x: Double)Double
scala> twice (fun, 10)
res2: Double = 40.0
并且相当于将op替换为fun，将x替换为10，即fun(fun(10)) 结果是 40.
And it will be equivalent to replacing op with fun, and replace x with 10, that is fun(fun(10)) and the result will be 40.
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