Add Using When Clauses In A With Construct 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/).
 
-_779 TILs and counting..._
+_780 TILs and counting..._
 
 ---
 
@@ -159,6 +159,7 @@ _779 TILs and counting..._
 - [String Interpolation With Just About Anything](elixir/string-interpolation-with-just-about-anything.md)
 - [Unique Indexes With Ecto](elixir/unique-indexes-with-ecto.md)
 - [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)
 - [Word Lists For Atoms](elixir/word-lists-for-atoms.md)
 

elixir/using-when-clauses-in-a-with-construct.md

@@ -0,0 +1,21 @@
+# Using When Clauses In A With Construct
+
+Because Elixir's `with` construct supports the full power of the language's
+pattern matching, we can use `when` clauses to further narrow down our
+matches.
+
+For instance, if we want to match against the response to an API request,
+but only for response status codes in the 2xx range, we can do something
+like the following:
+
+```elixir
+with %{status_code: code, body: body}
+       when code >= 200 && code < 300 <- HTTPoison.get!(url),
+     {:ok, decoded_body} <- Poison.decode(body) do
+  {:ok, decoded_body}
+end
+```
+
+See the [docs for
+`with`](https://hexdocs.pm/elixir/Kernel.SpecialForms.html#with/1) for more
+details.