1.ex

defmodule A do
  # What you can write in Elixir
  @spec a(String.t()) :: integer
  def a(b) when is_bitstring(b) do
    if b == "hello" do
      1
    else
      2
    end
  end

  @spec a(any) :: false
  def a(_), do: false
end

2.ex

# What Erlang see
@spec a(String.t() | any) :: integer | false

# Note, that what this suggests is that somehow a() an be given a string and 
# return false. This will never and can never happen.