Function body is nested too deep

learn to use with correctly

As you are mapping the return value of the function to itself, you can just avoid the inner case:

case NaiveDateTime.from_iso8601(valid_time) do
  {:ok, datetime} ->
    Somefunction(which give me true or false)

  {:error, _} ->
    "datetime is not valid"
end

In this case, I don’t think that with would help (but it might, depending on the code outside the outer case statement, which we don’t see).

In this particular case, @lucaong already provided the best suggestion. In general, we usually introduce a private function with several clauses in such a case.

valid_time
|> NaiveDateTime.from_iso8601()
|> do_validate_instead_of_case()

[...]

defp do_validate_instead_of_case({:ok, datetime}) do
  case Somefunction(which give me true or false) do
    ...
  end
end

defp do_validate_instead_of_case({:error, _}),
  do: "datetime is not valid"

Thanks :).