Add When Things Don't Match The With Statements as an elixir til

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

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+# 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.
+
+```elixir
+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`](https://hexdocs.pm/elixir/Kernel.SpecialForms.html#with/1) for more
+details.