Why immutable.Map's value type is covariant while mutable's one is not

Question

just read Scala collection implementation and noticed a subtile difference:

immutable HashMap : class HashMap[A, +B]()
mutable HashMap : class HashMap[A, B]()

Could you please explain me why the immutable's value type is covariant while the mutable's one is not ?

Thanks for your help

Probably not a duplicate, but related: https://stackoverflow.com/q/49410925/736957 — laughedelic, Mar 21 '18 at 18:02

score 6 · Accepted Answer · answered Mar 21 '18 at 18:15

If the mutable map were covariant, it would be possible to do something like this:

val m1: mutable.Map[String, Int] = mutable.Map.empty[String, Int]
val m2: mutable.Map[String, Any] = m1
m2 += ("foo" -> "bar")
m1("foo") // returns "bar" out of a Map[String, Int]???

score 3 · Answer 2 · answered Mar 21 '18 at 18:11

The mutable HashMap has methods taking B as argument, e.g. def update(key: A, value: B): Unit. Presence of such methods means B can't be covariant, or you could write

val map: mutable.HashMap[Int, AnyRef] = mutable.HashMap.empty[Int, String]
map.update(0, new AnyRef) // call update(Int, AnyRef) where (Int, String) is expected

The immutable one doesn't.

Why immutable.Map's value type is covariant while mutable's one is not

2 Answers2