when-things-dont-match-the-with-statements.md

When Things Don't Match The With Statements

You set up a series of match statements in a with construct as a way of avoiding a bunch of nested if statements. Inevitably you will be passing data through that doesn't meet all of the match criteria. By default, the with construct will short circuit and your program will continue from there.

You can, however, take more control over how you handle the failure cases by employing an else block. The else block works a lot like a case statement.

with %{status_code: 200, body: body} <- HTTPoison.get!(url),
     {:ok, decoded_body} <- Poison.decode(body) do
  {:ok, decoded_body}
else
  %{status_code: 401} ->
    reauthenticate()
  _ ->
    log_error()
end

Here we are able to anticipate a failure case and respond accordingly. For everything else, we have a generic action that we take.

See the docs for with for more details.