花括号中的奇怪行为与scala中的大括号

Question

我已经阅读了stackoverflow中的几个大括号和大括号的差异，例如What is the formal difference in Scala between braces and parentheses, and when should they be used?，但我没有找到我以下问题的答案

object Test {
  def main(args: Array[String]) {
    val m = Map("foo" -> 3, "bar" -> 4)
    val m2 = m.map(x => {
      val y = x._2 + 1
      "(" + y.toString + ")" 
    })

    // The following DOES NOT work
    // m.map(x =>
    //   val y = x._2 + 1
    //   "(" + y.toString + ")"
    // )
    println(m2)

    // The following works
    // If you explain {} as a block, and inside the block is a function
    // m.map will take a function, how does this function take 2 lines?
    val m3 = m.map { x => 
      val y = x._2 + 2         // this line
      "(" + y.toString + ")"   // and this line they both belong to the same function
    }
    println(m3)
  }
}

Answer 1

答案很简单，当你使用类似的东西时：

...map(x => x + 1) 

你只能有一个表达式。所以，像：

scala> List(1,2).map(x => val y = x + 1; y)
<console>:1: error: illegal start of simple expression
List(1,2).map(x => val y = x + 1; y)
...

根本行不通。现在，让我们对比一下：

scala> List(1,2).map{x => val y = x + 1; y} // or
scala> List(1,2).map(x => { val y = x + 1; y })
res4: List[Int] = List(2, 3)

甚至更进一步：

scala> 1 + 3 + 4
res8: Int = 8

scala> {val y = 1 + 3; y}  + 4
res9: Int = 8

顺便说一句，最后一个y 从未离开{} 的范围，

scala> y
<console>:18: error: not found: value y