Merge maps by key

Question

Say I have two maps:

val a = Map(1 -> \"one\", 2 -> \"two\", 3 -> \"three\")
val b = Map(1 -> \"un\", 2 -> \"deux\", 3 -> \"trois\")
         


        

Answer 1

So I wasn't quite happy with either solution (I want to build a new type, so semigroup doesn't really feel appropriate, and Infinity's solution seemed quite complex), so I've gone with this for the moment. I'd be happy to see it improved:

def merge[A,B,C](a : Map[A,B], b : Map[A,B])(c : (B,B) => C) = {
  for (
    key <- (a.keySet ++ b.keySet);
    aval <- a.get(key); bval <- b.get(key)
  ) yield c(aval, bval)
}
merge(a,b){Seq(_,_)}

I wanted the behaviour of returning nothing when a key wasn't present in either map (which differs from other solutions), but a way of specifying this would be nice.