Question

Given a Map[Int, Set[Int]], how can I modify a single value of the Map, generating a new one in the process, for example:

val x = Map(1 -> Set(1,2,3))
x(1) + 5 // This creates a new Set, but not a new Map

val y = x(1) change { x => x + 5 }
// The previous functionality is what I'm looking for
// z: Set[Int]] = List(Set(1, 2, 3, 5))

Answer 1

In scala 2.10:

implicit class ChangeableMap[K,V]( val m: Map[K,V] ) extends AnyVal {
  def change( k: K )( transform: V => V ): Map[K,V] = {
    m.get( k ).map{ v => m + (k-> transform(v)) }.getOrElse( m )
  }
}

Some test:

scala>val x = Map(1 -> Set(1,2,3), 2 -> Set(4,5))
x: scala.collection.immutable.Map[Int,scala.collection.immutable.Set[Int]] = Map(1 -> Set(1, 2, 3), 2 -> Set(4, 5))
scala> x.change(1) { x => x + 5 }
res1: Map[Int,scala.collection.immutable.Set[Int]] = Map(1 -> Set(1, 2, 3, 5), 2 -> Set(4, 5))

If you’re in scala 2.9, this will do:

class ChangeableMap[K,V]( m: Map[K,V] ) {
  def change( k: K )( transform: V => V ): Map[K,V] = {
    m.get( k ).map{ v => m + (k-> transform(v)) }.getOrElse( m )
  }
}
implicit def toChangeableMap[K,V]( m: Map[K,V] ) = new ChangeableMap[K,V]( m )