Map of Iterables - a generic way to add values to an iterable under the given key

3
votes

I can write the following for Vectors:

def add[K,V](map: Map[K,Vector[V]], key: K, values: Vector[V]): Map[K,Vector[V]] = {
  map + (key -> (map.getOrElse(key, Vector.empty) ++ values))
}

Usage:

scala> add(Map(1 -> Vector(1,2,3)), 1, Vector(4,5,6))
res1: Map[Int,Vector[Int]] = Map(1 -> Vector(1, 2, 3, 4, 5, 6))

The code for Sets is almost identical:

def add[K,V](map: Map[K,Set[V]], key: K, values: Set[V]): Map[K,Set[V]] = {
  map + (key -> (map.getOrElse(key, Set.empty) ++ values))
}

How to make a single function that works for all Iterables? I tried to write it like this:

def add[K,V](map: Map[K,Iterable[V]], key: K, values: Iterable[V]): Map[K,Iterable[V]] = {
  map + (key -> (map.getOrElse(key, Iterable.empty) ++ values))
}

But in this case I lost type info:

scala> add(Map(1 -> Set(1,2,3)), 1, Set(4,5,6))
res4: Map[Int,Iterable[Int]] = Map(1 -> Set(5, 1, 6, 2, 3, 4))

I tried the following:

def add[K,V,I[_] <: Iterable[_]](map: Map[K,I[V]], key: K, values: I[V]): Map[K,I[V]] = {
  map + (key -> (map.get(key).map(_ ++ values).getOrElse(values)))
}

But it didn't compile:

Cannot construct a collection of type That with elements of type Any based on a collection of type Repr. [error] map + (key -> (map.get(key).map(_ ++ values).getOrElse(values)))

2
scala

2 Answers

5
votes

You need to use CanBuildFrom to channel the type info about the used collection through the ++ call. See the overloads of ++ on TraversableLike for this.

The following works (not caring about details, like undefined keys).

def add[K,V,It <: TraversableLike[V,It]]
  (map: Map[K,It],k: K, vs: Traversable[V])
  (implicit bf: CanBuildFrom[It,V,It])
  :Map[K,It] = 
    map + (k -> (map(k) ++ vs))
2
votes

Ugly instanceof but works :)

def add[K,V,I[V] <: Iterable[V]](map: Map[K,I[V]], key: K, values: I[V]): Map[K,I[V]] = {
       val m: Map[K, Iterable[Any]] =  map + (key -> (map.get(key).map(_ ++ values.asInstanceOf[Iterable[V]]).getOrElse(values)))
       m.asInstanceOf[Map[K, I[V]]]
}

Results from scala console:

scala> def add[K,V,I[V] <: Iterable[V]](map: Map[K,I[V]], key: K, values: I[V]): Map[K,I[V]] = {
     |        val m: Map[K, Iterable[Any]] =  map + (key -> (map.get(key).map(_ ++ values.asInstanceOf[Iterable[V]]).getOrElse(values)))
     |        m.asInstanceOf[Map[K, I[V]]]
     | }
warning: there were 1 feature warning(s); re-run with -feature for details
add: [K, V, I[V] <: Iterable[V]](map: Map[K,I[V]], key: K, values: I[V])Map[K,I[V]]

scala> add(Map(1 -> Vector(1,2,3)), 1, Vector(4,5,6))
res0: Map[Int,scala.collection.immutable.Vector[Int]] = Map(1 -> Vector(1, 2, 3, 4, 5, 6))

scala> add(Map(1 -> Set(1,2,3)), 1, Set(4,5,6))
res1: Map[Int,scala.collection.immutable.Set[Int]] = Map(1 -> Set(5, 1, 6, 2, 3, 4))