Question

编译：

 def fibonacci():() => Int = {
        var first   = 1
        var second  = 2
        return () => {
            val oldFirst = first
            first = second
            second = second + oldFirst
            second 
        }
    }

这不是：

 def fibonacci():() => Int = {
     var first   = 1
     var second  = 2
     return ():Int => {
         val oldFirst = first
         first = second
         second = second + oldFirst
         second 
     }
 }

我明确地试图告诉它我正在返回一个Int，但我收到此错误：Illegal start of declaration，它指向first = second行。他们有什么不同？我使用的是Scala 2.8.1。

Answer 1

return (): Int => {...}在Scala中不是一个正确的表达式。如果要明确指定返回类型，则需要在之后放置声明（并且值将是匿名函数）：

def fibonacci():() => Int = { var first = 1 var second = 2 return ( () => { val oldFirst = first first = second second = second + oldFirst second } ) : () => Int }

但请注意，没有必要这样做。如果省略return，则根本不需要进行任何明确的类型声明：

def fibonacci() = { var first = 1 var second = 2 () => { val oldFirst = first first = second second = second + oldFirst second } }

Answer 2

Debilski是对的。还有两条评论：（1）return关键字不是必需的。默认情况下，最后一个表达式成为返回值。（2）如果你试图单独注释函数体的类型，那是可能的。代码变为：

def fibonacci2(): () => Int = {
  var first   = 1
  var second  = 2
  () => {
    val oldFirst = first
    first = second
    second = second + oldFirst
    second 
  }: Int
}

Scala如何以不同方式推断这两个函数？

2 个答案: