Question

Is there a shortcut to filter a Map keeping only the entries where the key is contained in a given Set?

Here is some example code

scala> val map = Map("1"->1, "2"->2, "3"->3)
map: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1, 2 -> 2, 3 -> 3)

scala> map.filterKeys(Set("1","2").contains)
res0: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1, 2 -> 2)

I am searching for something shorter than this.

Answer 1

Answering the Question

You can take advantage of the fact that a Set[A] is a predicate; i.e. A => Boolean

map filterKeys set

Here it is at work:

scala> val map = Map("1" -> 1, "2" -> 2, "3" -> 3)
map: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1, 2 -> 2, 3 -> 3)

scala> val set = Set("1", "2")
set: scala.collection.immutable.Set[java.lang.String] = Set(1, 2)

scala> map filterKeys set
res0: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1, 2 -> 2)

Or if you prefer:

scala> map filterKeys Set("1", "2")
res1: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1, 2 -> 2)

---

Predicates

It’s actually really useful to have some wrapper around a predicate. Like so:

scala> class PredicateW[A](self: A => Boolean) {
   | def and(other: A => Boolean): A => Boolean = a => self(a) && other(a)
   | def or(other: A => Boolean): A => Boolean = a => self(a) || other(a)
   | def unary_! : A => Boolean = a => !self(a)
   | }
defined class PredicateW

And an implicit conversion:

scala> implicit def Predicate_Is_PredicateW[A](p: A => Boolean) = new PredicateW(p)
Predicate_Is_PredicateW: [A](p: A => Boolean)PredicateW[A]

And then you can use it:

scala> map filterKeys (Set("1", "2") and Set("2", "3"))
res2: scala.collection.immutable.Map[java.lang.String,Int] = Map(2 -> 2)

scala> map filterKeys (Set("1", "2") or Set("2", "3"))
res3: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1, 2 -> 2, 3 -> 3)

scala> map filterKeys !Set("2", "3")
res4: scala.collection.immutable.Map[java.lang.String,Int] = Map(1 -> 1)

This can be extended to xor, nand etc etc and if you include symbolic unicode can make for amazingly readable code:

val mustReport = trades filter (uncoveredShort ∨ exceedsDollarMax)

val european = { 
  val Europe = (_ : Market).exchange.country.region == Region.EU
  trades filter (_.market ∈: Europe)
}