Can you merge the maps first and then apply the function?

Map.merge(a, b, fn _key, value1, value2 -> {value1, value2} end)
|> Map.new(fn
  {key, {value1, value2}} -> {key, f(value1, value2)}
  {key, value} -> {key, f(value)}
end)

It could get a bit cleaner if Elixir had an equivalent of Ruby’s transform_values, then it wouldn’t be necessary to repeat the key manually.

I’m guessing you want to use Stream because of t hef function? In that case I think you could first use Stream.map and then convert the result back into a map.

map1 = %{a: 1, b: 2}
map2 = %{b: 1, c: 1}
fun = fn x, y -> x + y end

for {k1, v1} <- map1, {k2, v2} <- map2, reduce: %{} do
  acc ->
    if k1 == k2 do
      Map.put(acc, k1, fun.(v1, v2))
    else
      acc
      |> Map.put(k1, v1)
      |> Map.put(k2, v2)
    end
end

edit: after reviewing it myself this is not a great solution.

This one iterates map2 for each key/value pair in map1. That can become expensive rather quickly.

a = %{1=>1, 2=>2, 4=>4}
b = %{0=>0, 1=>11, 2=>22, 5=>55}
c = %{}
keys = Map.merge(a, b) |> Map.keys

for key ← keys do
case {Map.take(a, [key]), Map.take(b, [key])} do
{a_value, b_value} when map_size(b_value) == 0 → Map.put(c, key, f.(a_value))
{a_value, b_value} when map_size(a_value) == 0 → Map.put(c, key, f.(b_value))
{a_value, b_value} → Map.put(c, key, ff.(a_value, b_value))
end
end

Would something like this work?

I think you’ll get (k1+k2) *2 only if you can make sure non of the map values is whatever you store in the map as an intermediate representation for “there were matching keys with two separate values”.