I don’t think there is such a way, you’re better off just writing two function variants for atom and binary argument.

The worst I could come up with is String.to_existing_atom(to_string(foo))

I think this is what @dimitarvp suggested:

hash_map = %{foo: "Foo", bar: "Bar"}

defmodule HashMap do
  def get(hash_map, key) when is_atom(key), do: hash_map[key]
  def get(hash_map, key) when is_binary(key), do: hash_map[String.to_existing_atom(key)]  
end

{atom, string} = {hash_map |> HashMap.get(:foo), hash_map |> HashMap.get("bar")}

# Output
# {"Foo", "Bar"}

If you pass non existent keys in, you get nil values back, no exception.

P.S. I just wrote it for practice, as I am learning Elixir.