Question

I have just noticed that whilst I need clarifying parens when adding a pair to a map, I don’t need them when doing a re-assignment:

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Get me some values

scala> var n = Map.empty[Int, String]
n: scala.collection.immutable.Map[Int,String] = Map()

Trying to add to the map with no clarifying parentheses:

scala> n + 1 -> ""
<console>:30: error: type mismatch;
 found   : Int(1)
 required: (Int, ?)
              n + 1 -> ""
                  ^

Fails as I expected it to. But doing the same via a re-assignment does not:

scala> n += 1 -> ""

scala> n
res12: scala.collection.immutable.Map[Int,String] = Map(1 -> "")

What is going on? Why is this not failing? Is scalac adding parens itself?

Answer 1

It’s just a precedence issue. + and - have the same precedence. = is lower. So Scala sees (n + 1) -> "" in the former case, and n += (1 -> "") in the latter (which is then transformed to n = (n + (1 -> "")) according to the normal rules for assignment operators.