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.