Call By Value
: 对函数实参求值，且仅求一次

Call By Name
: => 函数实参在函数体内被用到时都会求值

var value = 0
def valueIncr() = {
  value+=1
  value
}
def test1(a:Int) = a+a
def test2(a: =>Int) = a+a

test1(valueIncr())
res205: Int = 2
value
res206: Int = 1

// valueIncr() 调用两次
// valueIncr() + valueIncr()
test2(valueIncr())
res207: Int = 5

value
res208: Int = 3