How does Scala's mutable Map update [map(key) = newValue] syntax work?
scala, scala-collections
Solution
This is an example of the `apply`, `update` syntax.
When you call `map("Something")` this calls `map.apply("Something")` which in turn calls `get`.
When you call `map("Something") = "SomethingElse"` this calls `map.update("Something", "SomethingElse")` which in turn calls `put`.
Take a look at this for a fuller explanation.
Problem
I'm working through Cay Horstmann's Scala for the Impatient book where I came across this way of updating a mutable map. ``` scala> val scores = scala.collection.mutable.Map("Alice" -> 10, "Bob" -> 3, "Cindy" -> 8) scores: scala.collection.mutable.Map[String,Int] = Map(Bob -> 3, Alice -> 10, Cindy -> 8) scala> scores("Alice") // retrieve the value of type Int res2: Int = 10 scala> scores("Alice") = 5 // Update the Alice value to 5 scala> scores("Alice") res4: Int = 5 ``` It looks like `scores("Alice")` hits `apply` in `MapLike.scala`. But this only returns the value, not something that can be updated. Out of curiosity I tried the same syntax on an immutable map and was presented with the following error, ``` scala> val immutableScores = Map("Alice" -> 10, "Bob" -> 3, "Cindy" -> 8) immutableScores: scala.collection.immutable.Map[String,Int] = Map(Alice -> 10, Bob -> 3, Cindy -> 8) scala> immutableScores("Alice") = 5 <console>:9: error: value update is not a member of scala.collection.immutable.Map[String,Int] immutableScores("Alice") = 5 ^ ``` Based on that, I'm assuming that `scores("Alice") = 5` is transformed into `scores update ("Alice", 5)` but I have no idea how it works, or how it is even possible. How does it work?