是否有理由将scala中的赋值计算为Unit？

Question

  

可能重复：
  What is the motivation for Scala assignment evaluating to Unit rather than the value assigned?

为什么scala中的赋值会评估为Unit？

var s:String = null

s = "hello"     // works
s += " world" 

(s = "hello") += " world" // dramatic problem

或括号中有什么神奇的东西吗？