Scala Map: mysterious syntactic sugar?

I have just found out this syntax for a scala Map (used here in mutable form)

val m = scala.collection.mutable.Map[String, Int]()
m("Hello") = 5
println(m) //PRINTS Map(Hello -> 5)

Now I'm not sure whether this is syntactic sugar built in to the language, or whether something more fundamental is going on here involving the fact that a map extends a PartialFunction. Could anyone explain?

From stackoverflow

• If you mean (it would be nice if you could be more explicit)

  m("Hello") = 5
  

  that is intended syntactic sugar for

  m.update("Hello", 5)
  

  independent of what m is. This is analogous to

  m("Hello")
  

  which is syntactic sugar for

  m.apply("Hello")
  

  (I'm just reading "Programming in Scala".)

  oxbow_lakes : Could you point me to the page in Programming in Scala; unless I'm being blind I can't find it. I'm really not convinced that this is syntactic sugar at all

  oxbow_lakes : Can you indicate where the "update" method is in the hierarchy? I can't even find it!

  starblue : It's on page 40.

  oxbow_lakes : That's the bit in "Programming in Scala" which says that m("Hello") = 5 is equivalent to m.update("Hello", 5). But I can't actually see that Map has a method "update" on it.

  oxbow_lakes : Ah, it's on DefaultMapModel!

• @starblue is correct. Note that you can also do rather creative things with update such as returning values other than what was assigned. For example:

  val a = Map(1 -> "one")      // an immutable Map[Int, String]
  val b = a(2) = "two"
  val c = b(5) = "five"
  val d = c(1) = "uno"
  
  d == Map(1 -> "uno", 2 -> "two", 5 -> "five")       // => true
  

  This works because immutable.Map#update returns an instance of the new Map. It looks a little odd to the C-trained eye, but you get used to it.

  oxbow_lakes : So "m(K) = V" is syntactic sugar for m.update(K, V). How can one tell that this is the case? What other methods may have syntactic sugar applied to them? And I can't even see a method update on Map (or any of its inheritees) in the scaladoc. How would I even know that Map had such a method?