表达式就是一个语句块，包含一条或者多条语句
表达式的返回值是 最后一条语句的执行结果

var a = 1
var b = 2
// c = 2
var c = if(a>b) a else b
// c = 5
c = {
a = a+1
b = b+1
a+b
}

可以把表达式当做参数来传递
block: =>Unit    没有入参,返回类型Unit 

def runIt(block: => Unit){
  println("before run")
  block
  println("after run")
}

runIt {
  println("code1 run")
  println("code2 run")
}


var a = 1;var b = 2; a+b
var c = {var a = 1;var b = 2; a+b}
var c = {
var a = 1;
var b = 2; 
a+b
}

var max  =  a > b match{
  case true => a
  case false => b
}

var c = a match{
  case 1 | 2 |3 => 0
  case 10 => 1
}

a = 5
不配会报错  scala.MatchError: 5 (of class java.lang.Integer)

通配符模式
case _ => ...