斯卡拉：=>参数

Question

什么是（代码：=>单元）参数在此阶函数来完成从斯卡拉文档：

def unless(exp: Boolean)(code: => Unit): Unit = if (!exp) code 
unless(x < 5) { 
    println("x was not less than five") 
} 

Link

Answer 1

的=>语法函数的参数列表内意味着该参数本身就是一个函数（称为高阶函数）。那个函数签名说的是unless是一个带布尔值的函数，并且采用一个不带参数的函数并返回Unit。下面是一些例子：

// This is a function that takes as a parameter that is a function that takes an Int, and returns a Boolean 
def foobar(f: Int => Boolean) = ??? 

// It might be called like this: 
foobar(i => i/2 == 0) 

// Which is the same as this 
def isEven(i: Int): Boolean = { 
    i/2 == 0 
} 
foobar(isEven)