Flattening a map of sets - scala

Question

I am trying to flatten a map where the keys are traversables, in the sense that:
Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')
should flatten to:
Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)
Here is what I did:
def fuse[A, B, T <: Traversable[A]](mapOfTravs: Map[T, B]): Map[A, B] = {
val pairs = for {
trav <- mapOfTravs.keys
key <- trav
} yield (key, mapOfTravs(trav))
pairs.toMap
}
It works. But:
Is there a simpler way to do this?
I'm not very comfortable with the Scala type system and I'm sure this can be improved. I have to specify the types explicitly whenever I use my function:
val map2 = Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')
val fused2 = fuse[Int, Char, Set[Int]](map2)
val map1: Map[Traversable[Int], Char] = Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')
val fused1 = fuse[Int, Char, Traversable[Int]](map1)
P.S.: this fuse function does not make much sense when the key traversables have a non-null intersection.

Answer 1

This is basically what you're doing in the for comprehension, but simplified a little bit:
def fuse[A, B, T <: Traversable[A]](mapOfTravs: Map[T, B]): Map[A, B] = {
mapOfTravs.flatMap({ case (s, c) => s.map(i => i -> c) })
}
Not much you can do about the types, I'm sure there's some type lambda shenanigans that you can do, I'm just not sure how to do them...
UPDATE
Here's a slightly better for version, same as the above flatMap:
def fuse2[A, B, T <: Traversable[A]](mapOfTravs: Map[T, B]): Map[A, B] = {
for {
(keys, value) <- mapOfTravs
key <- keys
} yield key -> value
}

Answer 2

Like #Azzie, I was thinking zip, but maybe Azzie has the advantage with those zees.
scala> val m = Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')
m: scala.collection.immutable.Map[scala.collection.immutable.Set[Int],Char] = Map(Set(1, 2, 3) -> A, Set(4, 5, 6) -> B)
scala> (m map { case (k, v) => k zip (Stream continually v) }).flatten.toMap
res0: scala.collection.immutable.Map[Int,Char] = Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)
scala> (m map { case (k, v) => k zipAll (Nil, null, v) }).flatten.toMap
res1: scala.collection.immutable.Map[Any,Char] = Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)
scala> m flatMap { case (k, v) => k zip (Stream continually v) }
res2: scala.collection.immutable.Map[Int,Char] = Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)
It's not obvious how to generalize it nicely.

Answer 3

This looks horrible and using 0 is kind of cheating but it does the job
m.map( {case (s,c) => s.zipAll(Set(c),0,c)} ).flatten.toMap

Answer 4

Since I'm apparently on a "hideously generic implicits" kick lately:
import scala.collection.MapLike
import scala.collection.TraversableLike
import scala.collection.generic.CanBuildFrom
implicit class Map_[
A,
B,
T1 : ({type L[X] = X => TraversableLike[A, T2]})#L,
T2,
M1 : ({type L[X] = X => MapLike[T1, B, M2]})#L,
M2 <: MapLike[T1, B, M2] with Map[T1, B]
](map: M1) {
def fuse[M3](implicit cbfM: CanBuildFrom[M2, (A, B), M3]) : M3 =
map.flatMap({ case (k, v) => k.toTraversable.map((_, v)) })
}
Examples:
scala> Map(Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B').fuse
res: scala.collection.immutable.Map[Int,Char] =
Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)
scala> Map(Array(1, 2, 3) -> 'A', Array(4, 5, 6) -> 'B').fuse
res: scala.collection.immutable.Map[Int,Char] =
Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)