I have the following small bind (or flat_map) implementation for error tuples.

@spec bind({:ok, a} | {:error, reason}, function(a) :: {:ok, b} | {:error, reason}) ::
        {:ok, b} | {:error, reason}
      when a: any, b: any, reason: term
def bind({:ok, value}, func) when is_function(func, 1), do: func.(value)
def bind({:error, reason}, _func), do: {:error, reason}

When I use this function as follows

@spec safe_div(integer, integer) :: {:ok, float} | {:error, :zero_division}
def safe_div(_, 0) do
  {:error, :zero_division}
end

def safe_div(a, b) do
  {:ok, a / b}
end

OK.bind({:ok, 4}, &safe_div(3, &1))

I see the following warning.

test/integration.ex:5:no_return
Function run/0 has no local return.
________________________________________________________________________________
Please file a bug in https://github.com/jeremyjh/dialyxir/issues with this message.

Failed to parse warning:
[{:"(", 1}, {:"{", 1}, {:atom_full, 1, '\'ok\''}, {:",", 1}, {:atom_part, 1, 'a'}, {:"}", 1}, {:|, 1}, {:"{", 1}, {:atom_full, 1, '\'error\''}, {:",", 1}, {:atom_part, 1, 'r'}, {:atom_part, 1, 'e'}, {:atom_part, 1, 'a'}, {:atom_part, 1, 's'}, {:atom_part, 1, 'o'}, {:atom_part, 1, 'n'}, {:"}", 1}, {:",", 1}, {:atom_part, 1, 'f'}, {:atom_part, 1, 'u'}, {:atom_part, 1, 'n'}, {:atom_part, 1, 'c'}, {:atom_part, 1, 't'}, {:atom_part, 1, 'i'}, {:atom_part, 1, 'o'}, {:atom_part, 1, 'n'}, {:"(", 1}, {:atom_part, 1, 'a'}, {:")", 1}, {:::, 1}, {:"{", 1}, {:atom_full, 1, '\'ok\''}, {:",", 1}, {:atom_part, 1, 'b'}, {:"}", 1}, {:|, 1}, {:"{", 1}, {:atom_full, 1, '\'error\''}, {:",", 1}, {:atom_part, 1, 'r'}, {:atom_part, 1, 'e'}, {:atom_part, 1, 'a'}, {:atom_part, 1, 's'}, {:atom_part, 1, 'o'}, {:atom_part, 1, 'n'}, {:"}", 1}, {:")", 1}, {:->, 1}, {:"{", ...}, {...}, ...]


Legacy warning:
test/integration.ex:12: The call 'Elixir.OK':bind({'ok', 4},fun((_) -> {'error','zero_division'} | {'ok',float()})) breaks the contract ({'ok',a} | {'error',reason},function(a)::{'ok',b} | {'error',reason}) -> {'ok',b} | {'error',reason} when a :: any(), b :: any(), reason :: term()
________________________________________________________________________________
test/integration.ex:42:unused_fun
Function fetch_key/2 will never be called.
________________________________________________________________________________
done (warnings were emitted)

I can’t make head or tail of it. Any help would be appreciated.
You can see the full code on this PR. Add remaining type specs by CrowdHailer · Pull Request #54 · CrowdHailer/OK · GitHub