Mutable map with multiple values in Scala - scala

Question

I want to have a map so that:
I can update it -> mutable
I can have multiple values -> old multi map
I.e. I need to have a collection of the form:
k1 -> v1, k1 -> v2, ..., k2 -> v1, k2 -> v2, k2 -> v3, etc.
At the end of the day, what I need is to collect all this information as:
k1 -> Seq(v1,v2), ..., k2 -> Seq(v1,v2,v3)
How can I do this in Scala?
Edit
I tried this in Scastie:
val m = new scala.collection.mutable.Map[Int,Int] with scala.collection.mutable.MultiMap[Int,Int]
m += (1 -> 2)
m += (1 -> 3)
m
But I get:
illegal inheritance;
<$anon: Int => Int with Int => scala.collection.mutable.Set[Int]> inherits different type instances of trait Map:
scala.collection.mutable.Map[Int,scala.collection.mutable.Set[Int]] and scala.collection.mutable.Map[Int,Int]
trait MultiMap in package mutable is deprecated (since 2.13.0): Use a scala.collection.mutable.MultiDict in the scala-collection-contrib module

Answer 1

When people say "avoid using mutable structures" in scala, it especially applies to those, who have to ask questions about how to create a multimap.
I mean, it's really no rocket science:
val foo = mutable.Map[Int, List[Int]]().withDefault(_ => Nil)
foo += 1 -> (1::foo(1))
foo += 1 -> (2::foo(1))
foo += 1 -> foo(1).filterNot(Set)
foo += 1 -> foo(1).tail
tuples.foreach { case (k,v) => foo += k -> (v::foo(k)) }
//etc.
But if you have to ask how to do this, chances are you are going to end up using it wrong (which is very easy with mutable structure). I strongly encourage you to stick with immutable collections (and avoid mutable state in general) until you got enough command of the language to be able to definitively identify situations where mutable is actually the correct solution.

Answer 2

As it was discussed in the comments section, you can proceed with MultiDict from scala-collection-contrlib library. Example:
import scala.collection.mutable
def convert[K, V](iterable: Iterable[(K, V)]): mutable.MultiDict[K, V] = {
iterable.foldLeft(mutable.MultiDict.empty[K, V])(_ addOne _)
}
val list = List("k1" -> "v1", "k1" -> "v2", "k1" -> "v3", "k2" -> "v1", "k3" -> "v1")
val map = convert(list)
println(map.sets)
Will print: HashMap(k1 -> HashSet(v1, v2, v3), k2 -> HashSet(v1), k3 -> HashSet(v1))
Scatie: https://scastie.scala-lang.org/uULFEKUwTVaSHLpJosnXYw