Why is there no mutable TreeMap in Scala?

Question

Is it lack of time, some technical problem or is there a reason why it should not exist?

Answer 1

There may be performance reasons for a mutable TreeMap, but usually you can use an immutable map in the same way as you would a mutable one. You just have to assign it to a var rather than a val. It would be the same as for HashMap, which has mutable and immutable variants:

val mh = collection.mutable.HashMap[Int, Int]()
var ih = collection.immutable.HashMap[Int, Int]()
mh += (1 -> 2)
ih += (1 -> 2)
mh // scala.collection.mutable.HashMap[Int,Int] = Map(1 -> 2)
ih // scala.collection.immutable.HashMap[Int,Int] = Map(1 -> 2)