While not integer/number specific I like the idea of Guards in Elixir:

    def sum(a, b) when is_integer(a) and is_integer(b) do
      a + b
    end

    def sum(a, b) when is_list(a) and is_list(b) do
      a ++ b
    end

    def sum(a, b) when is_binary(a) and is_binary(b) do
      a <> b
    end

    sum 1, 2
    #=> 3

    sum [1], [2]
    #=> [1,2]

    sum "a", "b"
    #=> "ab"