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

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_784 TILs and counting..._
+_785 TILs and counting..._
 
 ---
 
@@ -163,6 +163,7 @@ _784 TILs and counting..._
 - [Updating Values In A Map](elixir/updating-values-in-a-map.md)
 - [Using When Clauses In A With Construct](elixir/using-when-clauses-in-a-with-construct.md)
 - [Virtual Fields With Ecto Schemas](elixir/virtual-fields-with-ecto-schemas.md)
+- [When Things Don't Match The With Statements](elixir/when-things-dont-match-the-with-statements.md)
 - [Word Lists For Atoms](elixir/word-lists-for-atoms.md)
 
 ### Git

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

@@ -0,0 +1,30 @@
+# 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.